intrinsically safe (is) active power supplieseprints.qut.edu.au/15896/1/mark_walpole_thesis.pdf ·...

INTRINSICALLY SAFE (IS)

ACTIVE POWER SUPPLIES

by

Mark Edward Walpole

Assoc. Dip. Elec. Eng., B Eng. (Hons.)

Submitted for the Degree of

Master of Engineering (research)

Queensland University of Technology

Faculty of Built Environment and Engineering

School of Electrical and Electronic Systems Engineering

Brisbane March, 2003

Keywords (ii)

Mark Walpole, Intrinsically Safe (IS) Active Power Supplies, M.Eng. Thesis, QUT 2003

intrinsic safety, intrinsically safe, active power supply, modelling, equivalent circuit,

intrinsic safety Standards, intrinsic safety assessment method

Abstract (iii)


Intrinsically safe (IS) active power supplies subjected to certain transient load

conditions can deliver power to a circuit at significantly higher levels than indicated

on their nameplate ratings. During a transient load such as an intermittent short-

circuit, energy is transferred from the power supply to the short-circuit and an

electrical arc may form when the short-circuit is applied or removed. This poses a

spark ignition risk as energy is transferred from the arc to the surrounding

atmosphere.

Currently various International and Australian Standards define the performance

requirements for IS electrical apparatus. A duly accredited laboratory is required to

establish the intrinsic safety compliance of an apparatus with the Standards. It

involves an assessment of the apparatus and may include testing. The assessment

of the apparatus determines adequate segregation, separation, construction, and

selection of components. The tests performed on the apparatus include a

temperature rise test and in some cases, the sparking potential of the circuit is

tested using the spark test apparatus (STA). Testing the sparking potential of active

power supplies to establish compliance adds significantly to the time and costs

involved in establishing compliance.

A new alternative assessment method is proposed in this report to augment or

replace the testing phase of the compliance certification process for active power

supplies. The proposed alternative assessment method (PAAM) is derived from a

determination of the steady-state and transient output characteristics of the active

power supply under consideration. Parameters such as peak output current, time

constant of peak current decay, and the output voltages at these times are

measured from the circuit's output characteristics. These measurements can

subsequently be used to derive the topology and component values of an equivalent

circuit. The resulting equivalent circuit is then considered like a linear power supply

and the sparking potential can be determined using existing assessment methods.

This thesis investigates in detail the equivalent circuit of a number of direct current

(DC) active power supplies whose transient output characteristics exhibit

predominantly capacitive behaviour. The results of the PAAM using the equivalent

circuit are then compared with results achieved using the current testing procedure

with a STA.

Abstract (iv)


A small sample of active power supplies is used to generate data from which a

relationship between the current testing procedure and the PAAM can be

established.

The PAAM developed in this research project can be used as a pre-compliance

check by designers, manufacturers, or IS testing stations. A failure of this test would

indicate that the active power supply’s sparking energy is not low enough to be

regarded as intrinsically safe. The PAAM requires fewer resources to establish a

result than the STA. The benefits of a simplified spark ignition test would flow on

from designers and manufacturers to end users.

Table of Contents (v)


Chapter 1 Introduction................................................................1

1.1 Background .................................................................................................1

1.2 Research Objectives....................................................................................2

1.3 Research Program.......................................................................................2

1.4 Scope of Thesis ...........................................................................................3

1.5 Publications .................................................................................................5

Chapter 2 Review of IS Power Supplies and Intrinsic Safety..6

2.1 Evolution of Intrinsic Safety .........................................................................6

2.1.1 Mechanism of Electrical Arcs .....................................................................7 2.1.2 Mechanisms of Ignition...............................................................................7 2.1.3 Energy Transferred from the Electric Arc ...................................................9 2.1.4 Development of the Principles of Intrinsic Safety .....................................11

2.2 IS Power Supplies .....................................................................................13

2.2.1 Evolution of IS Power Supplies ................................................................13 2.2.2 Modern IS Power Supplies .......................................................................14 2.2.3 Design Methodologies of IS Power Supplies............................................15

2.3 Types and Terminology of IS Power Supplies ...........................................17

2.3.1 Three Types of IS Power Supplies ...........................................................17 2.3.2 Definition of IS Power Supplies Terminology ...........................................19

2.4 IS Active Power Supplies...........................................................................21

2.5 Intrinsic Safety Standards..........................................................................25

2.5.1 Current Australian and International Standards .......................................25 2.5.2 Comparison of AS 2380.7 and AS/NZS 60079.11 ...................................27 2.5.3 Participants in Ensuring Intrinsic Safety ...................................................27 2.5.4 Accredited Intrinsic Safety Testing and Certification Bodies ....................29

2.6 Certification, Assessment and Testing of IS Power Supplies ....................31

2.6.1 Certification – Determining Conformance to a Standard ..........................31 2.6.2 Assessment of IS Active Power Supplies.................................................32 2.6.3 Testing IS Active Power Supplies using the STA .....................................34

2.7 Summary ...................................................................................................36

Table of Contents (continued) (vi)


Chapter 3 Electrical Investigation of the STA......................... 37

3.1 Introduction to the STA.............................................................................. 37

3.2 Low Voltage Electric Arcs and the STA..................................................... 40

3.3 Periodic and Randomness of the STA ...................................................... 43

3.4 Electrical Circuit of the STA....................................................................... 47

3.5 Sensitivity of the STA ................................................................................ 49

3.6 Summary................................................................................................... 52

Chapter 4 Characteristics of IS Active Power Supplies......... 53

4.1 Sample IS Active Power Supplies ............................................................. 53

4.2 Steady-state Output Characteristics.......................................................... 54

4.3 Transient Output Characteristics ............................................................... 56

4.3.1 Measuring Transient Characteristics using the STA ................................57 4.3.2 Measuring Transient Output Characteristics using a Relay .....................60 4.3.3 Limitations in Measuring Transient Output Characteristics ......................62

4.4 Transient Characteristics of Sample IS Active Power Supplies................. 64

4.5 Summary................................................................................................... 69

Chapter 5 Development of the PAAM...................................... 70

5.1 Assessment Methods for IS Active Power Supplies .................................. 70

5.2 The RLC Equivalent Circuit Model ............................................................ 72

5.3 Experimental Verification of the RLC Equivalent Circuit Model ................. 74

5.4 The RC Equivalent Circuit Model .............................................................. 80

5.5 Experimental Verification of the RC Equivalent Circuit Model ................... 81

5.6 The Proposed Alternative Assessment Method (PAAM) ........................... 85

5.7 Limitations of the PAAM ............................................................................ 87

5.8 Summary................................................................................................... 91

Chapter 6 Experimental Evaluation of the PAAM................... 92

6.1 Sample IS Active Power Supplies ............................................................. 92

6.2 Sample Active Power Supply Parameter Measurements .......................... 92

6.3 Example Application of PAAM................................................................... 94

6.4 Comparison with Spark Testing Results.................................................... 99

6.5 Summary................................................................................................. 101

Table of Contents (continued) (vii)


Chapter 7 Conclusions and Further Research .....................102

7.1 Conclusions .............................................................................................102

7.2 Further Research.....................................................................................103

References 105

Appendices

A 1. Generic Block Diagram of IS Active Power Supply...........................107

A 2. Measured Output Characteristic using STA......................................108

A 3. Measured Output Characteristic using Relay ...................................110

A 4. No-load to Short-circuit Output Characteristic ..................................112

A 5. Ignition Curves for ‘well defined’ Circuits ..........................................113

Resistive circuits..........................................................................113

Group I capacitive circuits ...........................................................114

Group I inductive circuits .............................................................115

List of Tables (viii)


Tables

Table 2-1: UK. National Coal Board DC IS power supplies [3] ................................13 Table 2-2: Summary of SIMTARS recommended design methodology [9]..............16 Table 2-3: Definition of active and passive power supplies......................................19 Table 2-4: Definition of linear and non-linear power supplies ..................................19 Table 2-5: Defining the types of IS power supplies ..................................................20 Table 2-6: Maximum values of V and I for Group I active power supplies [11] ........22 Table 2-7: Relevant Acts and Regulations [17] ........................................................25 Table 2-8: Summary of SIMTARS intrinsic safety assessment procedure [9]..........33 Table 2-9: Summary of SIMTARS intrinsic safety testing procedure [9] ..................34 Table 4-1: Measured steady-state parameters – sample active power supplies .....55 Table 4-2: Measured transient parameters – test circuit with STA...........................59 Table 4-3: Measured transient parameters – test circuit with a relay.......................61 Table 4-4: Instantaneous voltage and current for inductors and capacitors.............63 Table 5-1: Component equations for the RLC equivalent circuit model ...................73 Table 5-2: Experimental RLC equivalent circuit model – component values ...........74 Table 5-3: Component equations for the RC equivalent circuit model .....................80 Table 5-4: Experimental RC equivalent circuit model – component values .............81 Table 6-1: Measured transient parameters of sample active power supplies ..........93 Table 6-2: PAAM calculating component values (RC equiv. cct. model) - PS 1 ......95 Table 6-3: PAAM component values (RC equiv. cct. model) for PS 1, 2 and 3 .......96 Table 6-4: Comparison of results - PAAM vs. STA testing.....................................100

List of Figures (ix)


Figures

Figure 2-1: Energy system of an electrical arc ...........................................................8 Figure 2-2: Ignition kernel growth vs. ignition energy and quenching distance ........10 Figure 2-3: Power supply circuit topologies and their V-I characteristic [12] ............18 Figure 3-1: Plan and elevation views of STA wire holder and cadmium disk [24] ....38 Figure 3-2: Oblique view of the STA wire holder and the cadmium disk ..................39 Figure 3-3: STA wire and cadmium disk making and breaking contact....................40 Figure 3-4: STA making contact - discharging a capacitive circuit ...........................42 Figure 3-5: Test circuit with STA and wire path for a single traverse .......................43 Figure 3-6: Measured output current (IO) and voltage (UO) for a single traverse......44 Figure 3-7: Periodic make and break of wires on the cadmium disk ........................45 Figure 3-8: Measured periodic make and break waveform ......................................45 Figure 3-9: Geometry of arc scribed by the wire on cadmium disk ..........................46 Figure 3-10: STA electrical circuit.............................................................................48 Figure 3-11: STA calibration circuit with current measuring resistance....................50 Figure 3-12: Measured V and I waveforms for the STA calibration circuit ...............51 Figure 4-1: Block diagram of sample IS active power supply DC stage...................53 Figure 4-2: Steady-state test circuit..........................................................................54 Figure 4-3: Steady-state output characteristics ........................................................55 Figure 4-4: Transient characteristics test circuit with STA........................................57 Figure 4-5: Measured transient output characteristics (STA) ...................................58 Figure 4-6: Transient characteristics test circuit with a relay....................................60 Figure 4-7: Measured transient output characteristics (relay) ..................................61 Figure 4-8: Power supply output capacitance – external discharge path .................63 Figure 4-9: Active power supply NL to FL transient characteristics..........................64 Figure 4-10: Active power supply FL to SC transient characteristics .......................65 Figure 4-11: Active power supply NL to SC transient characteristics.......................66 Figure 4-12: Active power supply NL to SC transient characteristics.......................68 Figure 5-1: PAAM - RLC equivalent circuit model topology .....................................72 Figure 5-2: Experimental RLC equiv. cct. and steady-state characteristic ...............75 Figure 5-3: Experimental RLC equivalent circuit – transient tests............................75 Figure 5-4: Over damped RLC equiv. cct. NL to SC transient characteristics..........76 Figure 5-5: Over damped RLC equiv. cct. SC to NL transient characteristics..........77 Figure 5-6: Under damped RLC equiv. cct. NL to SC transient characteristics........78 Figure 5-7: Under damped RLC equiv. cct. SC to NL transient characteristics........79 Figure 5-8: PAAM - RC equivalent circuit model topology .......................................80

List of Figures (x)


Figure 5-9: Experimental RC equiv. cct. and steady-state characteristic.................81 Figure 5-10: Experimental RC equivalent circuit - transient test circuit ....................82 Figure 5-11: Measured RC equiv. cct. NL to SC transient characteristics ...............82 Figure 5-12: Measured RC equiv. cct. SC to NL transient characteristics ...............83 Figure 5-13: Illustration of ignition curve safe and unsafe areas..............................86 Figure 6-1: Measured transient output current response for PS 1 ...........................95 Figure 6-2: PAAM RC equivalent circuit model for PS 1 ..........................................96 Figure 6-3: PAAM ignition curve plots for PS 1, 2 and 3 [24] ...................................97

Abbreviations (xi)


AIT Auto ignition temperature

AS Australian Standard

AS/NZS Australian and New Zealand Standard

BS British Standard

BSI British Standards Institute

BVS Berggewerkschaftliche Versuchsstrecke (Approval organisation)

CENELEC European Committee for Electrotechnical Standardisation

ETCC SIMTARS Engineering Testing and Certification Centre

FOS Factor of Safety

HSE (M) Health and Safety Executive (Mining)

IEC International Electrotechnical Commission

IS Intrinsically Safe

JASANZ Joint Accreditation System of Australia and New Zealand

LEL Lower Explosion Limit

MIC Minimum Ignition Current

MIE Minimum Ignition Energy

MEIC Most Easily Ignited Concentration

NATA National Association of Testing Authorities, Australia

NSW New South Wales

NZ New Zealand

NZS New Zealand Standard

PAAM Proposed alternative assessment method

PS Power supply

QLD Queensland

QUT Queensland University of Technology

RC Resistive and Capacitive

RLC Resistive, Inductive and Capacitive

SIMTARS Safety In Mines Testing And Research Station (Approval organisation)

SMRE Safety in Mines Research Establishment (Approval organisation)

STA Spark Test Apparatus

UEL Upper Explosion Limit

UK United Kingdom

UL Underwriters Laboratories (Approval organisation)

Statement of original authorship (xii)


The work contained in this thesis has not been previously submitted for a degree or

diploma at any other higher education institution. To the best of my knowledge and

belief the thesis contains no material previously published or written by another

person except where due reference is made.

Signed ……………………………………..

Mark Walpole

Date: 27 / 3 / 2003

Acknowledgements (xiii)


I would like to express my appreciation to my principal supervisor Dr. Tee Tang for

his patience, guidance, and wisdom.

I would like to thank the following for providing industry support for this research:-

- Australian Coal Association Research Program (ACARP) for providing the

scholarship and research grant,

- Safety In Mines Testing And Research Station (SIMTARS) for providing facilities

and technical support to perform this research and

- Oakey Creek Coal Mining Company for providing active power supplies.

Finally but by no means the least I wish thank my parents and brother who

continually provide me with support and encouragement in all my endeavours.

For my three children Alex, Ryan and Jessica.

- 1 -


Chapter 1 Introduction

1.1 Background

Intrinsically safe (IS) active power supplies have received some degree of notoriety

in recent times. In December 1998 during the re-certification of an AUSTDAC Pty.

Ltd. IS active power supply it was discovered that the device failed to meet

requirements set out in the Australian Standard (AS) for intrinsic safety AS 2380.7.

The IS active power supply involved was in then current use in hazardous areas of

underground coal mines throughout Australia. The New South Wales (NSW)

Department of Mineral Resources issued safety alerts (SA98-0 23/12/98 and

SA99/01 5/2/99) [1]. The cost to the underground coal mines is uncertain according

to Bell and Hookham [2] who stated more than 50 mines in NSW and Queensland

(QLD) were affected.

The safety alerts were issued following the results of tests performed at TestSafe

Australia (formerly known as, Londonderry Occupational Safety Centre) in NSW, an

accredited testing laboratory, using the Spark Test Apparatus (STA) [1]. The tests

revealed that the IS active power supply in question was capable of generating

incendive electrical sparks under certain operating conditions. In the subsequent

months, additional intrinsic safety certificates and mining approvals of other IS active

power supplies were revoked.

These events illustrated the onerous nature of the task that the intrinsic safety

accreditation laboratories and certification bodies have in establishing that

equipment submitted to them for certification complies with the relevant AS thus

ensuring the safety of these devices in hazardous areas.

Power supply manufacturers are continually pressured by the industry to provide IS

power supplies that can deliver more power. Active power supplies can deliver more

power and have been used extensively in non-intrinsically safe industries. Their

deployment in underground coal mines poses a number of challenges to the issue of

intrinsic safety. Current Australian and International Standards were written for

passive power supplies and do not adequately cover aspects of the assessment and

testing of active power supplies.

Chapter 1 Introduction - 2 -


In the remainder of this chapter, the objectives of this research project are presented

followed by the details of research program, the scope of the research, and a list of

associated publications.

1.2 Research Objectives

This research aims at establishing the intrinsic safety requirements for IS active

power supplies. The objectives of the research and investigation are summarised as

follows:-

• identify the different forms of IS power supplies currently in operation and

clearly define the properties of each type and the differences between active,

passive, linear and non-linear power supplies

• investigate the industry standard intrinsic safety assessment and testing

practices for IS active power supplies

• analyse IS active power supply circuitry to determine energy outputs likely to

cause gas ignitions under dynamic conditions

• formulate and test a proposed alternative assessment method (PAAM)

1.3 Research Program

The major milestones of this research project are presented under the following

headings with a brief description of the research activities undertaken.

Literature review - A literature search to establish the current body of knowledge and

any other ongoing research activities related to IS power supplies

Definition of IS power supplies – An investigation of both the static and dynamic

output characteristics of IS active power supplies as currently used by the coal

mining industry

A review of typical assessment and testing practices – with reference to the Safety

In Mines Testing And Research Station (SIMTARS) IS active power supplies

assessment and testing procedures used during the compliance process.



A discussion of technologies used to control the output of IS power supplies –

Including investigation of IS active power supply circuits in order to derive a

functional block diagram and identify critical intrinsic safety parameters.

Development of a PAAM for IS active power supplies - The STA was investigated to

determine the methods by which IS active power supplies are tested. This leads on

to development of an assessment method for IS active power supplies using the

output steady-state and transient characteristics of the IS active power supply.

Designing, building and testing - A small number of sample IS active power supplies

are subjected to the PAAM and test outcomes are compared to results derived from

spark testing using the STA.

1.4 Scope of Thesis

Chapter 2 incorporates a literature review, which in addition to identifying the main

issues related to IS active power supplies introduces the fundamental concepts of

gas ignition and spark generation. The literature review covers the statutory

requirements, National and International approval schemes, National and

International Standards, intrinsic safety assessment and testing practices, and

summarises the major works of researchers in these areas.

National and International testing stations were contacted and requested to

contribute to this research project by providing access to their policies, procedures

and instructions for the assessment and testing of IS active power supplies.

Unfortunately, only one response was received stating that there were no

procedures available. Based on the minimal response received, it is assumed that

these documents either do not exist or are unavailable for review. Access to

SIMTARS policies, procedures and instructions enabled a review of SIMTARS

assessment and testing procedures, which is presented in Chapter 2. Recent events

that impact on assessment and testing practices have also been reviewed and are

included in Chapter 2.



In Chapter 3, the STA is investigated to determine its electrical characteristics and

the methods by which the sparking potential of active power supplies are tested.

Parameters in the output characteristics of the active power supply are identified.

These parameters determine whether the power supply is intrinsically safe.

In chapter 4, a number of IS active power supply circuits are analysed to identify the

methods used to control the output energy. IS power supply manufacturers’ circuit

diagrams are not readily available and access to SIMTARS intrinsic safety

certification documentation is restricted by client privacy agreements. However, a

functional block diagram is developed for active IS power supplies.

A concept for a PAAM is developed in Chapter 5. It makes use of the output steady-

state and transient characteristics of the active power supply. The PAAM uses an

equivalent circuit developed to represent the active power supply. Using the

equivalent circuit, an IS active power supply can be assessed similarly to the

existing current practices used for passive power supplies.

In Chapter 6, the PAAM is tested and the results analysed in order to investigate the

possibility of a relationship between the assessment of the equivalent circuit and the

results of spark testing the IS active power supply. A number of commercially

available IS active power supplies are subjected to the PAAM and they are spark

tested using the STA. The results from the two methods are compared in order to

develop a possible correlation.

The PAAM developed in this research may reduce or eliminate the need for spark

testing IS active power supplies. In Chapter 7, the implications of this research and

possible further research direction are discussed.



1.5 Publications

During the period of research, the following papers were published:

(1) Turner, D., Barnier, G. and Walpole, M., “Assessment, Testing, and

Certification of Intrinsically Safe Active Power Supplies”, Proceedings of

Mining Health and Safety Conference 2000, Townsville, August 2000 http://www.qmc.com.au/docs/conferences/QMC_2000/conf_turnerwalpole.pdf.

(2) Walpole, M. and Tang, T., “Modelling Active Power Supplies for Intrinsic

Safety Assessment”, Proceedings of Australasian Universities Power

Engineering Conference AUPEC 2002, CD-ROM, Melbourne, October 2002.

