iec 60079-25 intrinsically safe systems

8/13/2019 Iec 60079-25 Intrinsically Safe Systems

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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.

!"#$% '(%)

!"# $ %& #' (")* &" +#,-.Satyanarayan Gangaram Pitroda

Invent a New India Using Knowledge

/0)"1 &2 324 #' 5 *)6Jawaharlal Nehru

Step Out From the Old to the New

7"#1&"8+9&"), 7:1&"8+9&")Mazdoor Kisan Shakti Sangathan

The Right to Information, The Right to Live

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lS/lEC 60079-25:2003

ftwiwii a5rt@a-mftamdmJTIndian Standard

ELECTRICAL APPARATUS FOR EXPLOSIVE GASATMOSPHERESPART 25 INTRINSICALLY SAFE SYSTEMS

ICS 29.260.20

@ BIS 2007BUREAU OF INDIAN STANDARDS


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Electrical Apparatus for Explosive Atmospheres Sectional Committee, ETD 22

NATIONAL FOREWORD

This Indian Standard (Part 25) which is identical with IEC 60079-25:2003 Electrical apparatus forexplosive gas atmospheres Part 25: Intrinsically safe systems issued by the InternationalElectrotechnical Commission (lEC) was adopted by Bureau of Indian Standards on therecommendation of Electrical Apparatus for Explosive Atmospheres Sectional Committee andapproval of the Electrotechnical Division Council.The text of IEC Standard has been approved as suitable for publication as an Indian Standard withoutdeviations. Certain conventions are, however, not identical to those used, in Indian Standards.Attention is particularly drawn to the following:

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

b) Comma (,) has been used as a decimal marker, while in Indian Standards, the currentpractice is to use a point (.) as the decimal marker.

In this adopted standard, references appear to certain International Standards for which IndianStandards also exist. The corresponding Indian Standards, which are to be substituted in theirrespective places are listed below along with their degree of equivalence for the editions indicated:

International Standard

IEC 60060-1 : 1989 High-voltage testtechniques Part 1: Generaldefinitions and test requirementsIEC 60079-0 : 2004 Electricalapparatus for explosive gasatmospheres Part O: GeneralrequirementsIEC 60079-11 : 1999 Electricalapparatus for explosive gasatmospheres Part 11: Intrinsicsafety i

Corresponding Indian Standard Degree ofEquivalenceIS 2071 (Part 1) : 1993 High-voltage test Identicaltechniques: Part 1 General definitions andtest requirements second revisionlS/lEC 60079-0 : 2004 Electrical doapparatus for explosive gas atmospheres:Part O General requirements

IS 5780 : 2002 Electrical apparatus for doexplosive gas atmospheres - Intrinsicsafety i Specification second revision

The technical committee has reviewed the provisions of the following International Standard referredin this adopted standard and has decided that it is acceptable for use in conjunction with thisstandard:hternational Standard Title[EC 60079-14 (2002) Electrical apparatus for explosive gas atmospheres Part 14: Electrical

installations in hazardous area (other than mines)Only the English language text has been retained while adopting it in this Indian Standard and assuch the page numbers given here are not the same as in the International Standard.For the purpose of deciding whether a particular requirement of this standard is complied with, thefinal value, observed or calculated expressing the result of a test, shall be rounded off in accordancewith IS 2 : 1960 Rules for rounding off numerical values revised . The number of significant placesretained in the rounded off value should be the same as that of the specified value in this standard.


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lS/lEC 60079-25:2003Indian Standard

ELECTRICAL APPARATUS FOR EXPLOSIVE GASATMOSPHERESPART 25 INTRINSICALLY SAFE SYSTEMS

1 Scope1.1 This part of IEC 60079 contains the specific requirements for construction andassessment of intrinsically safe electrical systems, type of protection i, intended for use, asa whole or in part, in explosive atmospheres in Group II locations. This standard ia intendedfor use by the designer of the system who may be a manufacturer, a specialist consultant or amember of the end-users staff.

1.2 This standard supplements IEC 60079-11, the requirements of which apply to electricalapparatus used in intrinsically safe electrical systems.1.3 The installation requirements of a Group II system designed in accordance with thisstandard are specified in IEC 60079-14.

