Disclosure to Promote the Right To Information

Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public.

इंटरनेट मानक

“!ान $ एक न' भारत का +नम-ण”Satyanarayan Gangaram Pitroda

“Invent a New India Using Knowledge”

“प0रा1 को छोड न' 5 तरफ”Jawaharlal Nehru

“Step Out From the Old to the New”

“जान1 का अ+धकार, जी1 का अ+धकार”Mazdoor Kisan Shakti Sangathan

“The Right to Information, The Right to Live”

“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता है”Bhartṛhari—Nītiśatakam

“Knowledge is such a treasure which cannot be stolen”

“Invent a New India Using Knowledge”

है”ह”ह

IS/IEC 61241-2-3 (1994): Electrical Apparatus for use inthe presence of combustible dust, Part 2: Test Methods ;Section 3 Method for determining minimum ignition engergyof dust/air mixtures [ETD 22: Electrical Apparatus forExplosive Atmosphere]

IS/lEe 61241-2-3: 1994

1WT 2 q-fi~ q~Rltfi

~ 3 ~/~ ftr~ <tT ~·f(llf g~(1Ff \3\i1i ~ m <tT~

Indian Standard

ELECTRICAL APPARATUS FOR USE IN THEPRESENCE OF COMBUSTIBLE DUST

PART 2 TEST METHODS

Section 3 Method for Determining Minimum Ignition Engergy of Dust/Air Mixtures

ICS 2926020

© BIS 2009

BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

April 2009 Price Group7

Electr ical Apparatus for Explosive Atmospheres Sectional Committee, ETD 22

NATIONAL FOREWORD

This Indian Standard (Part 2/Sec 3) which is identical with IEC 61241 -2-3 : 1994 'Electrical apparatus for usein ths presence of combustible dust - Part 2: Test methods - Sect ion 3: Method for determining minimumIgnition energy of dust/air mixtures ' Issued by the International Electrotechnical Commission (IEC) wasadopted by the Bureau of Indian Standards on the recommendation of the Electrical Apparatus for ExplosiveAtmospheres Sect ional Committee and approval of the Electrotechnical Division Council.

The text of IEC Standard has been approved as suitable for publicat ion as an Indian Standard withoutdeviations. Certain conventions are, however, not identical to those used in Indian Standards. Attention isparticularly drawn to the following:

a) Wherever the words 'International Standard ' appear referring to this standard, they should be read as'Indian Standard' .

b) Comma (,) has been used as a decimal marker in the International Standard while in Indian Standards,the current practice is to use a POint (.) as the decimal marker.

In this adopted standard, references appear to certain International Standards for wh ich Indian Standardsalso exist. The corresponding Indian Standards, which are to be substituted in their respective places arelisted below along with their degree of equ ivalence for the editions indicated:

International Standard

IEC 50 (301 ) : 1983 Int ernationalElectrotechnical VocabularyChapter 301 : General terms onmeasurements In electr icity

IEC 50 (302) : 1983 InternationalEtactrote chrucal Vocabulary ­Chapter 302 : Electrical measuringInstruments

Corresponding Indian Standard

IS 1885 (Part 80 ) : 1994 Electrotechnical vocabulary: Part 80 Generalterms on measurements in elect ricity

IS 1885 (Par t 81 ) : 1993 Electrotechnical vocabulary : Part 81 Electricalmeasunng instruments

Degree of Equivalence

Iden tical

do

The technical committee has reviewed the provrsions of the following International Standards refer red in thisadopted standa rd and has decid ed that they are acceptable for use in conjunct ion with this standard:

Internationa l Standard

IEC 50 (303) : 1983

ISO 4225 : 1980

Title

International Electrctechrucal Vocabulary - Chapter 303: Electronic measuringinstrumen ts

Air qua lity - Gene ral aspects - Vocabulary

Only the Engllstl language text has been retained while adopting it as an Indian Standard, and as such thepage numbers given here are not the same as In the IEC Publ icat ion.