- 6 -


Chapter 2 Review of IS Power Supplies and Intrinsic Safety

The general topic of intrinsic safety and its application has been well documented [3-

8]. By contrast only a limited amount of literature exists concerning the topics of

design, characterisation, assessment and testing of intrinsically safe (IS) power

supplies [9-16]. It should be noted that most of the available research literature

relating to IS power supplies has been generated by testing laboratories [9-16].

Section 2.1 of this chapter gives a brief history of intrinsic safety and reviews the

mechanism of electrical arcs, mechanism of ignition, energy transfer from the arc,

and the principles of intrinsic safety. The development of IS power supplies is

presented in Section 2.2 and the types and terminology defined in Section 2.3. IS

active power supplies including their recent research activities are presented in

Section 2.4. The application of the relevant Standards are summarised in Section

2.5. The certification process including the assessment and testing of IS power

supplies is presented in Section 2.6. In Section 2.7 the main themes of the literature

review are summarised.

2.1 Evolution of Intrinsic Safety

The history of intrinsic safety dates back to the period between 1912–15, during

which the British Safety in Mines Research Establishment (SMRE) and other

international research laboratories developed the explosion protection technique.

This action was triggered by a series of colliery accidents in England, involving

explosions of fire damp. Subsequent investigations identified that sparking contacts

made by a signalling system in an atmosphere of coal gas were the most probable

cause of explosion [3]. “Fire damp, a gas consisting mainly of methane is generally

associated with coal seams, being produced during the process of formation of coal”

[3]. Fire damp, also known as coal gas, can be released during mining activity or

may occur due to the proximity of the coal seam itself.

Chapter 2 Review of IS Power Supplies and Intrinsic Safety - 7 -


2.1.1 Mechanism of Electrical Arcs

When two electrodes are separated in air by a distance called the spark gap (dsg) as

illustrated in Figure 2-1 (a) a spark will occur if the applied voltage between the

electrodes exceeds the breakdown voltage of the dielectric and there is sufficient

supply of current [5].

When the applied voltage across the spark gap is reduced below the breakdown

voltage of the dielectric, the arc will extinguish unless the arc itself has altered the

dielectric strength. It is usual for the arc to alter the dielectric either by ionisation of

the molecules or by contamination as a result of combustion. Combustion by-

products such as carbon in the spark gap may result in a reduction of dielectric

strength.

The energy available to the arc across the electrodes is a function of voltage,

current and time. In the closed energy system of power supply, electrodes and arc

as illustrated in Figure 2-1 (a), the available electrical energy at the electrodes is

converted into heat, light, sound, and other forms of electromagnetic radiation as in

Figure 2-1 (b). The heat generated due to the resistance of the arc path can be

either conducted into the electrodes, or by convection/radiation/conduction into the

surrounding matter. In the case where the surrounding matter is a flammable

gaseous mixture, an explosive ignition occurs if there is sufficient energy associated

with the arc.

2.1.2 Mechanisms of Ignition

Ignition is defined as the initiation of combustion of a flammable material. An ignition

occurs when there is sufficient energy in the electric arc to cause flammable gas

molecules in close proximity to the arc to be heated to a point above their auto

ignition temperature (AIT). If no further energy is supplied from the electric arc at this

point, the ignition will be quenched. Should additional energy be supplied via the

electric arc then the ignition kernel grows as more flammable gas molecules are

heated above the AIT and ignite. If the energy supplied by the arc is sufficient for the

ignition kernel diameter to exceed the quenching distance, the thermal energy

generated by the ignition will become self-sustaining, and a process known as

explosion results [5].



Figure 2-1: Energy system of an electrical arc

The smallest amount of energy required to ignite the most easily ignited

concentration (MEIC) of the gas or vapour is called the minimum ignition energy

(MIE). Scientific research institutions have established the values for the MEIC and

MIE of the most commonly used flammable gasses and vapours [5].

When the concentration of the flammable gas is below the lower explosion limit

(LEL), or above the upper explosion limit (UEL), an ignition cannot occur. Between

the LEL and UEL, it is possible, depending on the amount of energy in the spark, to

generate an ignition. A number of factors, such as the volume of gas, temperature,

humidity, and atmospheric pressure, act to directly influence the MIE and thus the

(a) High voltage spark generator

(b) Magnified view of arc

Series resistance R

DC High VoltageSource

Electrode

Arc

Electrode holder

Electrode holder

Electrode

dsgContact separation or Arc lengthor Spark gap

Arc path - ionisation

Arc resistance

Heat conducted into electrodes

Heat conducted into electrodes

Electrode

Heat - conducted to gas near the arc so that the gas heats

Electrode

Arc path - ionisation

Light Sound

other electromagnetic radiation



LEL and UEL. The LEL and UEL are typically expressed as a percentage, given by

the normalised ratio of the volume of the flammable gas or vapours to the volume of

air. As an example, the explosive concentrations of Methane gas are between LEL

of 5% and UEL of 15.9% [17] and the MEIC is in the range of 5.6% and 9% [5]

(Note: values are dependent upon environmental conditions and test apparatus

used).

Ignition may also occur as a result of high surface temperatures. If the temperature

of a surface in contact with an explosive concentration of a flammable gas exceeds

the AIT an explosive ignition will occur. The AIT of Methane is 537 oC [17]. In

electrical circuits, heat is dissipated through the components due to the finite

resistance of the current path. The surface temperature of an electrical component

is dependent upon the power dissipation and the power rating of the component.

2.1.3 Energy Transferred from the Electric Arc

Among the optimal conditions required in order to produce an arc is the use of high

voltage in conjunction with an adequate current supply. Under these conditions, it is

easy to quickly establish and maintain the arc. In a spark generator, the electrodes

are shaped to a point using low ohmic material that minimises the conduction of

heat away from the arc. The electrodes are positioned so that the points face one

another and they are separated by the spark gap (dsg). When the electrode

separation is less or greater than a critical distance called the quenching distance

(dq) a significant increase in the applied energy is required to cause an ignition of

the MEIC of an explosive gas mixture. The quenching distance is related to the size

of the gas molecules. The quenching distance of the MEIC of Methane-air mixture is

between 2.03-2.50 mm.

According to Magison, the value of MIE for Methane gas (coal gas) is 0.28 mJ [5].

This is determined by using a high voltage capacitive discharge test apparatus and

conditions that optimised the transfer of energy from the arc to the explosive gas

mixture. Under these conditions, there is minimal energy loss from the arc so that

most of the electrical energy in the arc is transferred to the surrounding MEIC of

explosive gas mixture. Once sufficient energy is transferred from the arc into the

gas, an explosive ignition occurs. For the MEIC of Methane-air, the breakdown

voltage required across this spark gap is in the range of 8-10 kV. In practice, MIE is

only a concern in high voltage circuits.



The output voltages of IS power supplies are usually low, typically in the range of 5-

48V. The mechanism of ignition for low voltage conditions according to Magison [5]

is similar to that for high voltage. That is, for an ignition to occur, the energy

transferred from the electric arc to the MEIC of the explosive gas will need to exceed

the MIE of the particular explosive gas. At lower voltages, the available energy in the

electric arc is less than that for a high voltage arc. Consequently, the transfer of

energy is not at the same rate or efficiency as for a high voltage arc.

Figure 2-2: Ignition kernel growth vs. ignition energy and quenching distance

In the case presented in Figure 2-2 the spark gap (electrode separation) distance

(dsg) is decreasing at a constant rate assuming a constant velocity, approaching

zero upon physical contact. The ignition kernel diameter (dk) growth is dependent

upon the energy transferred to the gas (Egas) and the relative difference between the

ignition kernel diameter and both the quenching and spark gap distances. This case

is more complex than for a fixed spark gap distance and is explained by the three

phases in the ignition kernel growth. In the first two phases the spark gap distance is

assumed to be greater than both ignition kernel diameter and quenching distance.

The initial phase of ignition kernel growth occurs once the arc is established evident

by the increase in spark gap current (Isg) and results in energy transfer from the arc

to the gas. Ignition kernel growth is relatively slow as the energy transferred to the

gas is less than optimal since the ignition kernel diameter is less than quenching

distance.

0

10

20

30

40

50

0 60 50

50 100 40

150 30

200 20

250 10

300 0

0

100

200

300

400

500

Spark gap Voltage Usg

E gas (mJ)

Ignition kernel growth in the spark gap (contact closing)

Spark gap current Isg

Energy transferred to gas Egas

Time (µs) Spark gap dsg (µm)

Usg (V), Isg (A)

dk < dq

dk = dq

dk > dq

MIE

Ignition kernel growth

dk > dq and Egas > MIE results in explosion

Assumed contact closing velocity 0.2 m/s



During the second phase of ignition kernel growth sufficient energy has transferred

to the gas so that the ignition kernel diameter is a similar size to the quenching

distance. During this phase the ignition kernel growth is at a maximum as the energy

transferred to the gas is optimal.

In the third phase stage the ignition kernel growth slows as the energy transferred to

the gas is again less than optimal since the ignition kernel diameter is greater than

quenching distance.

If during the second or the beginning of the third phases the ignition kernel has

reached a size where it is self sustaining then an explosive ignition will occur.

However, the further into the third phase where energy transfer is less than optimal

the less likely an explosive ignition will occur. This is due to two factors. The first is

that as the ignition kernel grows an increasing amount of energy is required to heat

the increasing volume of gas molecules and to overcome the losses at the periphery

of the ignition kernel. Secondly in the case of the closing contacts where the ignition

kernel diameter equals or exceeds the spark gap distance then the electrode

heating will further reduce the effective energy transferred to the gas.

2.1.4 Development of the Principles of Intrinsic Safety

The basic concept behind intrinsic safety relies on incorporating energy limitation to

ensure that an explosive ignition cannot occur through either spark or thermal

ignition. Therefore there will be insufficient energy available to heat the components

and, should a spark occur, there will be insufficient energy within the circuits to

cause an explosive ignition.

In an electrical circuit the possible sources of spark ignition include:-

• discharge of energy in a capacitive circuit when the circuit is closed

• discharge of energy in an inductive circuit when the circuit is opened

• intermittent making and breaking of a resistive circuit

• hot wire fusing

Intrinsically safe equipment utilises energy limitation by limiting its current or voltage,

and/or the duration of their occurrence. The spark test apparatus (STA), also known

as the break flash apparatus, is used to determine the sparking potential of the



electrical circuit (refer to Chapter 3). Alternatively, should the electrical circuit be

‘well defined’, the sparking potential of the electrical circuit can be determined by

assessment using ‘ignition curves’ defined in the intrinsic safety Standards. The

‘ignition curves’ are presented in AS 2381.7 and AS/NZS 60079.11 which is derived

from the International Standard IEC79. These Standards include ‘ignition curves’

(refer Appendix A 5) for the following ‘well defined’ electrical circuits:-

• linear DC voltage source and series current limiting resistor

• linear DC voltage source and shunt capacitance with/without series current

limiting resistor

• linear DC voltage source and series air-cored inductor and current limiting

resistor

These curves are used in Chapter 6 to assess the intrinsic safety compliance of

active power supplies.

In an electrical circuit the possible sources of thermal ignition include:-

• heating of a small gauge wire strand

• glowing of a filament or track on a printed circuit board

• high surface temperature of components [9]

IS equipment utilise components whose power ratings have been de-rated such that

the AIT of the hazardous atmosphere is not exceeded. Temperature rise tests are

performed to identify potential thermal ignition sources.



2.2 IS Power Supplies

IS power supplies are designed, manufactured and certified to meet specific criteria

in accordance with Australian or International Standards. These Standards specify

the amount of energy that an IS power supply is permitted to deliver to the IS circuit.

In this section a brief history covers the period from the early IS power supplies

through to the modern day. The modern IS power supply features, as well as their

design methodologies are described.

2.2.1 Evolution of IS Power Supplies

Early IS power supplies used by the British coal mining industry had considerably

higher output current and voltages than those permissible today. The United

Kingdom (UK) National Coal Board designed and certified a range of mains fed DC

power supplies to be used by the underground coal mining industry [3]. These IS

power supplies and their specifications are summarised in Table 2-1.

Each of the mains fed IS power supplies had a standby battery as indicated by the

left hand set of arrows in Table 2-1 in the case of electrical supply failure. The right

hand set of arrows indicate the specified voltage of the IS circuit being driven by the

respective mains fed IS power supplies.

The UK National Coal Board power supplies were used extensively throughout the

industry until 1965, when the new and more sensitive German STA was introduced.

A number of the existing IS power supplies had their certification revoked after it

was found that they were capable of generating incendive sparks on the new STA.

Table 2-1: UK. National Coal Board DC IS power supplies [3]

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This table is not available online. Please consult the hardcopy thesis available from the QUT Library



During the period from 1965 to 1970 IS “apparatus became more complex,

especially with the introduction of semiconductors”[3], and, as a result, the relevant

Standards were tightened. The research performed during this period was on simple

IS power supplies where the limits of intrinsic safety were derived from experiments

using the new STA.

2.2.2 Modern IS Power Supplies

One of the most significant events in the history of electronics was the introduction

of semiconductors. This development had far reaching effects. “The introduction of

solid state electronic systems created a need for regulated and stabilised DC power

supplies…” [13]. “Intensive automation and remote control in modern coal mines has

induced an exponential increase in the number of electrical apparatus with type

protection “i” - intrinsic safety. This causes the need for high-power intrinsically safe

power supplies being able to supply as many apparatus as possible…” [11].

The development of IS power supplies follows that of general-purpose power

supplies. The technical advances in general-purpose power supplies include the use

of more complex feedback techniques that enable output current fold-back

protection and switch-mode techniques. Both of these techniques involve the use of

non-linear devices.

The features in the modern IS power supply include: -

• voltage regulation and stabilisation

• filters to remove electrical noise

• over-voltage protection using semiconductor crowbar protection

• overload and short-circuit current protection using current limitation

• sophisticated fault detection, shutdown, and reset circuitry

Modern IS power supplies are significantly more complex than their predecessors

and this adds significantly to the effort associated with designing and testing these

products. The costs associated with accreditation for intrinsic safety have also

increased accordingly [18], [19].



2.2.3 Design Methodologies of IS Power Supplies

Designing IS power supplies according to Magison [6] is an iterative process and he

defines the tasks as:

“1. - Establish the intrinsic safety design objective. This will define the material

and temperature classes and will determine the type of intrinsic safety.

2. - Design the product.

3. - Document the design for the certifying authority to save time and money by

easing the evaluation and certification process.

4. - Document the design in manufacturing drawings and specifications to make

it easy to control the details relevant to intrinsic safety and its certification

throughout the life of the design”.

Magison goes on to further clarify Task 2. In Magison’s treatment of IS power

supplies he states that the Standards for intrinsic safety contain graphs of

characteristics for resistive circuits. “This ignition characteristic is only valid for

power supplies whose V-I characteristic is a straight line; that is; the Thevenin

equivalent circuit is a voltage source in series with a resistor…” [6]. This is also

confirmed by Dill and Kanty who state “The intrinsic safety of a power supply with

current limitation by resistors can be simply assessed using published reference

curves” [11].

Green and Thurlow [13] stated that the principal methods of “The design of an

intrinsically safe power supply can be based on one of two methods: resistive

limitation or zener diode clipping”. The authors then go on to present the utilisation

of a third method which utilises semiconductors for current limitation and voltage

regulation. “By using electronic devices to limit the current or spark duration an

attempt has been made to avoid the disadvantages of resistive limitation” [11].

An alternate approach proposed by SIMTARS uses an eight-step design procedure

presented in Table 2-2. This method is more detailed and is specifically aligned to

the intrinsic safety Standards AS 2380.7 and AS/NZS 60079.11.



Table 2-2: Summary of SIMTARS recommended design methodology [9]

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2.3 Types and Terminology of IS Power Supplies

IS power supplies are categorised by the method used to limit the output voltage

and current. It is applicable to both battery and mains fed power supplies. The

intrinsic safety Standards define mains fed IS power supplies as associated

apparatus. Associated apparatus include both non-IS and IS circuits where the non-

IS circuits cannot adversely affect the IS circuits. In the case of a mains fed IS

power supply it is only the IS low voltage output stage of the power supply that is

considered in this thesis. In this section three types of IS power supplies are

discussed followed by the definitions of the terminology relating to IS power

supplies.

2.3.1 Three Types of IS Power Supplies

Johannsmeyer and Kraemer [12] refer to three basic topologies of IS power

supplies: linear, trapezoidal, and rectangular. These are the descriptions of the

geometric shape of the output voltage versus output current (V-I) characteristics as

shown in Figure 2-3. The V-I characteristics illustrate the effect on output voltage as

a slowly decreasing resistive load (RL) is applied to the output terminals. As the

resistive load decreases from infinity, the output current increases from zero.

Linear power supplies are typified by straight line output V-I characteristics where

the gradient is determined by the series current limiting resistor (R) in Figure 2-3 (a).

The electronic components in the output stage of a linear power supply are passive

and are ‘well defined’.

The output stage of a linear power supply does not contain energy storage

components such as capacitors or inductors. The compliance process is by

assessment and the use of the ignition curves published in the Standards as

discussed in Section 2.1.4.

Trapezoidal power supplies have two linear sections. The first of these is where the

zener diode voltage regulator limits the output voltage for a range of load currents.

The second is where the load current exceeds the range that the zener diode can

regulate. The series current limiting resistor (R) determines the gradient of the

second section. The electronic components in the output stage of a trapezoidal



power supply with the exception of the zener diode are passive and are ‘well

defined’.

Figure 2-3: Power supply circuit topologies and their V-I characteristic [12]

The output stage of a trapezoidal power supply is similar to the linear power supply.

Where it does not contain energy storage components, the compliance process is

by assessment and the use of the ignition curves. In the case where the output

stage contains energy storage components the sparking potential of the circuit will

need to be tested using the STA.

Rectangular power supplies also have two linear sections. The first section is where

voltage regulation occurs for a range of load currents up to the maximum output

current. The second section is a current limited section where the output voltage is

reduced as the current exceeds the maximum output value and enters overload.

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This figure is not available online. Please consult the hardcopy thesis available from the QUT Library



The two linear sections indicate two modes of operation of rectangular power

supplies. The normal operation is a constant voltage mode and under fault

conditions a current limiting mode with voltage reduction.

The electronic components in the output stage of a rectangular power supply can be

active or passive and generally include energy storage components. The

combination of energy storage and active components makes the dynamic

behaviour difficult to determine. Consequently, the compliance process requires

both assessment and spark testing using the STA.

2.3.2 Definition of IS Power Supplies Terminology

A number of terms associated with IS power supplies are in common use without

any clarification in the published literature. The terminology applicable to power

supplies is tabulated in Table 2-3 and Table 2-4 defined by the author.

Power Supply Description Passive A power supply that does not include internal components for

either voltage or current regulation. Active A power supply that includes internal components used for

voltage, current, or a combination of both voltage and current regulation.

Power Supply Description Linear Steady-state characteristics of output voltage vs. output

current is a single straight line. Non-linear Steady-state characteristics of output voltage vs. output

current is not a straight line. May include multiple straight line segments.

In Table 2-5 the types of IS power supplies have been described in both the

terminology used by Johannsmeyer and Kraemer [12] and the more common

terminology of Table 2-3 and Table 2-4.

Table 2-3: Definition of active and passive power supplies

Table 2-4: Definition of linear and non-linear power supplies



IS Power Supply Description Linear A linear passive power supply.

Steady-state characteristics of output voltage vs. output current has a single straight line.

Trapezoidal A non-linear active power supply. Steady-state characteristics of output voltage vs. output current has two straight line segments:

1. Voltage regulation segment (Normal operation) 2. Non-regulated segment (Overload operation)

Rectangular A non-linear active power supply. Steady-state characteristics of output voltage vs. output current has two straight line segments:

1. Voltage regulation segment (Normal operation) 2. Constant current segment (Overload operation)

It is the non-linear active power supply that produces a rectangular output

characteristic that is the focus of the remainder of this thesis.

Table 2-5: Defining the types of IS power supplies



2.4 IS Active Power Supplies

IS active power supplies characterise themselves by their ability to regulate the

output voltage whilst the current demanded is within a specified range. If an

excessive amount of current is demanded, or a short-circuit occurs, the voltage will

drop rapidly to ensure that energy delivered to the circuit is below the minimum

energy required to ignite the specified explosive atmosphere.

As IS active power supplies commonly have a rectangular output characteristics

they cannot be assessed using the ignition curves published in the Standard and

their intrinsic safety must be determined by performing spark ignition testing using

the STA.

The V-I characteristics of the three types of IS power supplies discussed in Section

2.3.1 are steady-state characteristics. Steady-state characteristics are determined

from the static behaviour or the behaviour due to slow variations.

“The difference between static and dynamic characteristics in power supply units

usually results from the presence of capacitors and from the finite bandwidth of

semiconductor elements” [14]. The transient characteristics of IS power supplies are

important as a significant amount of energy in an IS active power supply can be

delivered from the energy storage components to the output terminals under fault

conditions.