2 Normative referencesThe following referenced documents are indispensable for the application of this, document.For dated references, only the edition cited applies. For undated references, the latest editionof the referenced document (including any amendments) applies.IEC 60060-1, High-voltage test techniques Part 1: General definitions and test requirementsIEC 60079-0, Electrical apparatus for explosive gas atmospheres - Part O: GeneralrequirementsIEC 60079-11:1999, Electrical apparatus for explosive gas atmospheres Part 11: Intrinsicsafety iIEC 60079-14:2002, Electrical apparatus for explosive gas atmospheres Part 14: Electricalinstallations in hazardous areas other than mines3 DefinitionsFor the purposeselectrical systems,IEC 60079-11.3.1

of this document, the following definitions, specific to intrinsically safeapply. They supplement the definitions which are given in IEC 60079-0 and

intrinsically safe electrical systemassembly of interconnected items of electrical apparatus, described in a descriptive systemdocument, in which the circuits or parts of circuits, intended to be used in an explosiveatmosphere, are intrinsically safe circuits

1


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lS/lEC 60079-25:20033.1.1certified intrinsically safe electrical systemelectrical system conforming to 3.1 for which a certificate has been issued confirming that theelectrical system complies with this standard3.1.2uncertified intrinsically safe electrical systemelectrical system conforming to 3.1 for which the knowledge of the electrical parameters of theitems of certified intrinsically safe electrical apparatus, certified associated apparatus, simpleapparatus and the knowledge of the electrical and physical parameters of the interconnectingwiring permit the unambiguous deduction that intrinsic safety is preserved3.2descriptive system documentdocument in which the items of electrical apparatus, their electrical parameters and those ofthe interconnecting wiring are specified

3.3system designerperson who is responsible for the descriptive system document, has the necessarycompetence to fulfil the task and who is err@owered to enter into the commitments on behalfof his employer3.4maximum cable capacitance (Cc)maximum capacitance of the interconnecting cable that can be connected into an intrinsicallysafe circuit without invalidating intrinsic safety3.5maximum cable inductance (Lc)maximum inductance of the interconnecting cable that can be connected into an intrinsicallysafe circuit without invalidating intrinsic safety3.6maximum cable inductance to resistance ratio (LCU?C)maximum value of the ratio inductance (.LJ to resistance (RJ of the interconnecting cable thatcan be connected into an intrinsically safe circuit without invalidating intrinsic safety3.7linear power supplypower source from which the available output current is determined by a resistor. The outputvoltage decreases linearly as the output current increases3.8non-linear power supplypower supply where the output voltage and output current have a non-linear relationshipEXAMPLE A supply with a constant voltaga output up to a constant current limit controlled by semiconductors.

4 Descriptive system documentA descriptive system document shall be created for all systems. The descriptive systemdocument shall provide an adequate analysis of the level of safety achieved by the system.

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lS/lEC 60079-25:2003Annex E comprises examples of typical diagrams, which illustrate the requirements of thedescriptive system document.The minimum requirements are as follows:a) block diagram of the system listing all the items of apparatus within the system;b) a statement of the group subdivision, temperature classification, category and ambienttemperature rating in accordance with Clauses 5, 6 and 7;c) the requirements and permitted parameters of the interconnecting wiring in accordancewith Clause 8;d) details of the earthing and bonding points of the systems in accordance with Clause 9.When surge protection devices are used, an analysis in accordance with Clause 10 shall

also be included;e) where applicable the justification of the assessment of apparatus as simple apparatus inaccordance with IEC 60079-11 shall be included. In particular, where several pieces ofsimple apparatus are included, the analysis of the summation of their parameters shall beavailable;f) a unique identification of the descriptive system document shall be created;g) the system designer shall sign and date the document.

5 Grouping and classificationIntrinsically safe electrical systems shall be placed in Group II as defined in IEC 60079-0. Thesystem as a whole or parts thereof shall be given a further subdivision of the classification asappropriate.Apparatus within a Group II intrinsically safe system, intended for use in explosiveatmospheres, shall be given a surface temperature class in accordance with IEC 60079-0 andIEC 60079-11.NOTE 1 In Group II intrinsically safe electrical systems, or parts thereof, the subdivisions A, B, C may be differentfrom those of the pati]cular intrinsically safe electrical apparatus and associated electrical apparatus included inthe system.NOTE 2 Diffewnt parts of the same intrinsically safe electrical system may have different subdivisions (A, B, C).The apparatus used may have different surface temperature classes and different ambient temperature ratings.