For the purpose of deciding wheth er a par ticular requirement of this standard is complied with, the finalvalue. observed or calculated expressmq the result of a test. shall be rounded off in accordance withIS 2 : 1960 'Rules for roun dinq off numencal values (revised)' . The number of sign ificant places retained inthe rounded off value should be the same as that of the specified value in this standard,

isnsc 61241-2-3: 1994

Indian Standard

ELECTRICAL APPARATUS FOR USE IN THEPRESENCE OF COMBUSTIBLE DUST

PART 2 TEST METHODS

Section 3 Method for Determining Minimum Ignition Engergy of Dust/Air Mixtures

Scope

This section of IEC 1241·2 specifies a method of test to determine the minimum ignitioneuerqy of a dust/air mixture by an electrically generated high-voltage d .c . spark. This testmethod is intended to develop data to be used in deciding whether or not combustibledust/air mixtures are considered to be ignitable with respect to electrical discharge. It isintended that the dust be tested in a form (particle size, moisture content, etc .) represent­ing cond itions of actual use so that assessment of the hazard present can be madeIgnition energies determined by this method would be compared with ignit ion energies ofother dusts to assess the relat ive hazard with regard to ignition by an electrical or electro ­static discharge. thereby permitting decisions to be made on the suitability of electricalapparatus for installation in areas where combustible dust is present

The test method is not suitable for use with recognized explosives , gunpowder. dynamite.explosives which do not require oxygen for combustion: pyrophoric substances. orsubstances or mixtures of substances which may under some circumstances behave in asimilar manner Where any doubt exists about the existence of a hazard due to explosiveproperties . an indication may be obtained by placing a very small quant ity of the dust inquestion on the heated surtace of the apparatus described in section 1 of the IEC 1241-2·1 ,heated to 400 DC.

NOTE - Precautions sho uld be taken to sa feguard the health 01 personnel conducting the tests againstthe nsk 01 lire . explos ion and /or the l'trects. inc lud ing tOXIC effects. 01 combustion . Compliance with this mterna tional standard does not Itself conler immunity from legal obligations .

Annex B of this section includes guidance on the significance of minimum ignition energywith respect to electrostatic discharges .

2 Normative references

The follow ing normative documents contain provisions which , through reference in thistext. constitute provisions of this sect ion of IEC 1241-2 . At the time of publication,the editions indicated were valid . All normative documents are subject to revision, and part iesto agreements based on this section of IEC 1241-2 are encouraged to investigate theposs ibility of applying the most recent editions of the normative documents indicatedbelow . Members of IEC and ISO maintain registers of currently valid International Standards.

iec 50(301. 302 , 303) : 1983, International Electrotechmcal Vocabulary

- Chapter 301 : General terms on measurements in electricity

- Chapter 302 : Electncal measuring instruments

- Chapter 303 : Electronic measuring instruments

ISO 4225 : 1980. Air Quality - General aspects - Vocabulary

1-40 BISINDl09

ISIIEC 61241-2-3: 1994

3 Dellnltlons

For the purposes of th is section of IEC 1241 -2. the def initions in IEC 50{301. 302 .303)and also the following apply.

3 1 dust : Small solid particles that settle out under their own weight but that may remainsuspended in air for some time .

NOTE - Thl5 defIni tion Include5 what are defined In ISO 4225 85 "dust" and "gril" .

3.2 combustible dust: Dust that is ignitable in mixtures with air.

NOTES

M"lure5 01 combu5ltble du st ,n a'r are .gnllable only between certain concentration limits.

2 Combusnble dusts arfl capable 01 being ,gnl ted by exlernal ignition sources and will continue to burn atatmospheric lemperatures but they WIll only spontaneously Ign ilP above their min imum ignition temperatures .

3.3 spark oiscnarqe : Transient electric discharge which takes place between twoconductors wh ich are at different potentials A spark is a discrete discharge that br idgesthe gap between the conductors in the form 01 a single ionization channel.

3.4 minimum Ignition energy (of a combustible dust/air mixture) : Lowest energy ofspark (as measured by the procedure in th is standard) that is capable of igniting the mostsensi tive dust/air mi xture with sustained combustion .