Tomlinson and Widginton [14] investigated the dynamic behaviour of power supplies

by examining the slew rate of the power supply. This is the rate at which the output

voltage recovers after a transient short-circuit load is removed. It was discovered

that power supplies with output voltage slew rates below 200 V/µs appeared to be

safer as higher values of MIC were required to cause an ignition.

The authors highlighted the need for great care when testing the intrinsic safety of

constant-current power supplies exhibiting limited slewing rates as the slewing rate

could be effectively increased by the addition of common circuit loads.

The authors experimental circuit was a power supply with limited slewing rate. The

addition of an external shunt capacitance reduces the voltage slewing rate providing



a spark quenching effect. This spark quenching effect seems to make the circuit

safer. By reducing the value of the external shunt capacitance the slewing rate

increases and it is possible that the circuit will become unsafe. There is a certain

range of external shunt capacitance values where the spark quenching effect

provides additional safety. Intuitively the addition of series resistance to a circuit

should make the circuit less incendive. However, in this case, the addition of series

resistance between the power supply and the external shunt capacitor made the

circuit incendive again as the slew rate is increased.

Dill and Kanty [11] researched the dynamic behaviour of IS power supplies with the

aim of reducing the time taken to design or modify and test power supplies. Dill and

Kanty suggested that “one possibility for solving this problem is to analyse the

dynamic behaviour of the electronically regulated (active) power supplies. The

results are then used to decide whether or not a new spark-ignition (STA) test for

intrinsic safety has to be made”.

The method presented by Dill and Kanty [11] is a comparative method, which relies

on having previously obtained data from the power supply in question. “If the static

values are higher than before, all previous tests have to be repeated. If the static

values are unchanged or lower, the dynamic behaviour has to be checked. The

analysis is made with a substitute load, which simulates dynamic events. Most

suitable for this purpose are electronic load modules, which can be regarded as

resistors whose values can be controlled with frequencies up to some 100 kHz” [11].

Dill and Kanty went on to explain in detail the points on the steady-state output V-I

characteristics to perform the dynamic tests.

Dill listed the maximum output voltage and current for power supplies with active

current limitation as shown in Table 2-6. The designer or manufacturer can expect

difficulty in obtaining intrinsic safety compliance for power supplies with ratings

exceeding these limits.

Table 2-6: Maximum values of V and I for Group I active power supplies [11]

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The concept of performing dynamic tests on active power supplies to determine their

intrinsic safety such as those performed by Tomlinson and Widginton [14], and Dill

and Kanty [11] are further explored as part of this research in Chapter 4.

Dill presented a paper [10] with the theme of regulated IS power supplies and then

applied five examples to highlight some of the deficiencies in the Standards. A

summary of the examples and their findings are given below:-

Example 1 – Wrong use of curves - Dill highlights that the application of the

ignition curves is only to very simple circuits and that they do not apply to

regulated power supplies.

Example 2 – Inductances with shunt diodes - Dill argued the case of not

using a zener diode as a shunt across an inductive coil such as a solenoid.

Shunt diodes are used to provide a discharge path for the stored energy in

the inductor. Zener diode shunts in comparison to normal diode shunts will

make the solenoid act faster. The case presented is of an electronically

regulated power supply supplying an inductor with a back to back zener

diode shunt in series with the STA. He concluded that, “In the open loop,

measured across the terminals of the STA, the voltage will be the addition of

the supply voltage and the zener voltage. The arcs will receive more

energy…” [10].

Example 3 – Regulated power supplies - “By using electronic limiting devices

for current or spark duration it has been tried to avoid the disadvantages of

the resistive current limitation” [10]. Dill explains the necessity of using the

STA and in addition a current regulating device. “This device simulates a

load, which reduces the slewing rate of the voltage in the circuit just like a

three-pin regulator in a subsequent electronic device could do, and which is

similar to the effect of an inductance. … Finally, it is necessary to say, that

also the maximum values for external inductance and capacitance C for

regulated current and voltage limitation cannot be taken from the curves in

the standard ” [10].

Example 4 – Influence of capacitors in parallel - Dill explained that shunt

capacitance across a regulated power supplies only has a “… spark

quenching effect only in a certain range of values, where the effect prohibits



ignition” [10]. The addition of resistance between the power supply and

capacitor increases the slew rate and the risk of ignition.

Example 5 – Combination of L and C - Dill highlighted a pitfall for the unwary

in assumptions made from reading the certificate. “But no standard and

nearly no certificate tells you, that the maximum permissible external

inductance (LEXT) is determined for external capacitance (CEXT) = 0 and the

maximum permissible CEXT is determined for LEXT = 0 ” [10]. Dill states that

this is the reason the German test houses decided to always certify with

values that can be combined. This is applicable to all IS apparatus with entity

parameters including IS power supplies.

A number of IS active power supplies circuit diagrams were analysed but due to

SIMTARS privacy agreements and proprietary information no documentation is

available for inclusion in this thesis. These IS active power supply circuits and the

following two IS active power supplies were used to derive a functional block

diagram presented in Appendix A 1:-

J.J Sammarco [18] developed a regulated IS, rechargeable power supply for

portable electronic equipment for underground use. The power supply uses a

number of semiconductor devices including a series current regulator and

silicon controlled rectifier (SCR) crowbar protection. The regulated output is

DC 5 V at 4 A.

United States Patent #4438473 is an IS active power supply, “…employing

a binary current interrupter connected between the power source and the

electrical load” [19]. The circuit employs a semiconductor switch to isolate

the output and utilises a flip-flop to reset the switch, and uses semiconductor

current and voltage regulation.



2.5 Intrinsic Safety Standards

Legislation and subsequent Standards on intrinsic safety have been used in

England since 1911 [3]. In Australia, the British Standards Institute (BSI) Standard

for IS apparatus (BS 1259) was used until 1968, when AS 1829 was introduced [7].

The current Australian and International Standards are discussed in Sections 2.5.1.

In Section 2.5.2 the two current Australian Standards AS 2380.7 and AS/NZS

60079.11 are compared. This is followed by an explanation of the roles and

responsibilities of the parties involved in ensuring intrinsic safety. The final section

introduces the Australian third party testing and accreditation bodies.

2.5.1 Current Australian and International Standards

In the Australian legislation, the Statutory Acts presented in Table 2-7 are used to

define the legal responsibilities related to underground mining and the use of

electricity. These Acts refer to Australian Standards publications and make these

Standards legal documents. A national scheme is used to manage and monitor

compliance to the relevant legislation.

“The standards for intrinsic safety on principal are the result of research work. Most

of this work was done in the years from 1960 – 1980….” [10]. There are currently

two Australian Standards applicable to the design and construction of intrinsic safety

apparatus and they are the recently introduced AS/NZS 60079 series which will

eventually replace the AS 2380 series as it is phased out.

The AS/NZS 60079 series is a direct adoption of the International Electrotechnical

Commission (IEC) IEC 79 series. The AS 2381 series covers the selection,

installation and maintenance of intrinsic safety equipment. A Handbook covering

Table 2-7: Relevant Acts and Regulations [17]

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electrical equipment for hazardous areas has also been published by Standards

Australia.

The current Australian Standards pertaining to intrinsic safety are as follows [17]:-

• AS 2380.1 –1989 Electrical equipment for explosive atmospheres -

Explosion-protection techniques Part 1: General Requirements

• AS 2380.7 –1987 Electrical equipment for explosive atmospheres -

Explosion-protection techniques Part 7: Intrinsic safety i

• AS/NZS 2381.1 – 1999 Electrical equipment for explosive atmospheres -

Selection, installation and maintenance Part 1: General requirements

• AS 2381.7 – 1989 Electrical equipment for explosive atmospheres -

Selection, installation and maintenance Part 7: Intrinsic safety i

• AS/NZS 60079.0:2000 Electrical apparatus for explosive atmospheres

Part 0: General Requirements

• AS/NZS 60079.11:2000 Electrical apparatus for explosive atmospheres

Part 11: Intrinsic safety i

• Standards Australia, HB13 - 2000 Handbook Electrical equipment for

hazardous areas

The relationships between the various international bodies and committees that

govern the International Standards are quite complex. A number of authors [7, 20]

have questioned this complexity and referred to the many vested commercial and

political interests involved. In brief the IEC Standards (IEC 60079-x series) are used

as a basis for the European Committee for Electrotechnical Standardisation

(CENELEC) Standards (EN 50 0xx series). Each CENELEC Standard is adopted

and renumbered to a British Standard (BS 5501.x series).

With the exception of the United States of America (USA) nearly all other nations

are progressing towards the adoption of the International Standard [7]. The adoption

by Australia and other nations of the IEC 60079-x series of Standards is a significant

step toward the development of harmonised International Standards.

Dill [10] highlighted a number of deficiencies in the intrinsic safety Standards and in

the certification documents. In his preamble, Dill subtly criticised the committee’s

responsible for the Standards for their lack of contact with researchers in the field

and failure to incorporate the latest knowledge in the Standards.



During the course of this research program, the new Queensland Coal Mining

Safety and Health Act 1999 (see Table 2-7) and associated regulations were

invoked. The main change relevant to IS active power supplies is that Mines

Department approvals are no longer required in Queensland. Mine managers now

have the responsibility of ensuring certified IS active power supplies are fit for their

intended purpose.

2.5.2 Comparison of AS 2380.7 and AS/NZS 60079.11

The differences between the two Australian Standards for IS power supplies are:-

• minimum value of voltage for simple circuits has increased from 1.2 V (AS

2380) to 1.5 V (AS/NZS 60079)

• reduction in the number of assessment curves from ten curves catering for

construction materials (AS 2380) to six curves (AS/NZS 60079)

• minor variations of the values in the ignition curves

Both Standards still fail to sufficiently clarify the measurement of let through energy

when testing crowbar (over-voltage) protection circuitry in IS power supplies. Both of

the Standards prescribe an upper limit but do not define how the measurement is to

be performed.

The nameplate information for IS apparatus requires improvement. The method

used by the German testing bodies (of quoting the limitations of the ranges for

external inductance and capacitance together) would reduce the potential for the

unwary to inadvertently connect an IS device to an unsafe cable or load. For IS

power supplies additional parameters need to be included which define the V-I

characteristics of the power supply as well as internal resistance, inductance and

capacitance [12].

2.5.3 Participants in Ensuring Intrinsic Safety

The legal roles and responsibilities of the parties involved in ensuring intrinsic safety

are defined within the Statutory Acts and associated Standards. In this section,

these roles are discussed at length and the costs associated with the intrinsic safety

process are highlighted in order to clarify the participation of the various stake

holders.



Designers and/or suppliers of intrinsic safety equipment have a responsibility to

ensure that their equipment is both functional and safe. The design strategy of a

commercial product traditionally involves compromise between design and

manufacture cost, device performance, and market pricing structures while still

satisfying the requirements of the intrinsic safety Standards. Market demands, new

technologies and competition all act to influence designers in their quest to produce

a saleable item. The certification process also imposes a considerable cost burden,

which must be considered. These costs are all ultimately passed on to the product

purchaser.

The factors that determine the cost and/or duration of the certification process falling

within the responsibility of the party seeking certification are [15]:-

• type of certification requested

• quality of the design and manufacture of the equipment

• nature and complexity of the equipment

• level of pre-compliance review

• quality, completeness and accuracy of the associated documentation

• time taken to modify and resubmit the equipment if required

• quality and responsiveness of the communications between the party

seeking certification and the accreditation body

The role of the third party certification body is to assess and test where necessary to

determine conformance to an Australian and/or International Standard. The services

provided by certification bodies are utilised by designers, suppliers, and users of

intrinsic safety equipment. The assessment, testing and certification process are

themselves covered by relevant Standards to which the certifying body must

conform to ensure that it retains its accreditation, i.e. its authority to certify

equipment.

In Australia, any testing of explosion protected equipment must be covered by the

National Association of Testing Authorities, Australia (NATA) laboratory

accreditation and the certification activities accredited by the Joint Accreditation

System of Australia and New Zealand (JASANZ). The main factors that determine

the cost and/or duration of the certification process, which are the responsibilities of

the certification body are [15]:-



• assessment, testing and certification processes, which are the methods used

to determine conformance to the requested Standard

• quality and responsiveness of the communications between the certification

body and the party seeking certification

• the current work volume of the certification body

The users of the intrinsic safety equipment have a responsibility to ensure that their

equipment is functional and safe throughout the serviceable life of the equipment.

This responsibility includes the application and usage, maintenance and repair of

the equipment, establishment and maintenance of documentation, and other

statutory and inspectorate requirements. It also includes timely response to

addressing any issues arising from publication of safety alerts, product recalls, and

requests for re-certification.

Australian and International Standards bodies set the requirements by which

certification is determined. They have a responsibility to maintain these Standards,

while responding to industry trends, and advances in technology. They also need to

ensure that the Standards remain relevant with acceptable levels of risk associated

with the use of the equipment in specified hazardous locations.

Australian has a number of industry associations such as the Australian Coal

Association (ACA) and the Association of Electrical and Electronic Manufacturers

Australia (AEEMA), which promote, lobby and influence matters that impact upon

industries using IS equipment.

2.5.4 Accredited Intrinsic Safety Testing and Certification Bodies

Third party testing bodies are used to establish that a particular apparatus or system

complies with the specified Standard. There have been examples of differences in

the interpretation of the Standards between the third party testing bodies, both on an

international and national level [21]. The impact on the NSW coal mining industry,

because of the Safety Alerts issued in 1998, also raised questions by industry

observers on the assessment, testing and certification process [21], [2].

In Australia the two main accredited laboratories capable of certifying IS equipment

are Safety In Mines Testing And Research Station (SIMTARS) and TestSafe



Australia. Both SIMTARS and TestSafe Australia issue certificates and approvals for

conformance with Australian Standards and legislative requirements for [22]:-

• Certificates of Conformity for Groups I and II explosion protected electrical

equipment

• Certificates of Conformity for electrical equipment used in NSW or

Queensland coal mines, and others

Safety In Mines Testing And Research Station (SIMTARS) acts as a semi-

autonomous, professionally independent division of the Queensland Government's

Department of Natural Resources and Mines. The testing, calibration, certification

and other specialised services for electrical equipment used in hazardous locations

is carried out by the Engineering Testing and Certification Centre (ETCC). Evidence

of conformity issued by SIMTARS include [23]:-

• NATA reports for Australian and equivalent International Standards

• Certificates of conformity to intrinsic safety Standards to AS 2380.7, AS/NZS

60079.11, and others

Some of the major International accredited testing bodies include:-

• Health and Safety Executive – mining (HSE (M)) in Britain

• Berggewerkschaftliche Versuchsstrecke (BVS) in West Germany

• Underwriters’ Laboratories Inc (UL) in America

These testing bodies are authorised to certify to the International intrinsic safety

Standard and their own national Standard [7].



2.6 Certification, Assessment and Testing of IS Power Supplies

The certification process in Australia confirms compliance to one of the two

Australian Standards AS 2380.7 or AS/NZS 60079.11. A testing and certification

body determines conformance to the Standards by circuit analysis, spark ignition

testing, or a combination of both. Conformance to the thermal ignition requirements

of the Standards can be determined by temperature rise tests. In practice, a

significant part of the certification process involves assessment and testing.

Generally, the various testing and certification bodies regard their procedures and

assessment methods as proprietary information. These are therefore not generally

available to the public. As part of this investigation, the author has summarised

SIMTARS intrinsic safety assessment and testing procedures and they are

presented in Sections 2.6.2 and 2.6.3.

The certification process requires a number of reviews to be performed at critical

points within the assessment and testing process. During the latter stages of the

process, a final review takes place and if compliance is confirmed an appropriate

certificate is issued.

2.6.1 Certification – Determining Conformance to a Standard

AS 2380.7 or AS/NZS 60079.11 categorises IS electrical apparatus initially by their

location relative to the hazardous area. IS electrical apparatus are able to be located

within a hazardous area. Associated equipment must be located in a safe area but

the interconnecting wiring may enter the hazardous area.

The Standards then further categorise IS electrical apparatus by whether the

equipment is self-contained, part of a system, or entity concept equipment [24].

Generally IS power supplies are categorised for accreditation as associated

electrical apparatus and are certified as entity concept equipment or as part of an

integrated system. Associated electrical equipment require the following output

parameters to be defined: Maximum output voltage (UO), Maximum output current

(IO), Maximum external capacitance (CO), Maximum external inductance (LO), and

Maximum external inductance to resistance ratio (L/R).



2.6.2 Assessment of IS Active Power Supplies

As the complexity of the electrical/electronic circuitry increases it becomes more

difficult and less reliable to determine the equipment’s conformance to the relevant

Standards by analysis alone. The interaction between the load and the active

components within the active power supply cannot be easily analysed. It is for this

reason that use of the ‘ignition curves’ is not applicable for IS active power supplies

[24].

In most situations IS active power supplies are subjected to a combination of both

circuit analysis and spark ignition testing using the STA. A summary of SIMTARS

general intrinsic safety assessment procedure is shown in Table 2-8 [9]. IS active

power supplies are subjected to this assessment procedure with the exception of the

initial part of Step 7. At this point IS active power supplies assessed by SIMTARS

are subjected to spark testing using the STA as described in Section 2.6.3.

There is scope to make a number of improvements in the assessment process by

increasing the pre-submission work and documentation. This would be performed by

suitably qualified designers. During the assessment procedure outlined in Table 2-8

a number of steps repeat the work performed earlier by the designer. These steps

are listed as follows:-

1. Identify all sources of energy

2. Identify components on which intrinsic safety depends

4. Segregation of components by creepage and clearance distances

5. Circuit Calculations - ratings for all components

6. Circuit Parameters - maximum voltages and currents determined

7. Identification of potential ignition sources - sparking and heating

The additional work outlined above, if performed and documented by designers,

would provide a self-review. A second benefit is a reduction in the assessment time

and cost. Magison [6] identified this in his design methodology Task 3 as presented

in Section 2.2.3.



Table 2-8: Summary of SIMTARS intrinsic safety assessment procedure [9]

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2.6.3 Testing IS Active Power Supplies using the STA

It is essential to ensure the intrinsic safety of the active power supply is tested using

the STA under numerous variations of the load parameters and over the full range of

its output characteristics to ensure that incendive sparking is not possible. These are

time consuming tests and are not required for linear power supplies. The sparking

potential of linear power supplies with ‘well defined’ circuits can be determined by

assessment alone using ignition curves.

A summary of the SIMTARS general intrinsic safety testing procedure is presented

in Table 2-9 [9].

It is an exhaustive process to establish whether a circuit is IS at all possible circuit

configurations and values under both normal and fault conditions. If the apparatus is

being certified under the Entity Concept, then in addition to establishing the IS

status, Entity Concept parameters (LO, CO and L/R ratio) must also be determined

using a trial and error method.

Table 2-9: Summary of SIMTARS intrinsic safety testing procedure [9]

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Of the research performed on the STA the main areas of interest relate to the

mechanics and materials used for the electrode wire and contact disk. In addition,

some research has been done on the application of the STA to testing high current

IS apparatus [25]. In Chapter 3 properties of the STA are investigated further.

There is scope for possible improvement in the testing phase of the certification

process. The testing procedure outlined in Table 2-9 is a lengthy and tedious

exercise, contributing significantly to the costs involved. The STA uses up to four

wires located in the wire holder. These tungsten wires are subjected to flexing and

after a period they either bend or break off. The wire holder rotates at 80 rpm and,

therefore, it is difficult to monitor the state of the wires during a test. Wires bend or

break frequently and, if unobserved, this may require the test to be repeated. By

sensing the current flow using additional circuitry, broken wires may be easily

identified. This would allow a test to halt immediately and the wire to be replaced.

The test could then be resumed with minimum lost time.

There are a number of requirements stipulated in the Standards that must be

maintained throughout the test for the results to be valid. These items include:-

• concentration of the explosive testing gas

• flow rate of explosive test gas through the testing chamber

• nominated voltage and current to the device under test

• number of revolutions of the wire holder

• occurrence of an explosive ignition

• results of the pre and post STA sensitivity check

Partial automation, data acquisition, and recording would improve the operation and

efficiency of the testing phase of the certification process.



2.7 Summary

Hazardous areas with explosive atmospheres are common in industries such as

processing, manufacturing, and in underground coal mines. Statutory requirements

may stipulate that IS devices must be used. IS power supplies have been designed,

manufactured and certified to meet specific criteria in accordance with intrinsic

safety Standards. These Standards specify the amount of energy that the IS power

supply is permitted to deliver to the IS circuit.

The intrinsic safety accreditation process may involve both assessment and testing

to determine conformance to the intrinsic safety Standards. Increase in the

complexity of modern IS active power supplies has complicated the assessment and

testing process and extended the time taken to determine conformance.

The sparking potential of linear and trapezoidal type power supplies can be

determined using ignition curves included in the intrinsic safety Standard.

Researchers have observed significant levels of output energy when subjecting

rectangular type (active) power supplies to dynamic load conditions. The Standard

specifies that the sparking potential of active power supplies must be determined

using the STA. The STA is used to determine whether this amount of energy can

cause an explosive ignition.