6 System categories6.1 GeneralEach part of an intrinsically safe electrical system intended for use in a explosive atmosphereshall be placed in a category either is or itr in accordance with IEC 60079-11. Thecomplete system need not necessarily be placed in a single category.

The descriptive system document shall specify the category of the system or, wherenecessary, the category of different parts of the system.NOTE For example, where an instrument is primarily an nibinstrument but which is designed for the connectionof an is sensor, such as a PH measuring instrument with its connected probe, the part of the system up to theinstrument is niband the sensor and its connections is.Clause 11 contains details of the required assessment.


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lS/lEC 60079-25:20036.2 Category isWhere the requirements applicable to electrical apparatus of category ia (see 5.2 ofIEC 60079-11) are satisfied by an intrinsically safe system or part of a system considered asan entity, then that system or part of a system shall be placed in category ia.6.3 Category .ibWhere the requirements applicable to electrical apparatus of category nib (see 5.3 ofIEC 60079-11) are satisfied by an intrinsically safe system or part of a system consideredas an entity, then that system or part of a system shall be placed in category nib.

7 Ambient temperature ratingWhere part or all the intrinsically safe system is specified as being suitable for operationoutside the normal operating temperature range of -20 C and +40 C, this shall be specifiedin the descriptive system document.

8 Field wiringThe electrical parameters of the interconnecting wiring upon which intrinsic safety dependsand their derivation shall be specified in the descriptive system document, Alternatively, aspecific type of cable shall be specified and the justification for its use included in thedocumentation. Where a specific type of cable is specified, then it shall comply with therelevant requirements of IEC 60079-14.Where relevant, the descriptive system document shall also specify the permissible types ofmulticore cables as specified in IEC 60079-14, which each particular circuit may utilize. In theparticular case where faults between separate circuits have not been taken into account, thena note shall be included on the block diagram of the descriptive system document stating thefollowing: where the interconnecting cable utilizes part of a multicore containing otherintrinsically safe circuits, then the multicore must be in accordance with the requirements ofa multicore type A or B as specified in IEC 60079-14.

9 Earthing and bonding of intrinsically safe systemsIn general, an intrinsically safe circuit shall either be fully floating or bonded to the referencepotential associated with the hazardous area at one point only. The level of isolation required(except at one point) is to be designed to withstand a 500 V insulation test in accordance with6.4.12 of IEC 60079-11. Where this requirement is not met, then the circuit shall beconsidered to be earthed at that point. More than one earth connection is permitted on acircuit, provided that the circuit is galvanically separated into sub-circuits, each of which hasonly one earth point.Screens shall be connected to earth or the structure in accordance with IEC 60079-14, Wherea system is intended for use in an installation where significant potential differences (greaterthan 10 V) between the structure and the circuit can occur, the preferred technique is to use acircuit galvanically isolated from external influences such as changes in ground potential atsome distance from the structure. Particular care is required where part of the system isintended to be used in a zone O location.