35 Ignition . In the te st . Igntllon IS considered to have occurred when :

a pressure nse 01 at least 0.2 bar above any pressure introduced by th e ignit ingspark is measured on a closed vessel (e g 20 I sphere) ; or

a name which propagates at least 6 cm away from the spark position is observed inan open tube (e g Hartmann tube) .

3 6 Ignition delay time ' Time between dispersion of the dus t and the occurrence 01 th espark d ischarge

4 Test apparatus

4 1 Sp ark qe netation cvcun

Ann ex A describes some suuable lorms 01 circuit . all of which shall have the followingcharacteristics What follows deals only With the circu it :

inductance of d ischarge circuit : 1 mH to 2 mH except when the data is to be usedfor the assessment of electrostatic hazards when the inductance of the dischargecircuit shall not exc eed 25 IlH ;

ohmic resi stance of discharge circuit : as low as possible and not more than 512 ;

electrode material stainless steel . brass. copper or tungsten ;

electrode diameter and shape. 2.0 mm ± 0.5 mm. Electrodes with rounded tips canbe used to reduce corona effects that can occur with pointed electrodes . and Whichmay give mcorrect values of spark ene rgy. II pointed electrodes are used. coronaeffects should be carefully cons idered ;

2

ISIIEC 61241-2-3: 1994

- electrode gap : 6 mm (minimum) ;

- capacitors : low-inductance type, resistant to surge current;

- capacitance of electrode arrangement: as low as possible ;

- insulation resistance between electrodes : sufficiently high to prevent leakagecurrents.

NOTE - Typ ically, a minimum res islance between lhe electrodes 01 10'2 U is required lor a minimumign ition energy of 1 mJ . and 10'0 a for a minimum ignition energy 01100 mJ .

4.2 Test vessel

The recommended vessels are the 20 I sphere apparatus and the Hartmann tube . Thesevessels are described in references 16] and (7)". Other vessels can be used, provided thatthe calibration requirements in 6.2 are met.

5 Test sample

Tests shall be performed on samples in a state of preparation corresponding to that foundin practice under plant conditions.

Minimum ignition energy decreases with decreasing particle size . Tests shall be carriedout on samples having particle sizes that are consistent with, or finer than, the finestmaterial that can be present in the intended use.

For comparative tests the samples shall be prepared by a constant method with the objectof fixing particle size distribution and moisture content.

NOTE - Where the panicle sizes 01 !he malerial are nol known. lesls should be carried out on particle sizesless !han 63 Ilf'l.

Minimum ignition energy decreases with decreasing particle size . It should be checkedthat the particle sizes of the sample are representative of the finest material that canbe present in the plant. Tests should be carried out on material of particle size lessthan 63 11m.

6 Procedure

6.1 Briefdescnpuon

The combustible dust to be tested is uniformly dispersed in air at atmospheric pressureand temperature in a suitable apparatus, and the dust/air mixture is subjected to a sparkdischarge from a charged capacitor.

The energy value of the discharge is calculated from the formula :

W =0,5 C x I.F

where

Wis the stored energy in joules (J);

C is the total discharge capacitance , in farads (F) ;U is the voltage of the charged capacitor in volts (V).

, Figu res In square brackets reler 10 the bIbliography , annex C

3

ISIIEe 61241-2-3: 1994

NOTES

1 At spark energies above 100 mJ, the spark res istance can become 10 small that the circuit resistance isno Iong.r Mgligible compared with !tie spark res istance. particularly when the circuit contains an inductancecoil of the order of 1 mH oIn such ca..s the net spark energy can be obtained from the equation :

w. f/(l) U(I)dl

where

/(1) IS the spark current . and

U(I) the spark voltage, both 01 whi ch are obtained by measurement.

2 Further mtormation retanve to Ihe calculation ot spark energies is contained in annex A .

It is necessary to take account 01 the lollowing possible influences on the test :

dust/air mixture aerodynamics (e.g. Ignition delay time, dispersing pressure) ;

dust concentration:

voltage to which the capacitor is charged ;

capacitance ot the capacitor;

inductance 01 the discharge circuit;

ohmic resistance ot the discharge circuit;

materials and dimensions of the electrodes and the gap between the electrodes .