The dynamic behaviour of active power supplies has been investigated by a number

of researchers and this literature was reviewed. The determination of the output

energy when subjecting the active power supply to dynamic loads is further

investigated in this thesis.

In this chapter, two of the research goals of Section 1.2 have been fulfilled. -(a) the

different types of IS power supplies and the relevant terminology have been clearly

defined, and -(b) the existing practices for assessment and testing of active IS

power supplies have been reviewed and opportunities for improvement identified.

The remaining research goals require determining the limits of intrinsic safety for

active power supplies and to this end the principal instrument for determining the

sparking potential of IS circuits, the STA is investigated in the following chapter.

- 37 -


Chapter 3 Electrical Investigation of the STA The spark test apparatus (STA) is used to determine the sparking potential of the

intrinsically safe (IS) power supply by simulating fault conditions likely to occur in the

field. The main concern for spark ignition is the presence of exposed conductors that

can touch (make) and then separate (break). The STA is connected to the circuit

under test, produces a variety of ‘makes’, and ‘breaks’ at different velocities and

intervals. In this chapter, the STA is investigated to determine how the sparking

potential of a device under test is established.

Sections 3.1 and 3.2 give an introduction to the STA and identify how the sparking

potential of an IS apparatus is determined. In Section 3.3 the periodic nature and

randomness of the STA is discussed. This is followed by the measurements of the

electrical circuit of the STA in Section 3.4. In Section 3.5 the sensitivity of the STA is

discussed.

3.1 Introduction to the STA

The STA consists of a small gas chamber to which a flammable test gas of known

concentration is applied at a low flow rate. The chamber contains an insulated wire

holder and an insulated cadmium disk as shown in Figure 3-1.

The wires on the wire holder and cadmium disk simulate the electrical contacts in a

switch that makes and breaks the circuit under test [24]. The wire holder is able to

secure up to four wires and is positioned above the cadmium disk so that their

circumferences overlap, as shown in Figure 3-1 and Figure 3-2. The wires are

located equidistant around the edge of the wire holder and extend down so that they

can make contact with the surface of the cadmium disk. Only one wire is able to be

on the cadmium disk at any one time. The wire holder is driven at 80 rpm in a

clockwise (CW) direction and the cadmium at 19.2 rpm in a counter-clockwise

(CCW) direction. As the wire holder rotates, one of the four wires makes contact

with the cadmium disk, traverses the surface of the cadmium disk, and then

disconnects as illustrated in Figure 3-2. A short period lapses before the next wire

makes contact with the cadmium disk [24].

Chapter 3 Electrical Investigation of the STA - 38 -


Figure 3-1: Plan and elevation views of STA wire holder and cadmium disk [24]

The cadmium disk has two parallel grooves on its surface which cause the wire and

cadmium disk surface to intermittently break contact as the wire end leaves the

edge of the groove and then re-make contact when it reaches the other side of the

groove. The angle at which the wire departs from the edge of a groove and the

tension of the wire determines the speed of departure at the break, the speed of

arrival of the make, and the time period the wire is located in the groove [24].

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Figure 3-2: Oblique view of the STA wire holder and the cadmium disk

The ability to accurately replicate test results using the STA is affected by the

statistical probability that an explosive ignition may occur. Ignition probability is

based upon the occurrence of a potentially explosive ignition event every 1600

revolutions of the wire holder.

Wire path acrosscadmium disk

Wire holder

Wire

Chordalgrooves19.2 rpm

80 rpm

Cadmium disk



3.2 Low Voltage Electric Arcs and the STA

The STA facilitates the discharge of the energy storage components within the

electrical circuit under test by repetitively making and breaking the electrical circuit.

The sparks produced by the STA are located in the spark gap between the end of

the wire and the cadmium disk. The dielectric in the spark gap is the prescribed

flammable test gas at a known concentration. The conditions under which the end of

the wire and the cadmium disk make and break the circuit during the STA operation

are illustrated in Figure 3-3 (a) to (g).

Figure 3-3: STA wire and cadmium disk making and breaking contact

KEY: v = the relative velocity between the wire and the cadmium disk v1 = 208 mm/s v2 = 250 - 2000 mm/s dependent on angle to chordal groove v3 = 250 mm/s

Wire holder

Wire

Cadmium disk

Wire holder

Wire

Cadmium disk

Magnified view of wire across disk surface

disk surface

wire

Scoured disk surface

(b) Wire makes with side of disk

Wire holder

Wire

Cadmium disk

(a) Wire approaches side of disk

(c) Wire traverses surface of disk

v1 v1 v1

Wire holder

Wire

Cadmium disk

Cadmium disk groove

Wire holder

Wire

(d) Wire breaks with edge of groove

(e) Wire makes with edge of groove

Cadmium disk

v2 v2

Wire holder

Wire

Cadmium disk

(f) Wire approaches edge of disk

Cadmium disk

(g) Wire breaks with edge of disk

Wire holder

Wirev3 v1



When the voltage applied across the terminals of the STA is greater than the

breakdown voltage of the dielectric an arc is formed. The arc will persist only while

the applied voltage is greater than the breakdown voltage of the dielectric and

sufficient current is available.

It is assumed that the wire and cadmium disk have been separated for a period

sufficiently long for the capacitor to be charged. The discharge of the capacitor will

generally occur when the wire is making contact with the cadmium disk as in Figure

3-3 (a) and (b), and in greater detail in Figure 3-4.

The discharge of an inductive energy storage component will generally occur when

the wire breaks contact with the cadmium disk. This is after a period of time where

the wire and cadmium disk have been in contact for a sufficient period to allow the

inductor to be energised as in Figure 3-3 (d) and (f). Resistive circuits may also

cause spark ignition where there is an intermittent making and breaking of a high

current circuit as in Figure 3-3 (c).

The energy available from the apparatus under test at the terminals of the STA can

significantly exceed the minimum ignition energy (MIE) of the explosive test gas in

the STA chamber without causing an explosive ignition.

As discussed in Section 2.1.3 the energy available at the spark gap typically

exceeds the MIE as the amount of energy transferred from the arc to gas is highly

dependent upon the physical shape, arrangement and materials of the electrodes.

The exact amount of energy transferred from the arc to the surrounding flammable

test gas in the STA is difficult to determine exactly and is dependent upon:-

• distance between the wire and cadmium disk

• geometry of the wire end, disk surface or edge at the spark gap

• duration of the time spent near the quenching distance

• relative velocity between the wire and cadmium disk

• the instantaneous values of arc voltage and current



Figure 3-4: STA making contact - discharging a capacitive circuit

The only indication that sufficient energy has been transferred from the arc to the

surrounding explosive gas mixture is the occurrence of an explosive ignition. This

confirms that an amount of energy equal to or greater than the MIE of the explosive

gas mixture has been transferred from the electric arc to the test gas in the spark

gap between the wire end and the cadmium disk.

KEY: velocity v, separation distance d, and time t Rotational speed of wire holder = 80 rpm and wire path radius r = 24.84 mm Angular velocity ω = (2 * π * 80 rpm) / 60 = 8.38 rad/s Linear velocity v = ω * r = 8.38 * 24.84 = 208.10 mm/s

vd = d0

t = 0

Edge of Cadmium disk viewed from above.The wire is approaching the edge of the cadmium disk. The voltage across the wire and cadmium disk is insufficient to breakdown the spark gap distance d0.

Wire

(a)

(b) vd = d1

t = t1The voltage across the wire and cadmium disk is sufficient to breakdown the spark gap distance d1 and an arc is formed.

Arc

(c) vd = d2

t = t2The voltage across the wire and cadmium disk is sufficient to breakdown the spark gap distance d2 and the arc continues.

(d) vd = dq

t = t3 The separation distance d = dq and the arc is quenched.

(e) vd = 0

t = t4 The wire is now in contact with the disk and the separation distance d = 0.



3.3 Periodic and Randomness of the STA

When the STA is connected to the direct current (DC) resistive circuit as shown in

Figure 3-5 (a) and the wire on the cadmium disk encounters both of the chordal

grooves as shown in Figure 3-5 (b), the current (IO) and voltage (UO) waveforms are

depicted in Figure 3-6. The spikes in the waveforms indicate when the wire breaks

contact with the cadmium disk.

Figure 3-5: Test circuit with STA and wire path for a single traverse

The measured waveforms in Figure 3-6 for the output voltage and output current

show small variations whilst the wire is over the surface of the disk. These variations

are due to physical irregularities of both the wire and the disk. Irregularities may

include: scratches on the disk surface, loose particles on the disk surface, worn

edges on the side of the disk and grooves, wire bending and wire splitting.

Steady state measurements:-

STA contacts open UO = 10V, IO = 0 mA STA contacts closed UO = 42 mV, IO = 42 mA

(a) STA connected to resistive circuit.

Wire positions on path:- a – wire makes with disk b – wire breaks with edge of groove c – wire makes with edge of groove d – wire breaks with edge of groove e – wire makes with edge of groove f – wire breaks with disk

(b) Wire path traversing cadmium disk

Uo

STA

Io

Rm = 1

AB

Chnl. A - UoChnl. B - Io = Um / Rm

Uo

Um

OSCILLOSCOPE

Ω

R = 237 Ω

10 V

Wire path

abcdef

Cadmium disk rotation CCW



The period that the wire is over the surface of the disk is dependent on the rotational

speed of the wire holder and the free length of the wire. The waveforms in Figure

3-6 were measured with the wire holder rotating at 80 rpm. The end of a single wire

is on the disk for a period of 130.4 ms followed by a 57.1 ms period with the wire off

the disk before the next wire makes contact. The total period for the end of a wire is

187.5 ms. As the four wires are located equidistant on the periphery of the wire

holder then the 187.5 ms period corresponds to 90o CW rotation of the wire holder.

A 90o rotation of the wire holder through the 50:12 gearbox ratio results in a 21.6o

CCW rotation of the cadmium disk.

Figure 3-6: Measured output current (IO) and voltage (UO) for a single traverse

The STA is periodic based on 12.5 revolutions of the wire holder, corresponding to 3

revolutions of the cadmium disk. The periodic nature of the STA is illustrated in

Figure 3-7.

Note: Wire positions a,b,c,d,e and f correspond to Figure 3-5.

a b c d e f wire positions on path

Output current Io

0

10

20

30

40

50

0 30 60 90 120 150 ms

mA

Output voltage Uo

0

2

4

6

8

10

0 30 60 90 120 150ms

V



Figure 3-7: Periodic make and break of wires on the cadmium disk

The measured current waveform in Figure 3-8 depicts two revolutions of the wire

holder. The waveform shows eight periods corresponding to each of the four wires

in the wire holder traversing the cadmium disk. The first period (0 to 188 ms) shows

the wire intersecting the two chordal grooves on the cadmium disk. In the following

period (188 to 375 ms), the cadmium disk has rotated and the wire intersects only

one of the chordal grooves. During the fifth period at the 800 ms point, the cadmium

disk has rotated to an angle where the groove is tangential to the wire path. The

wire scrapes along one of the edges of the groove rapidly making and breaking the

circuit under test.

Figure 3-8: Measured periodic make and break waveform

0 1 2 3 4 5 6 7 8 9 10

Wire ON disk

Wire OFF disk

Period of wire holder 0.75 sec [12.5 revs per STA period]

Period of Cadmium disk 3.125 sec [3 revs per STA period]

Period of wire 0.1875 sec [50 wires across disk to STA period]

Period of STA 9.375 sec [wire holder at 80 rpm]

Time sec

O u tp u t c u rre n t Io

0

10

20

30

40

50

0 300 600 900 1200 1500 m s

m A

1 2 3 4 5 6 7 8 Period



The tungsten wire is harder than the cadmium disk and therefore, it scratches the

surface of the cadmium disk. As the STA is periodic, the scouring forms a pattern as

illustrated in Figure 3-9 (b). The wire path forms an arc on the cadmium disk and the

geometry of the arc has been determined graphically in Figure 3-9 (a). The pattern

in Figure 3-9 (a) matches that observed in the picture of the cadmium disk in Figure

3-9 (b).

When a new cadmium disk with a smooth surface is used, the STA has poor

sensitivity. A conditioning process is described in the intrinsic safety Standard. The

purpose of the conditioning process is to roughen the surface of the cadmium disk.

At the end of the conditioning process, a distinct pattern is observed on the surface

of the cadmium disk as observed in Figure 3-9 (b). This process gradually improves

the sensitivity of the STA to the point where it will successfully have an explosive

ignition using the calibration circuit.

Figure 3-9: Geometry of arc scribed by the wire on cadmium disk

The sensitivity of the STA varies and is dependent upon both its physical properties

and environmental conditions. The wire condition and humidity appear to be the

main factors contributing to variations in STA sensitivity. These are minimised by air

conditioning (i.e. controlling the humidity) of the atmosphere within the testing

laboratory and ensuring that the test gas is at constant temperature with a low

moisture content. Wires can be prepared to minimise splitting and require regular

cleaning and straightening. Wire replacement is recommended if any deterioration of

the wire is noticeable.

(a) Graphically determined using (b) photo of scoured

QuickCAD release 7 cadmium disk

Wire path on disk (arc)

Arc angle88.33 deg

Cadmium disk

Arc radius

21.34 mm

3.36

mm

15.35 mm



3.4 Electrical Circuit of the STA

The individual circuit loops within the STA were measured using an LRC meter to

derive the electrical circuit presented in Figure 3-10 (a). The distributed components

in Figure 3-10 (a) have been lumped together and are presented in Figure 3-10 (b)

which illustrates the equivalent circuit of the STA.

During the measurement, it was observed that the value of RWH and RCD varied as

the wire holder was rotated. This indicates that the brush contact resistance with the

rotating shaft varies. Average values for RWH and RCD are presented in Figure 3-10.

The Standard specifies maximum allowable values for the STA as self-capacitance

30 pF (contacts open), self-inductance 3 µH (contacts closed), and resistance of

0.15 Ω (contacts closed, measured at 1 A DC) [24]. The measured value for the

resistance is considerably higher than the permissible values specified in the

intrinsic safety Standard.

The small values of series inductance, resistance, and shunt capacitance do not

significantly load the circuit under test. The voltage available at the terminal of the

STA is present across the wire holder and cadmium disk before the wire making

contact with the side of the cadmium disk. The low internal resistance of the STA

ensures the maximum short-circuit current is present during the time that the wire is

in contact with the cadmium disk.



Figure 3-10: STA electrical circuit

WH – Wire holder CD – Cadmium disk av. – average value av. R WH = 1064 mΩ L WH = 0.5 µH R CONTACT = 34 mΩ C OPEN = 7 pF L CD = 1.1 µH av. R CD = 489 mΩ

(a) Electrical circuit (distributed component) of the STA R CLOSED = R WH + R CD = 1553 mΩ L CLOSED = L WH + L CD = 1.6 µH R CONTACT = 34 mΩ C OPEN = 7 pF

(b) Electrical circuit (lumped component) of the STA

WH

CD

External terminals

R WH L WH

C OPEN

R CD L CD

WH

CD

R CONTACT

R CLOSED L CLOSED

WH

CD

External terminals

C OPENWH

CD

R CONTACT



3.5 Sensitivity of the STA

The intrinsic safety Standard prescribes a test to determine the sensitivity of the

STA. The calibration circuit is a DC inductive circuit as shown in Figure 3-11 and is

intended to cause an explosive ignition in the STA test chamber. The sensitivity test

is performed before and after all spark ignition tests. If the post sensitivity test fails

then the spark ignition test is invalid.

Widginton states “… ignitions can arise simply because of variations which are

known to occur in the sensitivity of the spark test apparatus, or as a consequence of

the probabilistic behaviour of the spark test apparatus” [16]. The probabilistic

behaviour of the STA is termed the ‘probability of ignition’ which, for the ignition

curves included in the intrinsic safety Standard, represents a probability of 1 ignition

in approximately 400 revolutions of the wire holder (4 wires) resulting in

approximately 1000 sparks [16]. This equates to a probability of < 1% [5].

The transient conditions that occur during the break of the inductive calibration

circuit is the period when the stored energy in the inductor is delivered to the STA

terminals and ultimately to the wire end and cadmium disk. It is at these times that

peak energy and potential for ignition occur. Generally an explosive ignition occurs

near an instance of a rapid rise in the available energy and not during steady-state

periods. A rapid rise in available energy coincides with peak output current as the

circuit is opened and has a short duration related to the time constant of the circuit.

The steady-state circuit measurements are presented in Figure 3-11 and typical

waveforms for voltage and current for the calibration circuit during a non-explosive

ignition are presented in Figure 3-12 (a). As the STA calibration circuit is opened, an

arc was formed. The energy transferred during this period was less than the

minimum ignition energy (MIE) of the surrounding explosive test gas.

In the case presented in Figure 3-12 (b), an explosive ignition did occur as the circuit

was opened. The total amount of energy transferred to the test gas caused ignition

kernel growth to exceed the quenching distance and hence become a self-

propagating flame front. When an explosive ignition occurs the energy value

associated with the MIE of the test gas must have been transferred from the arc to

the test gas. During the explosive ignition the measured peak values of the output



voltage 197 V and output current exceeded 3.2 A resulting in the output power

exceeding 630 W for a short duration.

Figure 3-11: STA calibration circuit with current measuring resistance

Steady state measurements:-

STA contacts open UO = 24V, IO = 0 A STA contacts closed UO = 101 mV, IO = 100.8 mA

InductorUo

STA

Io

Rm = 1

AB

Chnl. A - UoChnl. B - Io = Um / Rm

Uo

Um

OSCILLOSCOPE

Ω

24 V

L

214 Ω

RLRS

24 Ω 92.9 mH



Figure 3-12: Measured V and I waveforms for the STA calibration circuit

Upper trace: Output voltage UO Lower trace: Output current IO (a) Typical output voltage and current - Wire is traversing chordal groove and as

the circuit opens a non-explosive ignition occurs

Upper trace: Output voltage UO Lower trace: Output current IO (b) Output voltage and current during explosive ignition - As the circuit opens a

explosive ignition occurs

Explosive ignition wire and disk separating

Non-explosive ignition wire and disk separating



3.6 Summary

In this chapter the research goal in Section 1.2 of investigating the intrinsic safety

testing practices for IS active power supplies has been fulfilled. The principal

instrument for testing and determining the sparking potential of active power

supplies and other electrical circuits is the STA. The STA replicates conditions that

are likely to occur in the field and hence is able to determine if the output energy

from the electric circuit is low enough for it to be regarded as conforming to IS

requirements.

The investigations carried out as part of this thesis have determined that the STA

has a periodic make and break sequence between the wires and cadmium disk.

However, the roughened surface of the cadmium disk results in variable conditions

throughout the duration of contact between the wire and the disk surface. The

chordal grooves on the cadmium disk provide a range of separation and approach

velocities between the wire and cadmium disk.

The STA is sensitive to the physical condition of the wire and cadmium disk. For

optimal performance straight wires where the wire end is without splits or deformity,

and a conditioned cadmium disk with a roughened surface is required. The STA is

also sensitive to environmental conditions, in particular to humidity.

The STA wire and cadmium disk apply an intermittent transient short-circuit and

open-circuit loads to the circuit under test. It is the transient response of the circuit

under test that determines the output power during this transient period and thus the

available output energy. In the case of active power supplies, the energy stored in

components within the active power supply is transferred to the STA during these

transient periods potentially creating a low energy electric arc. The amount of

energy available at the output of an active power supply is dependent on the

transient characteristics of the active power supply and this is investigated in

Chapter 4.

- 53 -


Chapter 4 Characteristics of IS Active Power Supplies In order to carry out the investigations described within this chapter a number of

direct current (DC) intrinsically safe (IS) active power supply samples were provided

by a local underground coal mining company. The IS active power supplies had

similar electrical circuit topology and varied only in their nominal DC output ratings.

The three sample IS active power supplies are identified as PS 1, PS 2 and PS 3,

and their nameplate ratings are presented in Table 4-1

In this chapter an analysis of the output steady-state and transient characteristics of

this type of IS active power supply is undertaken. The sparking potential of these

power supplies is identified and defined by parameters that are measured from their

steady-state and transient output characteristics.

4.1 Sample IS Active Power Supplies

Due to proprietary privilege, no documentation or circuit diagrams were available for

the sample IS active power supplies. The circuits were traced and analysed and a

generic block diagram is included in Appendix A 1. The functional block diagram of

the output stage of a sample IS active power supply circuit is presented in Figure

4-1. The IS active power supply includes active components in the voltage regulator,

current limiter, and over-voltage crowbar protection circuitry. In this particular IS

active power supply the current limiter includes the intrinsic safety control circuitry.