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lS/lEC 60079-25:2003The descriptive system document should clearly indicate which point or points of the systemare intended to be connected to the plant reference potential and any special requirements ofsuch a bond. This may be achieved by cross-references to IEC 60079-14. The point or pointsat which the intrinsically safe system is connected to the plant shall be determined inaccordance with IEC 60079-14.10 Protection against lightning and other electrical surgesWhere a risk analyais shows that an installation is particularly susceptible to lightning or othersurges, precaution shall be taken to avoid the possible hazards.If part of an intrinsically safe circuit is installed in a zone O in such a way that there is a risk ofdeveloping hazardous or damaging potential differences within the zone O, a surge protectiondevice shall be installed. Surge protection is required between each conductor of the cableincluding the screen and the structure where the conductor is not already bonded to thestructure. The surge protection device shall be installed outside but as near to the boundaryof zone O as is practicable, preferably within 1 m.Surge protection for apparatus in zones 1 and 2 shall be included in the system design forhighly susceptible locations.The surge protection device shall be capable of diverting a minimum peak discharge currentof 10 kA (8/20 ys impulse according to IEC 60060-1 for 10 operations). The connectionbetween the protection device and the local structure shall have a minimum cross-sectionalarea equivalent to 4 mm2 copper. The cable between the intrinsically safe apparatus in zone Oand the surge protection device shall be installed in such a way that it is protected fromlightning. Any surge protection device introduced into an intrinsically safe circuit shall besuitably explosion protected for its intended location.The use of surge protection devices which interconnect the circuit and the structure via non-linear devices such as gas discharge tubes and semiconductors is not considered toadversely affect the intrinsic safety of a circuit, provided that in normal operation the currentthrough the device is less than 10 @NOTE If insulation testing at 500 V is carried out under well-controlled conditions, then it may be necassary todisconnect tha surge suppression davices to prevent them invalidating the measuramant.Intrinsically safe systems utilizing surge suppression techniques must be supported by anadequately documented analysis of the effect of indirect multiple earthing, taking into accountthe criteria set out above. The capacitance and inductance of the aurge suppression devicesshall be considered in the assessment of the intrinsically safe system.Annex F illustrates some aspects of the design of surge protection of an intrinsically safesystem.


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lS/lEC 60079-25:2003

11 Assessment of an intrinsically safe system11.1 GeneralWhere a system contains apparatus which does not separately conform to IEC 60079-11, thenthat system shall be analysed as a whole. The system shall be analysed as if it were anapparatus. A category is system shall be analysed in accordance with the criteria of 5.2 ofIEC 60079-11. A category nib system shall be analysed in accordance with the criteria of 5.3of IEC 60079-11. In addition to the faults within the apparatus, the failures within the fieldwiring listed in 11.3 shall also be taken into account.NOTE 1 It is recognized that applying faults to the system as a whole is less stringent than applying faults to eachpiece of apparatus; nevertheless, this is considered to achieve an acceptable avel of safety.Where, all the necessary information is. available, it is permissible to apply the fault count tothe system as a whole even when apparatus conforming to IEC 60079-11 is being used. Thisis an alternative solution to the more usual straightforward comparison of input and outputcharacteristics of the separately analysed or tested apparatus. Where a system contains onlyseparately analysed or tested apparatus conforming to IEC 60079-11, the compatibility of allthe apparatus included in the system shall be demonstrated. Faults within the apparatus havealready been considered and no further consideration of these faults ia necessary. Where asystem contains a single source of power, then the output parameters of the power sourcetake into account the possible cable failures, and consequently these failures do not need tobe further considered. Annex A contains further details of the analyais of these simple circuits.Where apparatus can possibly interconnect separate intrinsically safe circuits, for example, aresistance thermometer with two separate resistance windings, then the interconnectedcircuits shall be assessed as a single circuit.When a system contains more than one linear source of power, then the effect of thecombined sources of power shall be analysed. Annex B illustrates the analysis to be used inthe most frequently occurring combinations.If an intrinsically safe system contains more than onesource of power, and one or more ofthese sources are non-linear, the assessment method described in Annex B cannot be u$ed.For this kind of intrinsically safe system, Annex C explains how the system analysis can bedone if the combination contains a single non-linear power supply.NOTE 2 If further expert advice is required, it should be sought from a body such as an accepted certificationbody (ACB) under the IECEX scheme.Figure 1 illustrates the principles of system analysis.

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lS/lEC 60079-25:2003

L=lnalyse anintrinsically safesystem

TI ,Is apparatus in

,---

accordance with 0IEC 60079-11being used?I J

Is only one linearpower supplyused?