To limit the expense ot testing. every apparatus uses electrodes composed ot a specificmaterial with standardized dimensions and minimum electrode gap. The ohmic resistanceot the discharge circuit shall be kept as low as poss ible (see clause 4) .

Starting with a spark energy that will reliably cause ignition 01 the dust being tested . thedust concentration and dust dispersion parameters (e.g . ignit ion delay time and dispersionpressure) are adjusted to establish the most ignitable dust cloud. Using the optimalcond itions tor ignition, the spark energy is successively halved. by adjusting the capa­citance ot the capacitor and/or the voltage to which it is charged. until no ignition occursin 20 successive tests.

NOTE - Whpn tests are earned oul uSing the 20 I sphere apparatus, the ignition delay lime should be 120 ms

The minimum ignit ion energy. Wm,n' lies between the highest energy. W" at which ignitionfails to occur in 20 successive attempts to ignite the dust/air mixture. and the lowestenergy , W2 , at which ignition occurs within 20 successive attempts .

4

IS/lEe 61241-2-3: 1994

62 Calibration

Calibration tests should be carried oul on Ihree reterence dusts which shall have beendried under atmospheric pressure at 50 "C tor 24 h prior to the measurements .

The results shall be within the following ranges :

lycopodium:

anthraquinone :

poly acrylonitrile :

Wm,n '" 5 to 15 mJ. mean particle diameter 31 urn:

Wm,n =2 to 6 mJ. mean particle diameter 18 J.1m ;

Wm IO

= 2 to 6 mJ, mean particle diameter 27 J.1m .

The dust dispersion parameters. including ignition delay time, for each sample shall benoted .

6.3 Test report

Where the test has been carried out in accordance with this standard. the test report shallprovide the information listed in 6.3.1 , 6.3.2 and 63.3 . Although the dust concentrationvalues associated with the limits of the range of minimum ignition energy should berecorded by the test laboratory, the values, expressed in terms of the amount of dust whichis weighed , divided by the volume of the explosion vessel, are not usually included in thetest report .

6.3.1 Product characterist ics

sample des ignat ion (name and chemical description if not implicit in the name) ;

sample origin or source ;

sample pre -treatment ;

characteristics data for part icle size distribution and moisture content if ava ilableand not already given by pre-treatment procedures.

63.2 Characteristics of the test apparatus

- tr iggering :

explos ion vessel ;

- dust-dispersion system:

- total inductance of the discharge circuit ;

charging voltage . electrode material and length of gap of the optimized dischargecircuit.

63 .3 Results

highest energy W1 at wh ich ignition does not occur ;

lowest energy W2

at which ign ition is obtained .

63 4 Report form

An example 01 a suitable form is given in figure A.1.

5

ISIIEC 61241-2-3: 1994

Annex A(informative)

Examples of spark-generating systems

A.l General

Clauses A.2. A.3. A.4 and A.S contain descriptions of four designs of spark-generatingcircu it suitable for use in this test. With any of these examples it is possible to use differentexplosion vessels . prov ided that the dust dispersion is optimized and that suitable pre­cautions are taken 10 order to prevent side -effects occurring in comparatively large vesselsfrom electrostatic charging phenomena during the dispers ion of the dust. These phenomenainclude additional charging/discharging of the capacitor .

If the storage capacitor is decoupled from the electrode during the charging process. theeffect of the decrease in voltage that will occur due to the increase in capacitance whenthe connection to the electrode is made. should be taken into account in calculating thespark energy . In all calculations of energy the total capacitance of the discharge circuitshould be used . and the voltage at the time of discharge .

A.2 Triggering by auxiliary spark using tnree-etectroce system

Figure A.2 illustrates the general arrangement of the test apparatus .

The essential component IS a three -electrode spark gap. The two electrodes forming themain spar1< gap (1) are 3.2 mm in diameter. their ends being reduced to a diameter of2.0 mm over a length of 20 mm. The free end of the auxiliary electrode (2) is angledtoward the main spark gap. the length of this angled portion being 20 mm. This electrodearrangement is installed in an open-top Hartmann tube and is also suitable for installationin other explosion vessels .