Figure 4-1: Block diagram of sample IS active power supply DC stage

Voltage Regulator

V reg

I Limit

Low passfilter

C C

Bridge rectifier

Low passfilter

AC Line supply side of circuit not shown

+

-CurrentLimiter

Crowbarprotection

IS DC Output

IS Cntrl

Output current sensing circuitry

Over voltage sensing circuitry

IS Control circuit

Chapter 4 Characteristics of IS Active Power Supplies - 54 -


4.2 Steady-state Output Characteristics

The steady-state output characteristics of three sample IS active power supplies

were measured and are presented in Figure 4-3 with the measured values of all

three sample power supplies presented in Table 4-1. The load resistance RLOAD in

the test circuit of Figure 4-2 (a) is slowly reduced from infinity (open-circuit) to short-

circuit. At each measuring point, the output voltage and current are recorded once

they have stabilised. The two linear sections of the steady-state output

characteristics in Figure 4-3 are emphasised to illustrate the differences between

no-load, full-load and short-circuit values.

The two linear sections in Figure 4-3 correspond to the two operating modes of the

sample IS active power supply. The normal mode of operation is from no-load to full-

load where the voltage regulator in Figure 4-1 maintains a constant output voltage

as the output current and load resistance varies.

The fault mode of operation is where the current demand exceeds the rated full-load

current. In this mode of operation the current limiter in Figure 4-1 limits the output

current to approximately the full-load value and reduces the output voltage for

further demands of output current as load resistance is reduced. The current limiter

effectively reduces the output power available.

Figure 4-2: Steady-state test circuit

(a) Steady-state test circuit (b) Steady-state measurement

DCActive PowerSupply

+

-

Uo

Io

R LOAD

FAULT MODE

Amps

Volts

IO FLIO SC

UO SC UO FL UO NL

NORMAL MODE

Full load (FL)Short circuit (SC)

No load (NL)

[Current limiting]

[Voltage regulation]



Figure 4-3: Steady-state output characteristics

Power supply identifier PS 1 PS 2 PS 3 Rated UO (V) 12 12 18 Nameplate Rated IO (A) 1 2 1.25

UO NL (V) 12.73 12.74 18.33 UO FL (V) 12.34 12.01 17.89 UO SC (V) 0.550 0.220 0.642 IO NL (A) 0 0 0 IO FL (A) 1.000 2.000 1.250

Steady-state measurements (Refer to Figure 4-2(b) for description) IO SC (A) 1.020 2.040 1.281

Note: Full-load measurements are with the output current set to the rated IO where UO = Output voltage

IO = Output current NL = No-load (RLOAD = open circuit) FL = Full-load (RLOAD = UO FL / IO FL Ω) SC = Short-circuit (RLOAD = 0 Ω)

Table 4-1: Measured steady-state parameters – sample active power supplies

PS 1, 2 and 3 Steady-state Output Characteristics

0

500

1000

1500

2000

2500

0 5 10 15 20Output voltage Uo (V)

Out

put c

urre

nt Io

(mA

)

PS 1 12V 1A PS 2 12V 2A PS 3 18V 1.25A



4.3 Transient Output Characteristics

While the steady-state output characteristics define the circuit behaviour to slow

variation in load, the transient output characteristics define the circuit behaviour to

rapid variation in load.

Transient output characteristics are determined by rapidly switching between two

load conditions and are used to analyse the dynamic behaviour of the IS active

power supply. The transient output characteristics include two plots, one of output

voltage versus time and the other output current versus time. Both output voltage

and output current values must be considered as they determine the instantaneous

output power and hence the output energy.

IS active power supplies with energy storage components can output significantly

higher amounts of instantaneous power than their steady-state output power when

subjected to transient load conditions. During a transient, energy from the energy

storage components is transferred to the output terminals and to the load and poses

a spark ignition risk.

IS active power supplies with predominantly capacitive energy storage components

such as the three samples investigated here exhibit this behaviour on circuit

closures. Of particular concern are intermittent short-circuits with low circuit

resistance. Under these conditions it is possible that a low voltage arc can be

formed at the site of the short-circuit as described in Section 2.1.3.

The instantaneous output power of a power supply is dependent on the values of

output voltage and output current and can be determined by the following equation

[26]:

po(t) = vo(t) x io(t) … (4.1)

where po(t) = instantaneous power p at time t

vo(t) = instantaneous voltage v at time t

io(t) = instantaneous current i at time t



The output energy of a power supply is dependent upon the values of output voltage

and output current occurring during a defined period as determined by the following

equation [26]:

eo(t) = ⌡⌠ t1

t po(t) dt … (4.2)

where eo(t) = energy from time t1 to time t

As discussed in Section 2.1.3 energy from an arc is transferred to the proximate

atmosphere. All testing was carried out with a minimal air or test gas flow rate, after

the output energy integration is reset to zero after a reasonably long period of no

circuit current. An accumulative effect can occur if consecutive energy transfers

occur within a very short time period. Typically, the second energy transfer from the

energy storage components is smaller due to insufficient charging time to store any

significant amounts of energy.

4.3.1 Measuring Transient Characteristics using the STA

The transient output voltage and current of an IS active power supply were

measured using a storage oscilloscope connected across the STA as shown in

Figure 4-4.

Figure 4-4: Transient characteristics test circuit with STA

When the STA is operated, it was observed that consecutive wire and cadmium disk

closures did not always yield the same output voltages and output currents

waveforms as shown in Figure 4-5. The measured results of Figure 4-5 shows a

transition where the STA contacts closed providing a short-circuit. Although it is

RM – Current measuring resistor (1Ω)

time

time

t1 t2

t1 t2

Uo NL

Uo SC

Io Peak

Io SCIo NL

Uo(t)

Io(t)

Io AmpsRM


+

-

STAAB

Chnl. A - UoChnl. B - Io = UM / RM

Uo

UM

OSCILLOSCOPE

IO

Uo Volts

Chnl. B

Chnl. A



reasonable to expect the voltage and current to be the same it may not necessarily

be so due to debris on the disk, wire fatigue, or splitting of the wire end.

Figure 4-5: Measured transient output characteristics (STA)

IS active power supplies with predominantly capacitive energy storage components

pose a spark ignition risk during the contact closure transient period. The

parameters that define this transient period are; no-load output voltage, period from

non-zero output current to peak output current, peak output current and

corresponding output voltage values, duration of the output current decay from the

peak value to steady-state value, and the steady-state output current and

corresponding output voltage values. These parameters are measured from the

transient no-load to short-circuit load applied by the STA when the maximum peak

output current occurs.

The maximum peak output current sample is identified after recording a number of

sample waveforms. Transient waveforms are included in the sample if the energy

storage components are fully charged by a suitable open circuit time, and there is no

STA wire bounce on initial contact with the cadmium disk. This excludes contact

closures that occurred as the wire traversed the chordal grooves on the cadmium

7.61A IO 1.76A

0A 12.61V

9.44V UO

2.41V 0 V t0 t Peak=160µs tSS=7.12ms

Upper trace: Output current (IO) Lower trace: Output voltage (UO)



disk. The transient with the maximum peak output current is selected from the

recorded samples and the parameters measured. A sample size of 20 was found to

be statistically significant and typically included a sample with the maximum peak

output current.

This sample size was determined by repetitive experiments and obtaining a

population of measurements. Due to the inherent variations in the STA, numerous

measurements were required to derive a population. It was found that within any 20

consecutive samples of the population, a sample occurred where the peak output

current was equal to the maximum peak output current of the total population.

The measured parameter values of maximum peak output current and

corresponding voltage for the transient no-load to short-circuit load are presented in

Table 4-2. Data values of the sample transient voltage and current waveform were

tabulated in an Excel spreadsheet (see Appendix A 2) where the value for output

power was determined using equation (4.1). The output energy was determined

using equation (4.2) where the integration was approximated using the trapezoidal

method. Plots of output current, voltage, power and energy are also included in

Appendix A 2.

Initial contact of wire and cadmium disk Time Time IO(t) (A) UO(t) (V) PO(t) (W) EO(t) (mJ) t 1 0 0 12.61 0 0 t Peak 160 µs 7.61 9.44 71.84 5.5 t SS 7.12 ms 1.76 2.41 4.26 122.6

t 1 Contact makes and circuit closes at time t 1 t Peak Peak output current occurs at time t Peak

Times of interest

t SS Steady-state conditions occur at time t SS

The value of output energy when the peak output current occurs is approximately

5.53 mJ. This is considerably higher than the MIE of Methane 0.28 mJ [5]. This

amount of energy is a spark ignition risk, though as discussed in Sections 2.1.3 only

some of this energy is transferred to the ignition process.

Table 4-2: Measured transient parameters – test circuit with STA



4.3.2 Measuring Transient Output Characteristics using a Relay

To overcome variations inherent in the STA a relay contact was substituted for the

STA wire and cadmium disk as shown in Figure 4-6. A signal generator was used to

provide an independent square wave voltage to drive the relay coil. By varying the

signal generator frequency and voltage the relay closing speed could be adjusted to

a value between 0 to 200 mm/s. However, although contact bounce was a problem

at higher speeds. The relay contact applied a short-circuit load and a storage

oscilloscope was used to measure the output voltage and current during the

transition.

Figure 4-6: Transient characteristics test circuit with a relay

The measured results as shown in Figure 4-7 shows a transition where a short-

circuit was applied. During the experiment it was observed that the highest values of

peak output current occurred when the relay was operated manually with the relay

test button. This produced a slow closure of the relay contact with no contact

bounce. This led us to believe that a wetted contact may provide an alternate

solution to the problem of contact bounce.

RM – Current measuring resistor (1Ω)

Contactcloses at t1opens at t2

time

time

t1 t2

t1 t2

Uo NL

Uo SC

Io Peak

Io SCIo NL

Uo(t)

Io(t)

Io AmpsRM


+

-

AB


Uo

UM

OSCILLOSCOPE

IO

Uo Volts

Chnl. B

Chnl. A



Figure 4-7: Measured transient output characteristics (relay)

The same sampling strategy used in Section 4.3.1 was applied to obtain a sample

that included the maximum peak output current. The measured parameter values of

the maximum peak output current and corresponding voltage for the transient no-

load to short-circuit load are presented in Table 4-3: . Data values of the sample

transient voltage and current waveform were tabulated in an Excel spreadsheet

(refer Appendix A 3) where the value for output power was determined using

equation (4.1). The output energy was determined using equation (4.2). Plots of

output current, voltage, power and energy are also included in Appendix A 3.

Relay contacts closing Time Time IO(t) (A) UO(t) (V) PO(t) (W) EO(t) (mJ) t 1 0 0 12.78 0 0 t Peak 70 µs 8.56 9.09 77.8 3.9 t SS 6.02 ms 1.74 1.81 3.2 94.3

where t 1 ,t Peak , and t SS are defined in Table 4-2.

8.56A IO 1.74A

0A 12.78V 9.09V UO 1.81V 0V t0 t Peak=70µs tSS=6.02ms


Table 4-3: Measured transient parameters – test circuit with a relay



After comparing the sets of values in Table 4-2 and Table 4-3, it is evident that the

relay contact does not produce the same results as those for the STA. The relay

circuit has lower inductance and series resistance than the STA circuit. In Section

3.4 the STA was found to have series inductance and resistance. The STA’s series

impedance limits the current and the resistance damps the output characteristic

behaviour. The series inductance increases the time it takes to reach peak output

current and the resistance predominantly reduces the value of peak output current

although it can also affect the rise time.

In Section 3.2 the STA wire and cadmium disk closing speed for the initial contact

was calculated as 208 mm/s. The relay contact was closed manually at a much

slower closing speed resulting in higher values of peak output current. The contact

closing speed affects the rate of change of voltage and current thus the

instantaneous values of voltage and current. The relay method of measuring the

transient characteristics produces results that deliver higher values of output energy

in a shorter time period. This equates to a higher risk of sparking potential in terms

of intrinsic safety. As described in Sections 2.1.1 and 2.1.3 it is the initial rapid

increase in the available output energy that determines the amount of energy

transferred to an explosive test gas in close proximity to the spark gap.

To improve the correlation between the two sets of results, the relay contact circuit

could include values of inductance and resistance so that the relay circuit

impedance is the same as the STA. Alternatively a factor could be used to establish

the relationship between the two sets of results and to cater for the variations

inherent in the use of the STA. Statistical probability and analysis studies would be

required to determine the appropriate factor and this would require a suitable

sample size to attain acceptable values of confidence.

4.3.3 Limitations in Measuring Transient Output Characteristics

The short rise time of the peak output current approached the limit of the measuring

equipment used in this experiment. A digital storage oscilloscope with a 200 MHz

bandwidth and a sample rate up to 2.5 GS/s was used to measure the peak output

current. Variations in measured peak output current occurred as a result of

coincidence of the oscilloscope sample period and circuit closure. An oscilloscope

with a wider bandwidth, and higher sample rate would reduce variation in the

measurement of peak output current.



Measurement of output current is sensitive to changes in circuit resistance so it is

recommended that short wire lengths with a cross-sectional area of greater than

0.75 mm2, low resistance joints and clean relay contacts are utilised. The value of

peak output current is dependent on the impedance of the discharge path. The

discharge path is shown in Figure 4-8.

Figure 4-8: Power supply output capacitance – external discharge path

The discharge path includes the effective series resistance (ESR) of the energy

storage capacitors, wire resistances, wire contact resistances, relay contact

resistance, and a current measuring resistor (1 Ω). This resistance value was

selected as a conservative approximation to ensure that the proposed alternative

assessment method (PAAM) was more sensitive to ensure pass margins with high

levels of confidence. Parasitic inductance and capacitance are minimised by

separated short wire lengths.

When measuring the transient response of a circuit with either capacitance or

inductance, the instantaneous values of current and voltage are dependent upon the

rate at which the voltage or current is changing, as shown in Table 4-4.

RC = Effective series resistance (ESR) of C RM = Current measuring resistor

Inductor L eL = L di / dt vR = i * R i = 1/L ∫ eL dt i = vR / R E = vR + eL = i * R + L di / dt

Capacitor C i = C dvC / dt i = vR / R vC = 1/C ∫ i dt vR = i * R E = vR + vC = i * R + 1/C ∫ i dt = RC dvC / dt + vC

where E – applied source voltage i – instantaneous circuit current vR – instantaneous voltage across resistor R eL – instantaneous voltage across inductor L vC – instantaneous voltage across capacitor C

Table 4-4: Instantaneous voltage and current for inductors and capacitors

EC

i

vC

vR RL

i

e L

v R R E

RM


+

-

ContactAB


Uo

UM

OSCILLOSCOPE

RC

C IO



4.4 Transient Characteristics of Sample IS Active Power Supplies

Three transient output characteristics were obtained from the sample IS active

power supplies. Transient output characteristics were obtained for both the normal

and fault modes of operation to separately determine the transient behaviour of the

relevant blocks in Figure 4-1. The third transient output characteristics

encompassed both modes of operation and determined the transient behaviour

during the transition between operational modes. For each of the three transient

output characteristics a transient load is applied, the output voltage and current then

stabilise. This is followed by the removal of transient load so that there are two

transitions during a single test.

The transient output characteristics for the normal mode of operation are determined

by rapidly changing the load from no-load to full-load at time t1 and, after a period to

stabilise, rapidly removal of the load from full-load to no-load at time t2. The test

circuit and transient output characteristics for UO and IO are illustrated in Figure 4-9.

During the transition from full-load to no-load at time t2 in Figure 4-9 the output

voltage over shoots and has a damped oscillation as it stabilises at the steady-state

no-load output voltage as illustrated in the detail inset. Output current reduces

rapidly from the steady-state rated full-load value to zero. The oscillation near time t2

has a short period and decays quickly. During this transition, there is an increase in

the output energy which is a potential source for spark ignition. The parameters

defining the period near time t2 are the values of the first two peaks of output voltage

and the corresponding output currents.

Figure 4-9: Active power supply NL to FL transient characteristics

RFL – full-load resistance UO – output voltage Rm – current measuring resistor Um – voltage across Rm NL – No-load FL – Full-load IO - output current

Detail

Rm



+

-

R FL

A

BChnl. A - UoChnl. B - Io = Um / Rm

Uo

Um

OSCILLO-SCOPE

Uo(t)

time

timeIo(t)

Uo NLUo FL

Io FLIo NL

t1 t2

Uo Peak

Io Amps

t1 t2

Uo Volts



The transient output characteristics for the fault mode of operation are determined

by the transition from full-load to a short-circuit at time t1 and, after a period to

stabilise, a rapid reduction in the load from a short-circuit to full-load at time t2. The

test circuit and transient output characteristics for UO versus time and IO versus time

are illustrated in Figure 4-10.

During the transition from full-load to short-circuit at time t1 in Figure 4-10 the output

current rises rapidly to a peak followed by a non-linear decay to the steady-state full-

load current. The output voltage reduces rapidly from the steady-state full-load value

to the steady-state short-circuit value. During this transition, there is an increase in

output energy, which is a potential source for spark ignition. The parameters that

define the period near time t1 are the value of peak output current and the output

voltage.

Figure 4-10: Active power supply FL to SC transient characteristics

The test circuit and transient output characteristics of a no-load to short-circuit

transition are illustrated in Figure 4-11. During the transition from no-load to short-

circuit at time t1 in Figure 4-11 the output current rises rapidly to a peak followed by

a non-linear decay to the steady-state full-load current. The output voltage reduces

rapidly from the steady-state no-load value to the steady-state short-circuit value.

During this transition, there is an increase in the output energy, which is a potential

source for spark ignition. The parameters that define this period are the value of the

peak output current, the time constant of the exponential decay, and the output

voltages at these points.

RFL – full-load resistance UO – output voltage Rm – current measuring resistor Um – voltage across Rm FL – Full-load SC – Short-circuit IO - output current

Rm


+

-

R FL

A


Uo

Um

OSCILLO-SCOPE

time

time

Contactcloses at t1opens at t2 Uo SC

Uo FL

t1 t2

t1 t2

Io AmpsIo PeakIo SCIo FL

Uo(t)

Io(t)

Uo Volts



Figure 4-11: Active power supply NL to SC transient characteristics

Considering the transient output current responses in Figure 4-10 and Figure 4-11

the initial current rise at time t1 is attributed to the capacitive energy storage

components in the circuit of Figure 4-1 discharging into the short-circuit load. As the

contacts in the test circuit are closing at time t1 an arc is formed and output energy is

transferred to the arc.

The peak output current and the initial part of the decay near time t1 are caused by

the rapid discharge of the energy storage capacitors. The peak of the initial output

current rise is dependent upon the voltage across the energy storage capacitors and

the circuit resistance between the energy storage capacitors and the short-circuit.

The later part of the output current decay is due to the non-linear components in the

intrinsic safety control and current limiter circuit of Figure 4-1. As the output current

demand exceeds the rated value the IS control circuit drives the current limiter. This

increases its resistance to limit the output current, reduces the output voltage, and

limits the output power.

The time between time t1 and when the peak output current is reached is the

response time of the current sensing and intrinsic safety control circuit of Figure 4-1.

After the initial peak the output current returns to the steady-state short-circuit value.

The period between the peak output current and when steady-state short-circuit

values are reached is the response time of the intrinsic safety current limiter

circuitry.

On removal of the transient short-circuit at time t2 there is no evidence of output

voltage oscillation. The oscillation observed in the full-load to no-load transient

Rm – current measuring resistor UO – output voltage FL – Full-load Um – voltage across Rm SC – Short-circuit NL – No-load IO - output current

Rm


+

-


A


Uo

Um

OSCILLO-SCOPE time

time

t1 t2

t1 t2

Uo NL

Uo SC

Io Peak

Io SCIo NL

Uo(t)

Io(t)

Io Amps



characteristics of Figure 4-9 has been damped by intrinsic safety control and current

limiting circuits which are active during the initial part of the short-circuit to no-load

transition. The output current drops rapidly from the steady-state short-circuit value

to zero. This indicates that there are minimal inductive energy storage components

in the output stage of this type of IS active power supply.

Values of the no-load to short-circuit transient characteristics voltage UO(t) and

current IO(t) were tabulated in an Excel spreadsheet (refer Appendix A 4) where the

value for output power as shown in Figure 4-12 (b) was determined using equation

(4.1). The output energy as shown in Figure 4-12 (b) was determined using equation

(4.2) where the integration was approximated using the trapezoidal method to

calculate the area under the output power versus time curve.

The peak output power occurs with the peak output current shortly after time t1. The

output energy rises rapidly from time t1 to a knee and then continues to slowly

increase due to the steady-state output power. The value of the output energy at the

knee is the transient output energy rise and is the available energy in the arc that

potentially can be transferred to the surrounding explosive test gas. The time

between t1 and the knee is the duration of the arc. On the removal of the transient

short-circuit at time t2 the output power drops rapidly to zero and the output energy

ceases to rise.