ToYes

Use principles of System shall beIEC 60079-11 marked and testedLF in accordance withIEC 60079-11

Follow Annex A Createa descriptiveLF system documentI I

IAra the power Yes Follow Annex B Createsupplies linear? h a descriptivev F system documant

Not

Use guidance Createof Annex C andlor a descriptivetake expert advice .r system document1 I t

Figure 1- Systems analysis

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lS/lEC 60079-25:200311.2 Analysis of inductive circuitsWhere an apparatus has a well-defined inductance and resistance either by virtue of itsdocumentation or construction, then the safety of the inductive aspects of the system shall beconfirmed by the process defined in Annex D.11.3 Failures in field wiringWhen designing a system which requires consideration of the failure of the field wiring, thenthe following failures shall be applied:a) open circuit of any number of field wiring conductors;b) short circuit between any numbers of field wiring conductors and screens;c) failure to the bonded structure or armour at any point. For the purposes of this analysis,the return path in the structure or armour shall be considered to have zero impedance andnot to introduce any voltage or current into the circuit.The acceptable parameters of the interconnecting cables shall be calculated using a factor ofsafety of 1,5 in accordance with 10.4.2 of IEC 60079-11.11.4 Type verifications and type testsWhere it is necessary to carry out type verifications and/or type tests to establish thata system is adequately safe, then the methods specified in Clause 10 of IEC 60079-11shall be used.

12 MarkingAll apparatus within the system shall be readily identifiable. In the case of eimple apparatus,a traceable plant identification label is acceptable.The minimum requirement is that the relevant descriptive syetem document shall be readilytraceable. One acceptable technique is a clear instrument loop number, which identifies theloop documentation, which in turn Iiets the descriptive system document.If a system contains apparatue separately assessed or teeted in accordance with IEC 60079-11,such apparatus retains its original marking.Where a system is assessed as a whole and is found to conform to IEC 60079-11, then eachpiece of apparatus shall be marked in accordance with that standard.


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lS/lEC 60079-25:2003Annex A

(normative)Assessment of a simple intrinsically safe system

This simple analysis is only applicable when the system considered uses only one source ofpower.The process of determining the acceptability of the simple system, which is illustrated by theexample of Figure Al, shall be as follows:a) determine the category or group subdivision of the system by consideration of the

information for the two individual pieces of certified apparatus. The system adopts thelowest common denominator of the two pieces of apparatua. Hence, if either piece ofapparatus is nib, then the system is itY. The group subdivision is determined by the leastsensitive group IIC, IIB, 11A in order of decreasing sensitivity. In the example illustrated byFigure A. 1 the system becomes Ex ia IIC. It is permitted for different parta in the system tohave different categorization and classification. In these circumstances, the descriptivesystem document should clearly define the separate parts of the circuit

b) check the voltage, current and power parameters as follows:U. < Z.li10 S IiPO


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HAZARDOUS AREA NON HAZARDOUS AREA

.

Intrinsically safe apparatus

Ex ia IIC T4Ui 30 vIi 120 mAPi 1,2WLi IOKHCi 1 nF

System

Ex ia IIC

Cable parametersLC 3 mHLCIRC 54 pHlf2cc 82 nF

Associated apparatus

[Ex ia} IICU. 28 V10 93 mAP. 0,65 WLO 3 mHLCIRC 54 pHlL2co 83 nF

Figure A.1 - Interconnection of intrinsically safe apparatus with associated apparatus


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IS/lEC 60079-25:2003Annex B

(normative)Assessment of circuits with more than one source of power

This analysis is only applicable when the power sources considered use a linear resistivelimited output. It is not applicable to power sources using other forms of current limitation.IEC 60079-14, Annex B, contains a simplified procedure, which gives conservative results,which ensure a safe installation and may be used as an alternative to this annex.Where there is more than one source of power and the interconnections are made undercontrolled conditions so as to provide adequate segregation and mechanical stability inaccordance with IEC 60079-11, then the interconnections are considered to fail to open andshort circuit but not so as to reverse the connections or to change a series into a parallelconnection or a parallel connection into a series one. Interconnections made within a rack orpanel constructed in a location with adequate quality control and test facilities are an exampleof the degree of integrity required.Figure B. 1 illustrates the usual series combination. This series situation results in the opencircuit voltage U. being U1 + U2 but the possibility of the voltage being U1 - U2 is notconsidered. In considering the safety of the system, three voltages UI, U2 and U. = U1 + U2are considered together with their corresponding currents 11 and Z2 and the combined.