Follow ing the introduction into the mixture-generating device of the desired quantity ofdust. the tube is placed in position . The test capacitor C (20 pF to 10 000 pF) , whichstores the ignition energy. is charged by means of the high-voltage charging unit HVCUacross the charging resistance R which limits the charging current to 1 mAo The attemptsto ignite the dust/air mixture are initiated by means of the control facility CF . Initiation ofeach attempt involves . first of all . tr iggering the device which disperses the dust intosuspension. followed. after a predetermined interval. by the auxiliary spark and, with it. thetnggering of the main spark discharge by the test capacitor.

The energy of the aux iliary circu it is limited to not more than one-tenth of the energy of themain discharging circu it.

6

ISIIEC 61241-2-3: 1994

A.3 Triggering by electrode movement

Figure A3 illustrates the general arrangement 01 the test apparatus

PTFE stoppers (2) are fitted into the two electrode mounting ports In an open-topHartmann tube (1). These stoppers are bored in order to receive the electrodes (3) In amanner permitting them to be moved One of the electrodes, which is at earth potential,is attached to the measuring rod of a micrometer screw (4). The yoke of the micrometeris shortened, and is fastened to the modified Hartmann tube The other electrode, to whichthe high voltage is applied, is attached to the pushrod of a controllable, double-actingpneumatic piston (6) (piston nominal diameter: 35 mm; operating pressure: 600 kPa)which has a working travel of 10 mm, attachment being through a PFTE insulatingpiece (5). The high-voltage electrode is electrically connected to a capacitor (7) with avalue between 26 pF and 311 J..lF. The voltage to which this capacitor is charged IS

indicated by means of an electrostatic voltmeter (8) After disconnecting the high voltagegenerator (9) from the capacitor circuit, air is released electro-pneumatically fromthe pressure vessel (10) in which it is stored under pressure in such a way as to forma dust/air mixture by dispersing the dust in suspension After a delay, which is set withthe aid of a timing device (11). the high-voltage electrode is shot into the position definingthe spark-gap length. the energy stored in the capacitor then being liberated at thespark gap.

A.4 Triggering by voltage Increase (trickle-chargIng circuit)

Figure A.4 illustrates the general arrangement of the test apparatus.

The trickle-charging circuit is one of the simplest methods for producing sparks 01 knownenergy which are required for determining the minimum ignition energy of dust/airmixtures.

A high-voltage d.c. supply slowly raises the potential of the capacitor until a spark occursThe cycle is then repeated, giving a series 01 sparks. each of the same energy A currentlimiting resistor with a value between 10811 and 109 n is included in the circuit The potential across the capacitor is measured by an electrostatic voltmeter with a decoupllngresistor in series having a value between 108 nand 109 n Sparks 01 any energy levelIrom 1 mJ upwards can be readily produced using this circuit by varying the value of thecapacitor and. if necessary, the discharge voltage.

The settings for sparks of the required energy are determined before any dust IS placed In

the ignition chamber A capacitor of the appropriate value is chosen and a voltage in therange 10 kV to 30 kV selected. The voltage and electrode separation are then adjusted bytrial until sparks of the required energy. given by 0,5 CU2 occur at the electrodes In thisexpression, U is the voltage at which the spark occurs and C is the total capacitance atthe high-voltage electrode, which can be measured using normal ac. bridge methodsIn order to make an ignition test. the high-voltage electrode is earthed and the requiredquantity of the prepared dust is placed in the dispersion cup. The d.c. voltage supplyis then switched into the circutt, and, as sparks start to pass between the electrodes. Ihedust is dispersed by an air jet. It is noted whether ignition occurs and whether the flamepropagates away from the spark gap

The first tests are usually pertormed with a high-spark energy typically 500 mJ. If there isan ignition. the spark energy is then reduced in steps. and the test repeated until ignitiondoes not occur. as described in 6.1

7

ISIIEC 61241-2-3: 1994

A.S Triggering by auxiliary spark, using normal two-electrode system(trigger transformer In discharge circuit)

Figure A.S illustrates the general arrangement of the test apparatus .