Figure 4-12: Active power supply NL to SC transient characteristics

(a) Output current IO(t) and voltage UO(t)

(b) Output power PO(t) and energy EO(t)

12V 1A PS No-load to Short-circuit output characteristic

0

2

4

6

8

10

12

14

0 0.02 0.04 0.06 0.08 0.1

time (ms)

Ou

tpu

t vo

lts (U

o)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

Ou

tpu

t cu

rren

t (Io

) Am

ps

Uo Io

12 V 1A PS No-load to Short-circuit output characteristic

0.00

20.00

40.00

60.00

80.00

0 0.02 0.04 0.06 0.08 0.1

time (ms)

Out

put p

ower

(Po)

W

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

Out

put e

nerg

y (E

o) u

JPo Eo



4.5 Summary

The steady-state output characteristics of the sample IS active power supplies

identified a normal mode where the output voltage is regulated and a fault mode

where the output current is limited. Transient output characteristics can be

determined by measuring instantaneous output voltages and currents using a

storage oscilloscope and a relay contact to switch between two load conditions.

Output voltage and output current values can be measured during the transient

period.

The transient output characteristics of the sample IS active power supplies identified

a number of transient load conditions where the output power is significantly higher

than the maximum steady-state output power. During these transient load

conditions, there is a rise in the available output energy. This is a potential source of

spark ignition. During the no-load to short-circuit load transient period there was a

change over between the modes of operation of the sample IS active power supply,

from normal mode to fault mode and during this time the highest simultaneous

values of voltages and current were measured.

The sample IS active power supplies analysed in this section have transient output

characteristics consistent with circuits containing predominantly capacitive energy

storage components. The parameters that define the transient output current and

voltage in this chapter are used in Chapter 5 to develop a proposed alternative

assessment method (PAAM).

- 70 -


Chapter 5 Development of the PAAM The research performed by Dill and Kanty [11] established a way of determining the

sparking potential of a circuit utilising a comparative method. If the static and

transient output characteristics of an intrinsically safe (IS) active power supply were

recorded then any time later the sparking potential of that same power supply could

be determined by comparing its present static and transient output characteristics

with the recorded characteristics. The implications are that the steady-state and

transient output characteristics contain sufficient information to determine the

sparking potential of a circuit.

Three alternative assessment methods are discussed in this chapter. In Section 5.1

the first two methods are briefly described followed by a third method based on the

development of an equivalent circuit. The third method is the proposed alternative

assessment method (PAAM). In Sections 5.2 to 5.4 the equivalent circuits models

used in the PAAM are developed. The PAAM and its limitations are discussed in

Sections 5.6 and 5.7 respectively followed by the conclusions in Section 5.8.

5.1 Assessment Methods for IS Active Power Supplies

The first method is based on the determination of a finite value for the output energy

derived from the transient output characteristics of an active power supply. This

value of output energy could then be used to determine whether the active power

supply’s sparking potential is low enough to be regarded as intrinsically safe.

Whilst this appears to be a simple technique, consideration should be given to the

test conditions under which the transient output characteristics of the active power

supply are produced. The test conditions should be such that there is an optimal

transfer of energy from the electric arc to the surrounding test gas. The effective

amount of energy transferred to the ignition process needs to be determined and a

relationship established between the energy transferred to the test gas and the

sparking potential in terms of intrinsic safety limitations.

A determination of the effective amount of energy transferred to the ignition process

and the development of a relationship between this energy and the sparking

potential is beyond the scope of this thesis.

Chapter 5 Development of the PAAM - 71 -


The second method uses circuit analysis of the active power supply to determine the

maximum power transfer under transient short-circuit conditions. This would require

impedance matching between the internal impedance of the active power supply

and the impedance of the transient short-circuit. Under transient short-circuit

conditions the internal impedance of the active power supply can vary significantly.

An assessment based on analysis of dynamic impedance was deemed too complex

for consideration as a practical assessment method to determine sparking potential.

The third method entitled ‘proposed alternate assessment method‘ (PAAM) is

developed throughout the remainder of this thesis and features the modelling of an

IS active power supply via the use of an equivalent circuit. Ideally the equivalent

circuit would simplify the IS active power circuit, containing fewer components while

still producing the same output characteristics as the IS active power supply.

According to Dill and Kanty [11] the equivalent circuit can be used to establish the

sparking potential if it has the same steady-state and transient output characteristics

as the IS active power supply.

The existing assessment method using the ignition curves included in the intrinsic

safety Standard (refer Appendix A 5) is applicable to ‘well defined’ circuits. A ‘well

defined’ circuit is a circuit such as a direct current (DC) voltage source and

comprises of one of the following component combinations: a series resistor, or

resistor and inductor, or resistor and capacitor. If the equivalent circuit is one of

these ‘well defined’ circuits then its sparking potential and that of the IS active power

supply can be determined by using existing assessment techniques.

Two equivalent circuit models are presented in this chapter. The first equivalent

circuit discussed is entitled ‘RLC equivalent circuit model’ where the circuit topology

includes resistance, inductance and capacitance. The second equivalent circuit

discussed is simplified ‘RC equivalent circuit model’ as the circuit topology includes

only a resistance and a capacitance. The RC equivalent circuit model is a ‘well

defined’ circuit.



5.2 The RLC Equivalent Circuit Model

The RLC equivalent circuit model attempts to model both conditions that occur

where a transient output energy rise is observed during both the transient

application of a short-circuit and the transient removal of a full-load.

The circuit topology of the RLC equivalent circuit as presented in Figure 5-1 is

determined by analysing the output characteristics of the sample IS active power

supply. The steady-state output characteristics illustrated in Figure 4-2 show that the

full-load voltage is slightly less than the no-load voltage, indicating the existence of a

series resistance RS.

The first transient considered is the ‘no-load to short-circuit’ transition as described

in Figure 4-11. At time t1, when the short-circuit is applied, the current rapidly

increases from zero to a peak value, followed by a non-linear decay to the steady-

state short-circuit value. The voltage during this period decays from the steady-state

no-load voltage to the steady-state short-circuit voltage. This indicates a shunt

capacitive energy storage component C with a corresponding series resistance RC

which includes the effective series resistance (ESR) of the capacitor.

The second transient considered is the ‘full-load to no-load’ transition described in

Figure 4-9. At time t2, when the full-load resistance is removed, the current decays

from steady-state full-load value to steady-state no-load value, indicating a series

inductive energy storage component L with a corresponding series resistance RL.

The output voltage exhibits an overshoot followed by an oscillation that decays to

the steady-state no-load voltage, indicating a damped oscillatory circuit.

Figure 5-1: PAAM - RLC equivalent circuit model topology

US - DC voltage source RS - Source resistance L - Inductor RL - Inductor resistance C - Capacitor RC - Capacitor ESR resistance UO - Output voltage IO - Output current

+

-

RS + RL L

RC

CUS

Uo

Io



The damping factor (ξ) of a series RLC circuit exhibiting an under damped

oscillation can be estimated from the ratio of the magnitude of the first two

overshoots of the oscillation. The natural frequency of the oscillation (ωn) can be

estimated using the damped oscillation frequency (ωd) and the damping factor [27].

The characteristic equation for the damped second-order response can be solved so

that the damping factor and natural frequency are related to the series circuit

component values.

The component values for the RLC equivalent circuit model are determined using

the equations presented in Table 5-1. The parameters measured in Table 5-1 are

determined from the steady-state and transient characteristics of an IS active power

supply.

Component Equations Description US = UO NL UO NL = SS no-load circuit voltage

RS + RL = UO NLIO SC

- RLOAD RLOAD known (external component) Note(i) IO SC = SS short-circuit current

RC = UO NLIO Peak

Note (i) IO Peak = TS(i) peak output current Note(i)

Underdamped case ξ < 1 , ωd < ωn Critically damped case ξ = 1 Over damped case ξ > 1

L = (RS + RL + RC)

(2*ωn*ξ)

C = 1

(L*ωn2)

UO exhibits a damped oscillation UO exhibits an exponential like behaviour UO exhibits an exponential like behaviour

Damping factor ξ = log e

x1 x2

√(π2 - (log e x1 x2

) 2) [27]

Natural frequency ωn = ωd

√(1 - ξ2) [27]

where x1 = amplitude of first overshoot of TS(v) x2 = amplitude of first undershoot of TS(v) T = period of TS(v) oscillation ωd (damped frequency) = 1/T

SS – Steady-state characteristics in Figure 4-2 TS(i) NL – SC - Current transient characteristics in Figure 4-11 TS(v) FL – NL - Voltage transient characteristics in Figure 4-9 Note (i) - In some cases where RC or RL are calculated as low ohm values, special component types are selected such as a capacitor type with low ESR or manufactured such as an inductor with low internal resistance.

Table 5-1: Component equations for the RLC equivalent circuit model



5.3 Experimental Verification of the RLC Equivalent Circuit Model

Once the RLC equivalent circuit model was defined, component values were

determined for an over damped and under damped circuit. Experimental RLC

equivalent circuits were constructed using the component values listed in Table 5-2,

and tested to measure the steady-state and transient output characteristics.

The value for RS is higher than typically found in power supplies. A high value for RS

was used to ensure that the time constant involving the inductor was significantly

different from the time constant related to the capacitor. This would allow

identification of their respective affects on the circuit.

Value Component Over damped Under damped

US – DC voltage source 10 V 10 V RS – Series resistance 216 Ω 216 Ω L – Inductor (air cored) 92.8 mH 92.8 mH RL – Inductor resistance 24 Ω 24 Ω C – Capacitor 10.29 µF 972 nF RC – Capacitor (ESR) 0.91 Ω 5.2 Ω RM – Current measuring resistor 1.526 Ω 1.526 Ω ξ - Damping factor 1.27 0.4

The under and over damped experimental RLC test circuits presented in Figure 5-2

(a) produce the same steady-state characteristic for both the under and over

damped cases as shown in Figure 5-2 (b). The DC voltage source US has a current

limit that is activated as the current demand exceeds full-load value. The steady-

state characteristics for the under and over damped experimental RLC equivalent

circuits formed a rectangular shape consistent with an active power supply.

Table 5-2: Experimental RLC equivalent circuit model – component values



Figure 5-2: Experimental RLC equiv. cct. and steady-state characteristic

The under and over damped experimental RLC equivalent circuit output transient

characteristics are measured using the circuit shown in Figure 5-3. The DC voltage

source US had its current limiter de-activated for the measurement of the transient

output characteristics. In the case of the over damped experimental RLC equivalent

circuit the ‘no-load to short-circuit’ output transient is presented in Figure 5-4 and the

‘short-circuit to no-load’ output transient presented in Figure 5-5.

Figure 5-3: Experimental RLC equivalent circuit – transient tests

(Refer Table 5-2 for component values) (Measured values) (a) Experimental RLC equivalent circuit (b) Steady-state output characteristic

(Refer Table 5-2 for component values)

+

-

RS + RL L

RC

CUS

UO

IO

RLOAD


RM

AB


UO

UM

OSCILLOSCOPE

+

-

RS + RL L

RC

CUS

Uo

Io

Steady-state output characteristic RLC equivalent circuit model

0

10

20

30

40

50

0 2 4 6 8 10 12Output voltage (Uo)

Out

put c

urre

nt (I

o)m

A



Figure 5-4: Over damped RLC equiv. cct. NL to SC transient characteristics

The peak output current can be estimated from an analysis of the capacitor

discharge path by the following equation:

IO Peak ≅ UO NL

RC + RM ...(5.1)

The estimated value of peak output current for the over damped case is 4.1 A. It is

expected that this estimated value will be higher than the measured value due to the

exclusion of current path through the DC voltage source US and circuit inductances.

The measured value of 3.34 A is lower because of additional circuit loading by relay

contact resistance, oscilloscope probes, current sensing resistance, and parasitic

inductance.

Note(i) Io

41.59mA

23.15mA

0 A 9.99V UO

70mV 0 V t0 t Minimum=0.23ms tSS=2.04ms

t Peak=20µs Note(i) - Output current peak IO Peak (3.34 A at 20µs) not shown in this waveform

Upper trace: Output current (IO), Lower trace: Output voltage (UO)



Figure 5-5: Over damped RLC equiv. cct. SC to NL transient characteristics

When the experimental RLC equivalent circuit model (over damped) transient output

characteristics in Figure 5-4 and Figure 5-5 are compared with those measured from

sample IS active power supply. It is observed that the transient output current

characteristics of the RLC equivalent circuit model in Figure 5-4 near time tMinimum

falls below the steady-state value. The transient current characteristic of the sample

IS active power supply as in Figure 4-12 (a), at no stage falls below the steady-state

values. As a consequence, the output energy of the RLC equivalent circuit model is

significantly lower during this period due to the current IO(t) undershoot directly after

the peak output current at time t Peak.

As the short-circuit load is removed during the ‘short-circuit load to no-load’

transition at time t2 the output voltage characteristics of the experimental RLC

equivalent circuit model in Figure 5-5 replicates the behaviour of the sample IS

active power supply illustrated in Figure 4-11. The output power during this period

rapidly drops to zero as the circuit is opened.

IO

41.65mA

0A 9.97V UO

70mV 0V t0 tSS=14.76ms




In the case of the under damped experimental RLC equivalent circuit the ‘no-load to

short-circuit’ output transient is presented in Figure 5-6 and the ‘short-circuit to no-

load’ output transient presented in Figure 5-7. These output transient characteristics

attempt to replicate the behaviour of the sample IS active power supply at times t1

and t2.

Figure 5-6: Under damped RLC equiv. cct. NL to SC transient characteristics

Using equation 5.1, the estimated value of peak output current for the under

damped case is 1.49A. As discussed previously it is expected that this estimated

value will be higher than the measured value. The measured value of 89.5 mA is

lower than expected because of additional circuit loading as previously discussed.

The experimental RLC equivalent circuit model (under damped) transient output

characteristics in Figure 5-6 and Figure 5-7 are compared to those measured from

sample IS active power supply. The transient output current characteristics of the

RLC equivalent circuit model in Figure 5-6 near time t Minimum falls below the steady

state value. As with the over damped case the output energy of the RLC equivalent

circuit model (under damped) is significantly lower during this period. Electronic

89.47mA IO 41.74mA

7.29mA 0A

9.95V UO 70mV 0V t0 t Minimum=70µs tSS=1.81ms t Peak=10µs




circuit simulation of the RLC equivalent circuit model indicated that the inductance

(L) is primarily responsible this behaviour.

Figure 5-7: Under damped RLC equiv. cct. SC to NL transient characteristics

In Section 4.4 it was established that the output energy of the sample IS active

power supply during the transition from no-load to short-circuit load at time t1 is

significant and poses a spark ignition risk. This is a critical period and the RLC

equivalent circuit model would be required to accurately predict the behaviour of the

sample IS active power supply. A simplified model can be used to predict the

behaviour during the no-load to short-circuit load transition at time t1.

IO 41.63mA 0mA 15.27V

10.34V 10.01V 8.65V UO 0V t0 tOS=630µs tOS2=2.69ms tSS=4.28ms t US1=1.66ms




5.4 The RC Equivalent Circuit Model

The RC equivalent circuit model as shown in Figure 5-8 is the same as the RLC

equivalent circuit model with the exception that the inductance component has been

removed. The RC equivalent circuit component values can be determined from the

steady-state and transient characteristics of an IS active power supply.

Figure 5-8: PAAM - RC equivalent circuit model topology

Only the steady-state and transient ‘no-load to short-circuit’ characteristics of the

sample IS active power supply are required to determine the component values.

Table 5-3 shows the equations required.

Component Equations Description US = UO NL UO NL = SS output voltage

RS = UO NLIO SC

- RL RL known (external component) ISC = SS short-circuit current

RC = RS

(RS + RL)

IO SC.RS

IO Peak + IO SC - RL Note(i)

IO Peak = TS(i) peak output current

C = τ.(RS + RL)

(RS.RC + RS.RL + RL.RC) τ = time constant of TS(i) peak current decay

SS – Steady-state characteristics in Figure 4-2 TS(i) – Current transient characteristics in Figure 4-12 (a) Note (i) - In some cases where RC is calculated as low ohm values, special component types are selected such as a capacitor type with low ESR.

US - DC voltage source RS - Source resistance C - Capacitor RC - Capacitor ESR UO - Output voltage IO - Output current

Table 5-3: Component equations for the RC equivalent circuit model

+

-

RS

RC

CUS

Uo

Io



5.5 Experimental Verification of the RC Equivalent Circuit Model

A experimental RC equivalent circuit using component values as shown in Table

5-2. was constructed as shown in Figure 5-9 (a) and subsequently tested to

measure the steady-state output characteristics presented in Figure 5-9 (b).

Component Value US – DC voltage source 10 V RS – Series resistance 99.3 Ω C – Capacitor 10.29 µF RC – Capacitor (ESR) 0.91 Ω RM – Current measuring resistor 1.526 Ω

Figure 5-9: Experimental RC equiv. cct. and steady-state characteristic

The experimental RC equivalent circuit steady-state characteristics in Figure 5-9 (b)

is a rectangular shape consistent with an active power supply. The experimental RC

equivalent circuit output transient characteristics were measured using the circuit

shown in Figure 5-10. The ‘no-load to short-circuit’ output transient is presented in

Figure 5-11 and the ‘short-circuit to no-load’ output transient presented in Figure

5-12.

Table 5-4: Experimental RC equivalent circuit model – component values

(Refer Table 5-4 for component values) (Measured values) (a) Experimental RC equivalent circuit (b) Steady-state characteristic

+

-

RS

RC

CUS

Uo

Io

RLOAD

Steady-state output characteristic RC equivalent circuit model

020

40

60

80

100

120

0 2 4 6 8 10 12Output voltage (Uo)

Out

put c

urre

nt (I

o)m

A



Figure 5-10: Experimental RC equivalent circuit - transient test circuit

Figure 5-11: Measured RC equiv. cct. NL to SC transient characteristics

(Refer Table 5-3 for component values)

3.28A IO 105mA 0mA 10.04V UO

0.2V 0V t0 tSS=220µs tPeak=20µs

Upper trace: Output current (IO), Lower trace: Output voltage (UO )

+

-

RS

RC

CUS

Uo

IoContactcloses at t1opens at t2

RM

AB


UO

UM

OSCILLOSCOPE



Using equation 5.1, the estimated value of peak output current for the under

damped case is 4.1 A. As discussed previously it is expected that this estimated

value will be higher than the measured value. The measured value of 3.28 A is

lower as discussed previously.

Figure 5-12: Measured RC equiv. cct. SC to NL transient characteristics

The experimental RC equivalent circuit ‘no-load to short-circuit’ transient

characteristics in Figure 5-11 are compared to those measured from sample IS

active power supply in Figure 4-12 (a). The experimental RC equivalent circuit

model is able to predict the behaviour of the sample IS active power supply during

the ‘no-load to short-circuit’ transient period near time t1.

When the short-circuit load is removed at time t2 the RC equivalent circuit model has

an exponential voltage rise whereas the sample active power supply has a

exponential voltage with a faster rise time. This difference between the RC

equivalent circuit model and the sample IS active power supply can be ignored as

the output voltage near time t2, has a minimal effect on the output energy, since the

output current is zero.

IO 91.1mA 0mA 9.99V UO 0.33V 0V t0 tSS=5.19ms

Upper trace: Output current (IO), Lower trace: Output voltage (UO )



The value of the capacitance C in the RC equivalent circuit model is found to be

significantly lower than the physical value of the capacitance in the output stage of

the sample IS active power supply. This lower value of C is defined in this thesis as

the ‘effective capacitance’ of the active power supply. The current limiter of Figure

4-1 accounts for the difference between the ‘effective capacitance’ and the physical

value of the capacitance. The RC equivalent circuit is, in effect, modelling the non-

linear response of the current limiter. The response times of the current sensing

circuit, IS control circuit and current limiter have a significant effect on the transient

output energy.



5.6 The Proposed Alternative Assessment Method (PAAM)

Research performed by Dill and Kanty [11] was used as a basis for the research

carried out in this project. The aim was the development of an alternative

assessment method to determine the sparking potential of an active power supply.

This method is based on the use of an equivalent circuit model conforming to the

topology of one of the ‘well defined’ circuits as defined in the intrinsic safety

Standard. The output stage of a sample IS active power supply was modelled

utilising an RC equivalent circuit comprised of a small number of passive

components.

The steady-state and transient output characteristics can be obtained from a simple

test using a relay contact and a storage oscilloscope. Due to the transient nature of

the signals being measured, a storage oscilloscope with a suitable bandwidth or

sampling rate and input impedance is used in order to ensure the accuracy of the

measurements.

The response of the equivalent circuit throughout the period of application of a short-

circuit up to the occurrence of the peak output current is a function of the specific

characteristics of the short-circuit. These include the rate at which the contacts are

closing, the applied voltage, dielectric strength, and impedance of the discharge

circuit. These specifics of the short-circuit determine the time period between the

first conduction of current and the occurrence of peak output current, value of the

peak output current and corresponding output voltages.

The second stage of the transient response extends from the point where the peak

output current occurs to the establishment of steady-state circuit conditions. This is

due to the current sensing circuit, the IS control circuit and the current limiter within

the IS active power supply. The current sensing circuit, the current limiter and IS

control circuit have a finite response time. This information was used to develop the

RC equivalent circuit model. The RC equivalent circuit model replicates the

behaviour of the sample IS active power supply when subjected to a short-circuit.

The RC equivalent circuit model is an ‘equivalent linear power supply’ of the IS

active power supply under investigation.



If it is postulated that an active power supply can be adequately represented by a

RC equivalent circuit model, the active power supply unit can then be assessed

using the ignition curves in the intrinsic safety Standard. If the equivalent circuit

model is assessed as intrinsically safe that is inside the safe area as illustrated in

Figure 5-13 then the active power supply could also be considered intrinsically safe.

Pass margins for the voltage and capacitance would be used to monitor how close

an assessment is to the ignition curve thus providing an acceptable level of

confidence. In cases where pass margins are small, the assessment should be

confirmed using the STA. If the equivalent circuit model fails the assessment using

the ignition curves then the active power supply would not be regarded as

intrinsically safe. A series of appropriate ‘pass margins’ need to be established via

statistically means.

Figure 5-13: Illustration of ignition curve safe and unsafe areas

Ignition curve for capacitive circuit Note: This is an illustration and is not to be used for assessment

Group I capacitive circuits

0.01

0.1

1

10

100

1000

10000

1 10 100 1000 10000Minimum igniting voltage U (V)

Cap

acita

nce

C (u

F)

SAFE AREA

UNSAFE AREA



5.7 Limitations of the PAAM

The development of the PAAM to determine the sparking potential of active power

supplies in this thesis was based on a small sample size. All of the sample active

power supplies had similar circuit topologies, although they had different nominal

voltages and currents, as shown in Table 4-1.

Two of the sample active power supplies had output currents approaching the upper

limit recommended by Dill in Table 2-6. These two power supplies are examples of

active power supplies that approach the boundaries of intrinsic safety.

A series of further investigations, utilising a larger sample size and including a more

comprehensive variation in circuit topology and nominal output values are required

in order to establish reliability of the proposed alternative assessment method.

The speed of the relay contact operation has a direct impact on the test results and

requires further consideration. It was determined experimentally that the maximum

peak output current occurs when the relay contact is closed slowly with no contact

bounce. It is anticipated that for a different circuit topology, the relay closing or

opening speed may need to be altered to optimise the measurement of transient

behaviour.

The parameters measured from the transient output characteristics using the relay

contact are higher than those measured using the STA. The higher measured

values may cause PAAM result to fail the power supply or to indicate inadequate

pass margins. Although this is undesired, it is erring on the side of safety. The

existing assessment technique utilises a factor of safety (FOS) applied to both

output voltage and current to provide a safety margin. The PAAM may not require

the use of a FOS.

A storage oscilloscope with high a input impedance and a suitably large bandwidth

or high sampling rate is required to measure the transient behaviour. Repeatable

results can be achieved using the relay contact and storage oscilloscope. The

sampling strategy ensures the energy storage components have had enough time to

fully charge before the relay contact closes and the transient behaviour is measured.

Selection of the value of peak output current from a statistically significant sample



size will ensure that the maximum peak output current is measured along with its

associated time constant and output voltages.

The measured peak output current is sensitive to circuit impedance. Estimation of

the time constant of the non-linear peak output current decay affects the ‘effective

capacitance’ value used to determine the position on the ignition curve and hence

the PAAM assessment result and pass margins.

The non-linear behaviour of the output current decay closely approximates an

exponential decay during its initial phase but varies from typical exponential

behaviour as the output current stabilises at the steady-state short-circuit value. To

ensure that the time constant of the output current decay is accurately estimated a

trendline is selected so that it coincides with the initial values of output current decay

and always exceeds the output current value. As the trend line values are either

equal to or greater than the output current values the time constant is not under

estimated. Over estimation of the time constant can be a problem and will lead to

the PAAM result to be a fail or determine inadequate pass margin.

The initial technique used in this research to estimate the time constant was to

measure the period from the occurrence of the output current peak to the point

where the output current had reduced by 63.8 % of the difference between the peak

current value and the final steady-state short-circuit current value. This technique

was replaced by transferring values of the transient output current to an Excel

spreadsheet, plotting the characteristic and utilising the trend-line feature (refer to

Figure 6-1).

The application of additional external loads to the active power supply under test

was not considered within the scope of this research because IS devices, including

IS power supplies, can be assessed in isolation using the IS entity concept

approach.

Certification of active power supplies using the 'entity concept method' [24] requires

the establishment of maximum values of external circuit inductances and

capacitances are required to be determined. This is typically determined via the use

of the STA and application of external capacitances and inductances to the active

power supply, repetitive testing and alteration of the external component values until



ignition occurs. The external components represent the combination of distribution

cabling and IS devices (load) connected to the cable.

The addition of external load resistance, which is equivalent to increasing the series

shunt resistance reduces output current and hence improves the safety of the

circuit. As worst conditions are attempting to be determined additional resistance

should be minimised.

In the case of additional external load inductance, capacitance or both inductance

and capacitance, the distributed nature of the connected loads and the resistance

between the distributed elements provides a degree of current limitation. The

potential combination of energy from the output of the active power supply and the

energy storage components in the load is a concern. It is anticipated in this case

where the output current is predominantly a single order capacitive transient

behaviour the RC equivalent circuit model can be applied. In other cases an

alternate equivalent circuit model would need to be developed.

The PAAM has not been validated in circumstances where external components are

added to the active power supply under test. It is envisaged that further

development of the PAAM would include the identification of a number of PAAM

equivalent circuit models. These PAAM equivalent circuit models would cater for the

varying types of output characteristic behaviour, including single order inductive,

second order and higher order responses.

Applying the PAAM to other types of power supplies has not been validated. Where

the power supply to be tested exhibits similar output transient behaviour to the

power supplies already examined it is anticipated that the PAAM RC equivalent

circuit model can be applied. Where the power supply to be tested has different

transient characteristics to the power supplies examined then a number of options

are presented in the following paragraphs.

The first option is to approximate the result of the PAAM by utilising the RC model

with component values that result in transient characteristics which envelopes the

transient characteristics of the power supply under test. This will ensure that the

instantaneous values of current and voltage of the PAAM RC model always exceed

those of the power supply under test. The duration of envelope would be critical and

would have a direct effect on the pass margin confidence level. In the case where



the fail margin confidence level is low it is possible that the PAAM may have unfairly

produced a fail result so the next (second) option is recommended.

In the second option where the transient characteristic exhibits a first order

inductive, second and higher order behaviours a different PAAM equivalent circuit

will need to be developed. A number of modelling techniques that synthesise an

equivalent circuit from transient characteristics are well documented in control

theory literature [27].

Alternately the transient behaviour of the power supply under test may be analysed

piecewise by assessing adjacent periods of transient behaviour. In this case the

PAAM RC model or another equivalent circuit model is utilised and each piecewise

assessment result would need to have an adequate pass margin confidence level

for the overall PAAM result to be defined as a pass. The duration of adjacent

periods and the overall duration assessed would be critical and have a direct effect

on the pass margin confidence interval.



5.8 Summary

The transient response of an active power supply results from rapid changes in load

conditions. The amount of energy available at the output of an active power supply

during transient conditions is dependent on the capacitance and/or inductance of the

energy storage components, resistance between the energy storage components

and the output, trigger level of the IS control circuit, response time of the IS control

circuit, and characteristics of the current limiting device used in the IS control circuit.

The circuit topologies used in the output stage of IS active power supplies can be

modelled using an equivalent circuit. The circuit topology and component values of

the equivalent circuit can be determined by measurement of parameters associated

with the steady-state and transient output characteristics of the active power supply

under assessment. The desired equivalent circuit topology is one of the ‘well

defined’ circuit topologies which has a corresponding ignition curve defined in the

intrinsic safety Standard.

The sparking potential of the active power supply under assessment can be

determined by assessing the equivalent ‘well defined’ circuit using existing methods

and the appropriate ignition curve. If there is;

(a) an adequate pass margin, the active power supply passes,

(b) inadequate pass margin, the result is confirmed using the STA, and

(c) failure, the active power supply fails.

The acceptance of the PAAM requires further testing on a suitably sized sample of

active power supplies with sufficient variation in circuit topology and nominal output

ratings to establish a set of equivalent ‘well defined’ circuit models. Subsequently

suitable confidence intervals for the pass margins can be established by statistical

analysis.

In chapter 6 the PAAM is applied to the sample IS power supplies and the results

compared to spark testing the sample IS power supplies using the STA.

- 92 -


Chapter 6 Experimental Evaluation of the PAAM In this section, the proposed alternative assessment method (PAAM) for determining

the sparking potential of active power supplies is verified experimentally. A

comparison is made between the results of the PAAM to the results obtained from

testing using the spark test apparatus (STA).

6.1 Sample IS Active Power Supplies

The three sample intrinsically safe (IS) active power supplies described in Section

4.1 were used to validate the new assessment method. All of the sample IS active

power supplies have the same circuit topology as shown in Figure 4-1 with an

energy storage capacitance of 4000 µF. The nominal output voltage and current

ratings for each of these active power supplies are listed in Table 4-1.

The sample IS active power supplies PS 1 and PS 2 both have a rated output

voltage of 12 V. The rated output current for PS 1 is 1 A which is a mid-range value

whereas PS 2 has a rated output current of 2 A. The rated output current for PS 2

nears the recommended maximum listed in Table 2-6. The sample 18 V IS active

power supply PS 3 with a rated output current of 1.25 A exceeds the recommended

maximum current limit for any power supply in the range of 12.5 V to 24 V. Both

sample active power supplies PS 2 and PS 3 are examples of active power supplies

that approach and test the boundaries of intrinsic safety.

6.2 Sample Active Power Supply Parameter Measurements

The transient output characteristics of the sample IS active power supplies were

determined using a relay and storage oscilloscope as discussed in Section 4.3.2.

The parameters outlined in Section 4.3 were determined from the transient

waveforms. This experiment was repeated until a sufficient number of samples were

obtained. The maximum peak output current was identified from the sample

waveforms. The measured parameters for each of the sample active power supplies

are tabulated in Table 6-1.

Chapter 6 Experimental evaluation of the model - 93 -


Sample Identifier

UO NL (V)

UO SC* (V)

PS 1 12.7 1.1 PS 2 12.7 2.1 PS 3 18.3 1.3

* RM = 1 Ω UO – Output volts IO – Output current NL – No-load SC – Short-circuit

Sample Identifier

IO Peak (A)

IO SC * (A)

Period t1 to IO Peak (µs)

Period IO Peak to IO SC (µs)

PS 1 8.6 1.0 20 30 PS 2 6.8 2.0 20 30 PS 3 7.0 1.3 30 20

Note: Values are the worst case sample selected from a sample size of 30

The measured parameters in Table 6-1 were then used to derive component values

of the RC equivalent circuit model presented in Figure 5-4, using the formulae in

Table 5-3. The RC equivalent circuit component values for each of the sample IS

power supplies is presented in Table 6-3.

At this point in the PAAM each of the sample IS active power supplies has been

simplified to a RC equivalent circuit model. The RC equivalent circuit model is an

equivalent linear power supply.

Table 6-1: Measured transient parameters of sample active power supplies



6.3 Example Application of PAAM

The RC equivalent circuit model components for the sample IS active power supply

PS 1 are determined from the transient output current characteristics presented in

Figure 6-1.

The period of interest in the transient output current characteristics extends from the

maximum peak output current to the steady-state short-circuit output current. The

output current behaviour during this period is then matched to an exponential trend

line, as shown in Figure 6-1 and defined in the following equation:

IO Trend = (IO Peak - IO SS )e-t/τ + IO SS ...(6.1)

where IO Trend = Exponential model of active power supply output current IO

IO Peak = Peak output current (worst case sample)

IO SS = Steady state output current

τ = time constant of exponential decay

The criteria for the exponential trend line is that, the value of the trend line either

equals or exceeds the value of the output current at all times. It is the value of peak

output current and the initial period of the transient response that directly affects the

value of output energy at the knee of the curve, refer to Figure 4-12. As both output

current and voltage reduce with time, the latter part of the transient is less

significant.



Figure 6-1: Measured transient output current response for PS 1

Table 6-2 shows the calculations leading to the values of the RC equivalent circuit

model components for the PS 1. The time constant ‘τ’ of the transient output current

decay is used to determine the RC equivalent circuit model shunt capacitance

component value. Using the exponential trend line, the value of the time constant ‘τ’

is determined by the inverse of the coefficient of time ‘t’ in the exponential term.

Component Calculation (Refer to Table 5-3 for component equations)

US UO NL = 12.7 V RS UO NL

IO SC - RL =

12.71 - 1 = 11.7 Ω

RC RS(RS + RL)

IO SC.RS

IO Peak + IO SC - RL =

11.7(11.7 + 1) x

1x11.7

8.6 + 1 - 1 = 0.5 Ω

C τ.(RS + RL)(RS.RC + RS.RL + RL.RC) =

11.24x10-6 x (11.7 + 1)(11.7x0.5 +11.7x1 + 1x0.5) = 7.9 µF

The RC equivalent circuit model for the sample IS active power supply PS 1 is

presented in Figure 6-2.

Table 6-2: PAAM calculating component values (RC equiv. cct. model) - PS 1

PSU 1 12V1A Ouput Current (Io) vs Time

0

2

4

6

8

10

0 10 20 30

Time (microseconds)

Out

put c

urre

nt Io

(am

ps)

Measured transient response IO - Output current

Trend line for IO IO Trend = (8.6 -1.0) e (- 0.089x10E(-6) x t) + 1.0 τ = 1/0.089x10-6 = 11.24µs

Measured transient response IO Peak – Peak output current



Figure 6-2: PAAM RC equivalent circuit model for PS 1

Applying this same technique to PS 2 and PS 3 results in the RC equivalent circuit

component values as presented in Table 6-3.

Table 6-3: PAAM component values (RC equiv. cct. model) for PS 1, 2 and 3

Sample identifier US (V) RS (Ω) RC (Ω) C (µF) PS 1 12.7 11.7 0.5 7.9 PS 2 12.7 5.3 1.0 5.8 PS 3 18.3 13.1 1.8 3.0

The RC equivalent circuit model can now be assessed using the existing

assessment method. The RC equivalent circuit model circuit topology is a ‘well

defined’ circuit configuration included in the intrinsic safety Standard with its

respective ignition curve (Group I capacitive circuits) as presented in Figure 6-3.

Each sample IS active power supply was assessed using its values of US, RC, and C

as shown in Table 6-3 to determine a point of intersection on the appropriate ignition

curve, refer to Figure 6-3. The appropriate ignition curve was determined by the

value of RC. Where the value of RC occurs between the standard curves on the

graph, the ignition curve is interpolated.

If the point of intersection between US and C lies on the left hand side of the ignition

curve defined by RC, the pass margins are determined for both US and C. The pass

margin for US is the horizontal distance between the point of intersection and the

ignition curve. Similarly, the pass margin for C is the vertical distance between the

point of intersection and the ignition curve.

US - DC voltage source (12.71 V) RS - Source resistance (11.7 Ω) C – Capacitor (7.9 µF) RC - Capacitor ESR (0.5 Ω)

+

-

RS

RC

CUS

Uo

Io



Figure 6-3: PAAM ignition curve plots for PS 1, 2 and 3 [24]

halla




Where the pass margins are acceptable, the active power supply has passed

PAAM. Where the pass margins are not acceptable, the active power supply’s

sparking potential needs to be confirmed using the STA.

If the point of intersection of C and Us falls to the right hand side of the appropriate

ignition curve defined by RC, the active power supply has failed the PAAM. The

results of the PAAM are presented in Table 6-4 as a pass or fail with the respective

pass margins included.



6.4 Comparison with Spark Testing Results

The three sample IS active power supplies were also subjected to spark testing

using the STA. The STA was applied directly to the output terminals of the sample

IS active power supplies. The test conditions for the STA complied with the

requirements of Group I (underground coal mining) and using the FOS explosive

test gas Hydrogen with a concentration ratio of 52:48 with air.

Spark testing was carried out following the normal procedure, where the sensitivity

of the STA was checked before and after each test. Only one wire was used in the

STA wire holder to ensure that there was sufficient time available for the active

power supply to recover from the previous short-circuit condition and that the output

stage energy storage capacitors were fully recharged. Where there is one wire in the

wire holder the Standard prescribes 1600 revolutions of the wire holder. The STA

was connected to the terminals of the sample IS active power using one polarity for

the first 800 revolutions and then for next 800 revolutions with the polarity reversed.

The results of spark testing using the STA are presented in Table 6-4 as either a

pass or fail outcome. For a pass to occur no explosive ignitions occurred during

1600 revolutions of the wire holder. For a fail to occur an explosive ignition occurred

during the 1600 revolutions. PS 3 had an explosive ignition during STA testing. Two

further tests were performed on this power supply but no explosive ignition resulted.

This is an example of a situation where the ability of the STA to accurately replicate

test results becomes questionable.

The results presented in Figure 6-3 following the application of the PAAM of the

three sample IS active power supplies indicate that they all fall on the left hand side

of the appropriate ignition curve. Pass margins are then used to determine if the

PAAM outcome is a pass or fail.



PAAM STA Pass margins Sample

Identifier Result UO (V) C (µF) Results PS 1 PASS 20.3 242 PASS PS 2 PASS 27.3 294 PASS PS 3 PASS 41.7 42 FAIL

The sample active power supply PS 3 provided the only example of a deviation

between the results of the PAAM and STA testing. The capacitance pass margin for

PS 3 is significantly smaller larger than that of PS 1 and PS 2, and the voltage pass

margin for PS 3 is higher indicating that an a logical AND function i.e. both margins

are required. The STA testing result obtained for PS 3 place this active power

supply on the borderline of intrinsic safety.

The spark testing results using the STA were obtained using Hydrogen as the test

gas, which has a FOS of 1.5. As the pass margin has not been determined and the

confidence interval is unknown, PS 3 may not be intrinsically safe.

The results obtained using the PAAM do indicate a relationship with those obtained

using the STA. The establishment of pass margins and confidence intervals for the

PAAM would provide the data to help establish a mathematical definition of this

relationship. It may also be sufficient to define this relationship by correlating the

STA test results and the PAAM results using a larger sample of active power

supplies.

On initial inspection, it would be expected that PS 2 would have lower pass margins

than PS 1. Meaning that it is closer to the intrinsic safety limit. PS 2 has the same

rated output voltage as PS 1 but has twice the rated output current. However, further

investigation reveals that the peak output current during the ‘no-load to short-circuit’

transition is significantly higher for PS 1. As both PS 1 and PS 2 have the same

period between the occurrence of the peak output current and the steady-state

short-circuit value, significantly more energy is being transferred from PS 1 to the

short-circuit and constitutes a potentially higher risk as an spark ignition source. This

is reflected in the PS 1 being located closer to the ignition curve and hence the

lower pass margins.

Table 6-4: Comparison of results - PAAM vs. STA testing



The sample power supply PS 3 has a higher rated output voltage than either PS 1 or

PS 2. To be intrinsically safe it would be expected to have a lower effective

capacitance than either PS 1 or PS 2. PS 3 does a lower effective capacitance.

The PAAM capacitance pass margin for PS 3 is significantly lower than PS 1 or PS

2. The PAAM in this case recommends confirmation using spark ignition testing. The

subsequent spark testing would confirm the PAAM result as an incendive ignition

occurred and PS 3 failed.

As a result of applying the PAAM, the sample power supplies PS 1 and PS 2 pass

the spark ignition assessment phase of the intrinsic safety compliance process

whereas PS 3 fails.

6.5 Summary

The PAAM developed in this research project has been applied to a small number of

sample active power supplies. There appears to be a correlation between the results

produced by the PAAM and those obtained from spark testing using the STA.

Further research is required to establish the confidence intervals for pass margins

with consideration to FOS.

- 102 -


Chapter 7 Conclusions and Further Research

7.1 Conclusions

The types of intrinsically safe power supplies have been defined and categorised in

this research. The static and dynamic (transient) behaviour of active power supplies

that exhibit a predominantly capacitive behaviour has been investigated. Parameters

that define the amount of available output energy have been identified.

The methods used in the assessment and testing of active power supplies as part of

the intrinsic safety accreditation process have been reviewed, particularly in the

determination of the sparking potential of active power supplies. A number of

improvements to the current methods are proposed.

Substantial energy can be available at the output of active power supplies under

transient conditions. Transient conditions can occur during both normal operation

and fault conditions. It is the fault conditions that give rise to concerns associated

with the use of active power supplies due to the inherent energy stored within the

output stage. The active power supplies investigated, when subjected to intermittent

short-circuit fault conditions, capable of delivering output energy that pose a

significant spark ignition risk.

An alternate assessment method is proposed to determine the sparking potential of

active power supplies. The proposed alternative assessment method (PAAM)

determines the equivalent linear power supply for the active power supply under

test. The equivalent linear power supply is then subjected to the existing

assessment method using the ignition curves included in the Intrinsic Safety

Standard.

The results of applying the PAAM to the sample active power supplies were verified

by performing traditional spark potential testing using the STA. The results of the

PAAM show correlation with those derived using the STA, although the safety

margins will still need to be established.