This circu it cannot be used for tests without inductance. C is the discharge capacitor,having an initial voltage 01 U By having a range of capacitors Irom 40 pF and downwardsin steps 01 a lactor 01 10. and variable voltage Irom 1 000 V downwards (400 V to 500 V isa practical minimum level) , a wide select ion of 0,5 cll values is available. Initiation ofspark discharge at the desired moment, which is essential if synchronization of sparkdischarge with the format ion of a transient dust cloud is requ ired , is accomplished bymeans of the trigger circu it in which the capacitor CTr• a switch S, and the primary coil ofthe trigger transformer T constitute the essential elements . By closing the switch , a high ­voltage pulse of approx imately 15 kV peak value is induced in the secondary coil of thetransformer. causing breakdown of the spark gap G. and thereby discharge of the maincapac itor C Experience has shown that It is very diff icult to reduce the energy input to thespark gap by the trigger spark below 2 mJ to 5 mJ. For this reason, this trigger pr inc iple isonly applicable to spark energ ies above 5 mJ

The net spark energies generated for vanous comb inat ions of C and U are determinedin the convent ional way by measuring current and voltage at the spark gap as funct ionsof time and integrating the power-versus -time curve . The function of the diode D is toproduce urudirectional discharges only . The selt -induc tance of the secondary coil of thetrigger transformer should be 1 mH to 2 mH o

8

ISIIEC 61241-2-3: 1994

Min imum ignition energy of a du~lIa" mixlure measured according to lEe 1241 2 ·3

1) Product characteristics

sample designatIon :

sample o(lglO or source

sample pra -tre atrnant

particle size information .

(e .g median , range . maximum)

mo isture content

2) Tes' apparatus

tr igger ing .

sxplosron vessel

dust d ispersion system

tota l inductance '

electrode gap .

charg ing voltaqe

Result

<Wnll'~

Date Signature :

'----------------------------- ---- --- -- - - -'

Figure A.l - Specimen report form

9

IS/lEe 61241·2·3: 1994

(1 )

M

MV

Compressed airreservoir

l.

...-----tt-----.C

+-- Compressed air

AC ~ aUXIliary crrcuuCF : controt facIlityM • manometer

MV = magnetrc valveV = shut oil valve

C = test capacitorHVCU = charging unit

L • InductanceR • charging resistance

Figure A.2 - Apparatus for determining the minimum Ignition energiesof dust (schematic) - Triggering by auxiliary spark,using three-electrode system

10

~moen...N~...I

NI

Co)

I,

I

1( 10 )

( 1)

(3 1

~

(1 1) \

I V

~

(9\

, 1) , op", ,, -top HiI' tmil nn tub",(2) • PTFE stoppsrs(3) • electrodes(4) • microme ter scre w(5 ) • PTFE Insulati ng p,eeR(6) • double -act ing pneuma tic pist on(7) • capacuor(8) • eleclroslatlc volt meter(9) • high -voltage generato ' (5 to 10 KV )

(' 0) • prll5sure vessel( 11) • timing dsvice

~

~

Figure A.3 - Apparatus for determining the minimum Ignition energies of dust (schematic) ­Triggering by electrode movement