The PAAM developed in this research project can be used as a pre-compliance

check by designers, manufacturers, or IS testing stations. A failure of this test would

indicate that the active power supply’s sparking energy is not low enough to be

Chapter 7 Conclusions - 103 -


regarded as intrinsically safe. The PAAM requires fewer resources to establish a

result than the STA. A simplified spark ignition test like PAAM would be beneficial to

designers, manufacturers and end users.

7.2 Further Research

Verification of the PAAM and submissions

While this research project has endeavoured to establish a method of assessing the

sparking potential of an active power supply, the PAAM still requires exhaustive

testing and further validation by other concerned or specialist bodies. The PAAM

would then need to be promoted amongst national and international testing stations

in order to solicit further interest and promote acceptance. In addition, a submission

would need to be prepared and delivered to the appropriate committees of the

international, other national and the Australian Standards bodies.

Development of IS power supply barrier

In many intrinsic safety applications ‘ IS barriers’ are used to isolate intrinsic safety

circuits from non-intrinsic safe circuits. These ‘IS barriers’ are located in a safe area.

Applying this concept to power supplies resulted in the concept of an ‘IS power

supply barrier’.

An ‘IS power supply barrier’ would remove all of the intrinsic safety circuitry from the

power supply and locate them in a separate device. This device would be situated

between a conventional off-the-shelf power supply (non-IS) and the hazardous area.

The device would ensure that under all conditions there is insufficient energy in the

circuit to cause an incendive spark.

Development of an electronic IS testing device

The output energy available at the output terminals of an active power supply can be

determined from the relationship between voltage, current, and time. As these

variables can be measured from the circuit, the amount of energy available at a

potential spark can be determined. This is a measure of the potential spark energy

that can be transferred to the surrounding gas. This information, combined with the

Chapter 7 Conclusions - 104 -


known MIE of the standard testing gases and physical properties of a making or

breaking circuit, could be developed into a measuring device that indicates a

measure of intrinsic safety.

References - 105 -


References [1] NSW Dept of Mineral Resources, "Power Supplies Warning", Mine Safety

News, vol. June 1999

[2] S. Bell and M. Hookman, "Mines count cost of IS power supply bungle",

Australia's Longwalls, vol. March 1999

[3] J. Hall, Intrinsic Safety, England: Marylebone Press Ltd, 1985.

[4] Hillcon Consulting, "Ex - What is it?", What's New in Process Engineering,

vol. June 2000, 2000

[5] E. C. Magison, Intrinsic Safety, USA: Instrument Society of America, 1984.

[6] E. C. Magison, Electrical Instruments in Hazardous Locations, 4th ed, USA:

Instrument Society of America, 1988.

[7] Measurement Technology Limited (MTL), "Application Note 9003 - A user's

guide to intrinsic safety", MTL, Bedfordshire AN 9003, 1999.

[8] Pepperl + Fuchs, "In view of Intrinsic Safety", Pepperl + Fuchs Pty Ltd,

England 1989.

[9] G. Barnier, "Intrinsic Safety - Assessment and Testing", [Offline], 2000,

03/05/2001, Available: SIMTARS Intranet.

[10] W. G. Dill, "Intrinsic Safety: Design of complex systems, case studies",

presented at EuropEx - The first World Seminar on the explosion

phenomenon and on the application of explosion protection techniques in

practice, Brussels, Belgium, 1992.

[11] W. G. Dill and G. Kanty, "Analysis of the dynamic behaviour of electronically

regulated power supplies as a substitute for testing intrinsic safety by spark-

ignition tests in explosive mixtures", presented at 23rd International

Conference of Safety in Mines Research Institutes, Washington, DC, 1989.

[12] U. Johannsmeyer and M. Kraemer, "Interconnection of Non-Linear and

Linear Intrinsically Safe circuits", Physikalisch-Technische Bundesanstalt

(PTB), Braunschweig Report No. 13715 E, 1989.

[13] R. Thurlow and P. J. Green, "Progress in the design of intrinsically safe

power supplies", Mining Technology, vol. 59, 1977.

[14] R. Tomlinson and D. W. Widginton, "Effects of slewing rate when testing

intrinsically safe power supplies", Health and Safety Executive, Health and

Safety Laboratories, London Report No. 2, 1978.

References - 106 -


[15] D. Turner, G. Barnier, and M. Walpole, "Assessment, Testing and

Certification of Intrinsically Safe Active Power Supplies", presented at

Queensland Mining Industry Health and Safety Conference 2000,

Townsville, Australia, 2000.

[16] D. W. Widginton, "Intrinsic safety reference curves: Some recent

considerations.", presented at Fourth International Conference on Electrical

Safety in Hazardous Areas, London, 1988.

[17] Standards Australia, HB13 - 2000 Handbook Electrical equipment for

hazardous areas: Standards Australia International Ltd and Standards New

Zealand, 2000.

[18] J. J. Sammarco, "Intrinsically Safe 5-V, 4-A Rechargeable Power Supply",

Bureau of Mines, Pittsburgh Research Center, Pittsburgh Report No.

BUMINESIC9223, 1989.

[19] J. C. Cawley, M. D. DiMartino, T. J. Fisher, R. L. King, and M. H. Uhler,

"Power Supply for an Intrinsically Safe Circuit US Patent #4438473",

Washington, DC: Department of the Interior, 1981.

[20] P. S. Babiarz, "CEC, NEC, IEC, CENELEC: Harmony or discord?", INTECH.,

vol. V, 1998

[21] M. Hookman, "When IS is not safe", Australia's Longwalls, vol. March 1999

[22] TestSafe Australia, "TestSafe Australia - A Safety Testing and Research

Centre", [Online], 2000,, Available:

www.workcover.nsw.gov.au/testing/londonderry/about.html.

[23] SIMTARS, "SIMTARS - Safety in Mines Testing and Research Station",

[Online], 2000, Last update 28 August 1998, Available:

www.dme.qld.gov.au/simtars/index.htm.

[24] Standards Australia and Standards New Zealand, AS/NZS 60079.11:2000

Electrical apparatus for explosive gas atmospheres Part 11: Intrinsic safety i:

Standards Australia International Ltd. and Standards New Zealand, 2000.

[25] S. Halama, J. Cerri, and J. Bigourd, "Intrinsic safety of high intensity

sources", presented at 22nd International conference of safety in mines

research institutes, Beijing, China, 1987.

[26] E. Hughes, Hughes Electrical Technology, Sixth ed, England: Longman

Scientific & Technical, 1987.

[27] J. Schwarzenbach and K. F. Gill, System Modelling and Control, 2nd ed,

Britain: Edward Arnold Ltd, 1984.

Appendices - 107 -


A 1. Generic Block Diagram of IS Active Power Supply

C

Brid

ge

rect

ifier

Low

pass

filterIsol

atio

nPr

otec

tion

Filte

rTr

ansf

orm

er

ON/

OFF

AC Inpu

tVo

ltage

Sour

ce

Volta

ge

Regu

lato

r

V re

g

I reg

+ -Cu

rrent

Regu

latio

n&

IS C

ontro

l

Crow

bar

Prot

ectio

n

IS D

C O

utpu

tIS

Cnt

rl

Volta

ge

Sens

eCu

rrent

Sens

e

RR

VI

OUT

PUT

STAG

E O

F IS

ACT

IVE

DC P

OW

ER S

UPPL

Y

Appendices - 108 -


A 2. Measured Output Characteristic using STA ORM(YOKOGAWA) Data for transient output characteristic measurement using STA Number of data 700 Trigger point 23040 Trigger time 01-07-20 15:04 Sample rate 50 kHz Io =B17/$B$11 Tag name C01 C02 Uo =C17 Unit V V Po =G17*H17 No. Eo inst =(I17+I18)*0.5*$B$10*1000000

Eo accum =SUM($J$20:J20) sample interval 2.00E-05 sec resistance Rm 1.004 ohms

Lapsed Eo Eo Time Time Io Uo Po Inst Accum

Sample No. Urm V Uo V mSecs mSecs Amps Volts Watts u J u J

38760 0.01 12.61 314.40 0.0100 12.61 0.1256 2.39 38761 0.009 12.6 0.00 314.42 0.0090 12.6 0.1129 2.39 38762 0.01 12.61 0.02 314.44 0.0100 12.61 0.1256 2.26 38763 0.008 12.61 0.04 314.46 0.0080 12.61 0.1005 1.88 1.88 38764 0.007 12.61 0.06 314.48 0.0070 12.61 0.0879 11.53 13.41 38765 0.086 12.43 0.08 314.50 0.0857 12.43 1.0647 248.76 262.17 38766 2.026 11.8 0.10 314.52 2.0179 11.8 23.8116 955.62 1217.79 38767 7.559 9.53 0.12 314.54 7.5289 9.53 71.7503 1439.46 2657.25 38768 7.638 9.49 0.14 314.56 7.6076 9.49 72.1958 1440.40 4097.65 38769 7.641 9.44 0.16 314.58 7.6106 9.44 71.8437 1434.59 5532.24 38770 7.641 9.41 0.18 314.60 7.6106 9.41 71.6153 1430.22 6962.46 38771 7.635 9.39 0.20 314.62 7.6046 9.39 71.4070 1425.31 8387.77 38772 7.621 9.37 0.22 314.64 7.5906 9.37 71.1243 1419.10 9806.88 38773 7.601 9.35 0.24 314.66 7.5707 9.35 70.7862 1410.39 11217.27 38774 7.568 9.32 0.26 314.68 7.5378 9.32 70.2527 1402.16 12619.43 38775 7.553 9.3 0.28 314.70 7.5229 9.3 69.9630 1395.34 14014.77 38776 7.535 9.27 0.30 314.72 7.5050 9.27 69.5712 1386.78 15401.55 38777 7.509 9.24 0.32 314.74 7.4791 9.24 69.1067 1378.89 16780.44 38778 7.49 9.22 0.34 314.76 7.4602 9.22 68.7827 1371.40 18151.84 38779 7.468 9.19 0.36 314.78 7.4382 9.19 68.3575 1363.84 19515.68 38780 7.448 9.17 0.38 314.80 7.4183 9.17 68.0261 1356.84 20872.52 38781 7.432 9.14 0.40 314.82 7.4024 9.14 67.6578 1349.58 22222.09 38782 7.417 9.11 0.42 314.84 7.3875 9.11 67.2997 1342.43 23564.53 38783 7.394 9.09 0.44 314.86 7.3645 9.09 66.9437 1334.05 24898.57 38784 7.365 9.06 0.46 314.88 7.3357 9.06 66.4611 1324.86 26223.44 38785 7.341 9.03 0.48 314.90 7.3118 9.03 66.0251 1316.17 27539.60 38786 7.309 9.01 0.50 314.92 7.2799 9.01 65.5917 1307.24 28846.85 38787 7.274 8.99 0.52 314.94 7.2450 8.99 65.1327 1298.53 30145.37 38788 7.244 8.97 0.54 314.96 7.2151 8.97 64.7198 1289.12 31434.49 38789 7.201 8.95 0.56 314.98 7.1723 8.95 64.1922 1278.14 32712.63 38790 7.137 8.95 0.58 315.00 7.1086 8.95 63.6217 1267.10 33979.73

39114 1.751 2.4 7.02 321.48 1.7440 2.4 4.1857 83.64 122186.45 39115 1.748 2.4 7.04 321.50 1.7410 2.4 4.1785 83.40 122269.85 39116 1.748 2.39 7.06 321.52 1.7410 2.39 4.1611 83.36 122353.21 39117 1.754 2.39 7.08 321.54 1.7470 2.39 4.1754 83.81 122437.02 39118 1.752 2.41 7.10 321.56 1.7450 2.41 4.2055 84.60 122521.63 39119 1.758 2.43 7.12 321.58 1.7510 2.43 4.2549 84.73 122606.36 39120 1.75 2.42 7.14 321.60 1.7430 2.42 4.2181 84.31 122690.66 39121 1.755 2.41 7.16 321.62 1.7480 2.41 4.2127 84.01 122774.67 39122 1.752 2.4 7.18 321.64 1.7450 2.4 4.1880 83.59 122858.26 39123 1.752 2.39 7.20 321.66 1.7450 2.39 4.1706 83.48 122941.74

Appendices - 109 -


Measured output characteristic using STA

0

2

4

6

8

10

12

14

0 2 4 6

time (ms)

Out

put V

olts

(Uo)

0

2

4

6

8

Out

put C

urre

nt (I

o) A

mps

Uo Io

Measured output characteristic using STA

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7

time (ms)

Out

put p

ower

(Po)

W

0

20000

40000

60000

80000

100000

120000

140000O

utpu

t ene

rgy

(Eo)

uJ

Po Eo

Appendices - 110 -


A 3. Measured Output Characteristic using Relay ORM(YOKOGAWA) Data for transient output characteristic measurement using a relay Number of data 2001 Trigger point 5760 Trigger time 01-07-17 11:22 Sample rate 100 kHz Io =B17/$B$11 Tag name C01 C02 Uo =C17 Unit V V Po =G17*H17 No. Eo inst =(I17+I18)*0.5*$B$10*1000000




5717 0.03 12.78 0 57.17 0.0299 12.78 0.3823 3.17 3.17 5718 0.02 12.62 0.01 57.18 0.0199 12.62 0.2516 0.63 3.80 5719 -0.01 12.59 0.02 57.19 -0.0100 12.59 -0.1255 200.87 204.67 5720 4.018 10.06 0.03 57.20 4.0060 10.06 40.3002 566.52 771.19 5721 7.985 9.17 0.04 57.21 7.9611 9.17 73.0034 755.86 1527.05 5722 8.55 9.17 0.05 57.22 8.5244 9.17 78.1690 780.24 2307.29 5723 8.565 9.12 0.06 57.23 8.5394 9.12 77.8792 778.37 3085.67 5724 8.584 9.09 0.07 57.24 8.5583 9.09 77.7952 776.17 3861.84 5725 8.573 9.06 0.08 57.25 8.5474 9.06 77.4391 774.57 4636.41 5726 8.577 9.06 0.09 57.26 8.5513 9.06 77.4752 773.09 5409.50 5727 8.578 9.02 0.1 57.27 8.5523 9.02 77.1421 771.62 6181.12 5728 8.573 9.03 0.11 57.28 8.5474 9.03 77.1826 770.39 6951.51 5729 8.56 9.01 0.12 57.29 8.5344 9.01 76.8949 767.22 7718.73 5730 8.55 8.98 0.13 57.30 8.5244 8.98 76.5494 765.61 8484.33 5731 8.543 8.99 0.14 57.31 8.5174 8.99 76.5719 764.04 9248.37 5732 8.534 8.96 0.15 57.32 8.5085 8.96 76.2359 761.40 10009.77 5733 8.522 8.95 0.16 57.33 8.4965 8.95 76.0438 758.39 10768.16 5734 8.495 8.93 0.17 57.34 8.4696 8.93 75.6334 755.38 11523.54 5735 8.483 8.92 0.18 57.35 8.4576 8.92 75.4420 752.49 12276.02 5736 8.468 8.89 0.19 57.36 8.4427 8.89 75.0554 749.42 13025.45 5737 8.452 8.88 0.2 57.37 8.4267 8.88 74.8293 746.72 13772.17 5738 8.426 8.87 0.21 57.38 8.4008 8.87 74.5151 743.23 14515.40 5739 8.392 8.86 0.22 57.39 8.3669 8.86 74.1307 740.82 15256.21 5740 8.362 8.88 0.23 57.40 8.3370 8.88 74.0325 738.08 15994.29 5741 8.33 8.86 0.24 57.41 8.3051 8.86 73.5831 733.96 16728.25 5742 8.297 8.85 0.25 57.42 8.2722 8.85 73.2088 731.25 17459.50 5743 8.278 8.85 0.26 57.43 8.2532 8.85 73.0412 729.40 18188.90 5744 8.255 8.85 0.27 57.44 8.2303 8.85 72.8382 726.08 18914.97 5745 8.212 8.84 0.28 57.45 8.1874 8.84 72.3769 722.23 19637.20 5746 8.177 8.84 0.29 57.46 8.1525 8.84 72.0685 717.86 20355.06 5747 8.122 8.83 0.3 57.47 8.0977 8.83 71.5028 716.43 21071.49 5748 8.191 8.79 0.31 57.48 8.1665 8.79 71.7835 715.96 21787.44 5749 8.176 8.76 0.32 57.49 8.1515 8.76 71.4075 713.78 22501.23 5750 8.16 8.77 0.33 57.50 8.1356 8.77 71.3492 711.34 23212.57

6315 1.744 1.82 5.98 63.15 1.7388 1.82 3.1646 24.52 94246.53 6316 1.744 1.82 5.99 63.16 1.7388 1.82 3.1646 24.52 94271.05 6317 1.744 1.82 6 63.17 1.7388 1.82 3.1646 24.51 94295.56 6318 1.744 1.83 6.01 63.18 1.7388 1.83 3.1820 24.60 94320.16 6319 1.743 1.81 6.02 63.19 1.7378 1.81 3.1454 24.42 94344.57 6320 1.743 1.83 6.03 63.20 1.7378 1.83 3.1801 24.61 94369.18 6321 1.743 1.83 6.04 63.21 1.7378 1.83 3.1801 24.59 94393.78

Appendices - 111 -


Measured output characteristic using a relay

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6time (ms)

Out

put v

olts

(Uo)

-1012345678910

Out

put c

urre

nt (I

o)

Am

ps

Uo Io

Measured output characteristics using a relay

0102030405060708090

0 2 4 6 8time (ms)

Out

put p

ower

(Po)

W

-20000

0

20000

40000

60000

80000

100000O

utpu

t ene

rgy

(Eo)

uJ

Po Eo

Appendices - 112 -


A 4. No-load to Short-circuit Output Characteristic ORM(YOKOGAWA) Number of data 100 Trigger point 1920 Trigger time 01-06-26 15:12 Sample rate 100 kHz Io =B17/$B$11 Tag name C01 C02 Uo =C17 Unit V V Po =G17*H17 No. Eo inst =(I17+I18)*0.5*$B$10*1000000




1900 0.01 12.73 19 0.01 12.73 0.13 -0.63 1901 -0.02 12.74 19.01 -0.02 12.74 -0.25 1.26 1902 0.04 12.73 19.02 0.04 12.73 0.50 0.00 1903 -0.04 12.73 19.03 -0.04 12.73 -0.50 -4.40 1904 -0.03 12.72 19.04 -0.03 12.72 -0.38 -4.40 1905 -0.04 12.72 19.05 -0.04 12.72 -0.50 -1.26 1906 0.02 12.72 19.06 0.02 12.72 0.25 -0.63 1907 -0.03 12.72 19.07 -0.03 12.72 -0.38 -3.77 1908 -0.03 12.72 19.08 -0.03 12.72 -0.38 -3.77 1909 -0.03 12.73 19.09 -0.03 12.73 -0.38 -3.77 1910 -0.03 12.72 19.1 -0.03 12.72 -0.38 -3.14 1911 -0.02 12.73 19.11 -0.02 12.73 -0.25 -2.52 1912 -0.02 12.73 19.12 -0.02 12.73 -0.25 -2.51 1913 -0.02 12.72 19.13 -0.02 12.72 -0.25 -2.51 1914 -0.02 12.72 19.14 -0.02 12.72 -0.25 0.00 1915 0.02 12.72 19.15 0.02 12.72 0.25 -0.63 1916 -0.03 12.72 19.16 -0.03 12.72 -0.38 -0.63 1917 0.02 12.72 19.17 0.02 12.72 0.25 -0.63 1918 -0.03 12.72 19.18 -0.03 12.72 -0.38 -0.63 1919 0.02 12.73 0 19.19 0.02 12.73 0.25 106.71 106.71 1920 1.96 10.89 0.01 19.2 1.94 10.89 21.09 472.82 579.53 1921 7.91 9.4 0.02 19.21 7.82 9.4 73.47 455.95 1035.49 1922 4.17 4.3 0.03 19.22 4.12 4.3 17.72 95.06 1130.55 1923 1.1 1.19 0.04 19.23 1.09 1.19 1.29 11.80 1142.34 1924 0.99 1.09 0.05 19.24 0.98 1.09 1.07 10.61 1152.96 1925 0.99 1.08 0.06 19.25 0.98 1.08 1.06 10.89 1163.84 1926 1.05 1.08 0.07 19.26 1.04 1.08 1.12 11.31 1175.16 1927 1.06 1.09 0.08 19.27 1.05 1.09 1.14 10.94 1186.10 1928 0.99 1.07 0.09 19.28 0.98 1.07 1.05 10.78 1196.87 1929 1.02 1.1 0.1 19.29 1.01 1.1 1.11 11.04 1207.92

Appendices - 113 -


A 5. Ignition Curves for ‘well defined’ Circuits

Resistive circuits [24]

halla

This image is not available online. Please consult the hardcopy thesis available from the QUT Library

Appendices - 114 -


Group I capacitive circuits [24]

halla


Appendices - 115 -


Group I inductive circuits [24]

halla


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