...i

C", re nt ·IH"!''lItlng

res lSl or J19n :tl::ln

c r-arn osr

L

~moO'l~

~~~

I~I

W

~

lD

~

o---J\/\/' . . c

r\)DC

voltagesource

c ao acuo-_I-,-­

I

DeCO"P ! l ngU~reS·SIOr

Elect ro st at rc

vo ttrna ts r

Ind uctance, mH

Figure A.4 - Apparatus for determining the minimum Ignition energies of dust (schematic) ­triggering by voltage Increase

ISIIEC 61241·2·3: 1994

v

o

e

G

l__----'c ~ main capacitoreTr • capacitor in trigger circuit

0: diodeS" switchT " transformerG " spark gap

s

Figure A.5 - Apparatus for determining the minimum Ignition energies fordust (schematic) - Trigger transformer In discharge ctrctt

13

IS/lEe 61241-2-3: 1994

Annex B(informative)

Significance of minimum ignition energy

Although it is not within the scope of this standard to specify limits or safety precautionsas functions of the measured minimum ignition energies. it is appropriate to give someguidance on the significance of the measured values in relation to the safety of industrialinstallations.

Several discharge types need to be considered . These are:

- glow corona

brush

cone

propagating brush

spark

from pointed or small radius conductors

from bulk powder or insulating solids

from highly insulating granules

from polarized insulating surfaces

Detailed descriptions of these discharges. and their occurrence can be found in theliterature (11) . Their ignition capabilities are indicated in the table below in relation tominimum ignition energies measured by the present method.

Table 8.1 - Ignition capabilities of various types ofelectrostatic discharge

Minimum ignitionDischarge rype Ign ition capabJliry energy

mJ

Corona Ignition 0' dust clouds olher Ihan explosives Up 100.1not likely

BrushHyb rid mixtures and some very sensitive dusts

Up to 3may be Ignited

Cone Most combustible dusts may be igni ted Increase with volume01 material

Propagating Most combustible dusts may be ignited Typically up to severaljoules

Sparks Any combustible dusl may be ign ited No limit

--

Sparks are the most potent type of electrostatic discharge. and are capable of igniting awide range Of flammable mixtures . The basic precaut ion against the occurrence of incendivespark discharges is to ensure that all conducting parts of the plant and equipment andconductive products are securely earthed .

14

ISIIEC 61241-2-3: 1994

In order to give a wide margin of safety, and to allow for variations of resistance over aperiod of time, a resistance to earth of less than 108 n is advocated for conducting items.

The maximum safe electrical resistance (R) between a conducting part and earth can bedeterm ined from the maximum electrostatic charging current (I) In the system.

It is well established that spark discharges that occur at 100 V or less do not cause ignitionprimarily because the gap across which the spark passes is much smaller than the quenchingdistance.

Using Ohm's law and a maximum voltage of 100 V, the maximum safe resistance isgiven by:

The maximum charg ing current for most types of operations is 10-6 A so that the maxi ­mum safe resistance to earth in these cases is 1 X 108 n . In very energetic operations,charg ing currents may reach 10-4 A. In such instances the maximum safe resistancewould be 1 x 106 n.

In cases where proper earthing cannot be achieved, the minimum ignit ion energy can beused to determine whether earthing is essential. From knowledge of the charging rate,the electrical capacitance, duration of operation and leakage resistance, it is possible tocalculate the maximum amount of energy that can be stored . This can then be comparedwith the lowest min imum ignition energy from the materials handled in the plant. Alterna ­tively, if the maximum potential that can occur is limited by a fixed , narrow gap acrosswhich any spark will pass (for example, in some rotating machinery such as rotary valves)similar calculations can be made and compared with the measured minimum ignitionenergy . In all these cases it is essential that the lowest minimum ignit ion energy from thematerials being handled or processed isused in the comparisons.

The assessment and control of the other types of discharge requ ire expert knowledge, butgeneral guidance is available in the literature [1 t] .

Spark discharges from charged, unearthed metal parts of plant and equipment arecapacitative. To assess the ignition hazard in the case of such sparks, minimum ignitionenergy values can be determined by using a simple , plain capacitative discharge circuit .In some cases , the repeatability and reproducibility of results from such circuits can beimproved by the inclusion of an inductance of 1 mH in the discharge circuit. It should benoted, however, that this modification normally produces more incendive sparks than aplain capac itat ive discharge circuit and, therefore, lower minimum ignition energy values.This can lead to the adoption of precautions that are not strictly necessary, with theattendant costs .

15

IS/lEe 61241-2-3: 1994

Exper imental investigations on a great number of dusts. using Ign ition sources other thanelectric sparks . ind icate that the ranking order of the ignitabilities of dust clouds, obtainedby using the present test method for minimum ign ition energy, is also valid for other typesof ignition sources . Differences in the characterist ics of energy release are responsible forthe differences in the total amou nt of energy requ ired for ign it ing a given mixture byvarious ign it ion sources . A well -known examp le of the problem of drawing conclus ionsbased only on a comparison of total energies is the attempt to assess the incendivity ofbrush discharges in the case of dust clouds . Prev ious experiments have shown that brushdischarqes can ign ite explosible gas mixtures of minimum spark ignition energies of 4 mJ.However . so far it has not been possible to demonstrate that dust clouds of considerablylower minimum electr ic spark ignit ion energ ies than 4 mJ can be ignited by brushdischarges . One reason for this could be that in the case of brush discharges the dis­charge time is different

Dete rmination of absolute energies required lor igniti ng dust clouds is possible as far asignition by electric sparks is concerned ; provided the measurement method satisfies therequ irements stated earlier . In princ iple, the spatial and temporal distribution of the energyin the discharge constitutes the basic characteristic of the incendiv ity of any discharge .However . an equ iva lent energy can be ascr ibed to a discharge by equating it to the energy01 a spark discharge that gives the same incendivity as the discharge being considered .

The problem of determin ing equ ivalent energ ies lor mechanical sparks is discussedelsewhere [1 J and [2)

The considerat ions above are only applicable to combust ible dusts . When it comes to theincendiv uy of spark discharges in mixtures of a dust cloud and an explosible gas . the tech­meat literature should be consulted {31. (4), (5), (6) In case of doubt. the value for the gasalone should be used

16

GMGIP~ BtSiNDt09-JOO

Bureau of Indian Standards

BIS is a sta tutory institution established under the Bureau of Indian Standards Act, 1986 to promoteharmonious development of the activ it ies of standardization, marking and qual ity certif icat ion ofgoods and attending to connected matters in the country.

Copyright

BIS has the copy right of all its publicat ions. No part of the these publ icat ions may be reproduced inany form without the prior permiss ion in wr iting of BIS . This does not preclude the free use, in thecourse of implementing the standard, of necessary details, such as symbols and sizes, type or gradedesignations . Enquiries relating to copyright be addressed to the Director (Publications) , BIS.

Review of Indian Standards

Amendments are issued to standards as the need arises on the basis of comments. Standards arealso reviewed periodically ; a standard alongwith am end ments is reaff irmed when such review indicatesthat no changes are needed ; if the review indicates that changes are needed , it is taken up for rev ision.Users of Indian Standards should ascertain that they are in possession of the latest amendments oredition by referring to the latest issue of 'BIS Catalogue' and 'Standards: Monthly Additions '.

Th is Indian Standard has been developed from Doc: No . ETD 22 (5902) .

Amendments Issued Since Publication

Amend No . Date of Issue

BUREAU OF INDIAN STANDARDS

Text Affected

Headquarters :

Manak Bhavan , 9 Bahadur Shah Za lar Marg, New Delh i 110 002Telephones : 2323 0131 , 2323 3375. 2323 9402 Website: www.bis .org.in

Regional Offices :

Central Manak Bhavan, 9 Bahadur Shah Zafar MargNEW DELHI 110002

Telephones

1 2323 7617123233841

AHMEDABAD.BANGALORE.BHOPAL. BHUBANESHWAR.COIMBATORE. FARIDABAD.GHAZIABAD. GUWAHATI. HYDERABAD. JAIPUR. KANPUR. LUCKNOW. NAGPUR.PARWANOO. PATNA. PUNE. RAJKOT. THIRUVANANTHAPURAM .VISAKHAPATNAM.

Eastern

Northern

Southern

Western

Branches

1/14 C.I.T. Scheme VII M, V.I.P. Road . KankurgachiKOLKATA 70005 4

SCO 335-336, Sector 34-A , CHANDIGARH 160022

C.I.T. Campus. IV Cross Road . CHENNAI 600113

Manaka laya. E9 MIDC . Maret . Andhen (East)MUMBAI 400093

I 23378499,23378561! 2337 8626 , 2337 9120

j 26038431 260 9285

i 2254 1216, 22541442~ 22542519.22542315

" 2832 9295, 2832 7858I 2832 789 1, 2832 7892

,.o1IJ(j;Zco..,

PRINTED BY THE GENERAL MANAGER, GOVT. OF INDIA PRESS, NASHIK-422 006