~ = U,+U2oRI +R2

Each of the three equivalent circuits has to be assessed for safety using Table A. 1 ofIEC 60079-11. The value of Lo, Lo/Ro and Co must then be established for each circuit and themost onerous value used together with its relevant equivalent circuit.A factor of safety 1,5 shall be used in determining these values in all circumstances.NOTE Where the two voltages add, the combined circuit will decide the capacitive figure. However, theinductance and the LJRO ratio may ba determined by one of the separate circuits being considered on its own. Theminimum inductance does not always coincide with the maximum circuit current and the minimum LJRO ratio maynot be coincident with the minimum inductance.The matched power available from each of the equivalent circuits should be determined. Thematched power of the combined circuit is the sum of the power available from each circuitonly when the sources have the same output current.When the sources of power are connected in parallel as in Figure B.2, then the three currentsll; 12 and 10 = 11 + 12 have to be considered with their corresponding voltages U1, U2 and

u = UIR2 + U2RIo RI+ R2

11


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lS/lEC 60079-25:2003Each of the three equivalent circuits has to be assessed for safety .using Table A.1 ofIEC 60079-11. The values LO, LJRo and CO have to be established for each circuit and themost onerous value used together with its relevant equivalent circuit. The matched poweravailable from each of the three equivalent circuits must also be established. The matchedpower of the combined circuit is the sum of the power available from each circuit only whenthe sources have the same output voltage.Where two sources of power are connected to the same intrinsically safe circuit and theirinterconnections are not well defined by reliable interconnections as illustrated in Figure B.3,then there is a possibility that the sources of power can be connected in both series andparallel. In these circumstances, all the possible equivalent circuits have to be evaluated,following both the procedures set out. The most onerous output parameters and equivalentcircuits have to be utilized in establi~hing the integrity of the intrinsically safe system.The hazardous area apparatus may contain a source of power, which results in the apparatushaving significant output parameters, for example from internal batteries. When this occurs,the analysis of the system shall include the combination of this source of power with anysource of power in the associated apparatus. Such an analysis shall normally include thereversal of the interconnection because of the possible failure of the field wiring.Having established the representative equivalent circuits, then these circuits can be used as ifthere was a single source of power, and the procedure already discussed in Annex A can beused to establish whether the system as a whole is acceptably safe.When two or more sources of power with different output voltages are interconnected then theresultant circulating current can cause additional dissipation in the regulating circuits. Wherethe circuits have conventional resistive current limiting, then the additional dissipation is notconsidered to adversely affect intrinsic safety.

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lS/lEC 60079-25:2003

Alsoanalyse

U1 [,

and

U2 12

Alsoanalyse

u, 11

and

U2 12

----l~fwwerUO=U,+U2

u, +U210=R1+R2

---P-I

Figure 6.1- Sources of power

~7

outputlxwametersu, c,

1, L,

R, L,/R,

U2 C2

12 L2

R2 L R2

connected in seriesoutputparameters

Sourceof power1 1-9 +U,R2 + U. lU.= i?,+1+2 - J

Sourceof power21.=11+12

+ -

I J

u,I,

R,

U2

12

R2

c,

L,

L,/R,

C2

L2

L~R2

Figure 9.2- Sources of power connected in parallel


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lS/lEC 60079-25:2003HAZARDOUS AREA

Intrinsicallysafe apparatus

Key1 source of power 12 source of power2

NON HAZARDOUS AREAAssociatedapparatus

------+SeriesUO=U1+U2u, +U*Io=RI+ R2

I 2Parallel10=11+1*

U,R2 + U$,U.= R, +R2

u, c,

11 L,

R, L,/RI

Figure B.3 - Sources of power not deliberately connected

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lS/lEC 60079-25:2003Annex C

(informative)Interconnection of non-linear and linear intrinsically safe circuits

This subject has been under active consideration for some considerable time and is still;eveloping. This document is the considered opinion of a major test house and has beenextensively reviewed. It is the best knowledge currently available and is included so that widerexperience of its use can be obtained.The design and application of non-linear power supplies does require specialist knowledgeand access to appropriate test facilities, Where an authorized test house has satisfied itselfthat a particular source of power is adequately safe then it is permissible to design a systemin accordance with this standard. Any particular conditions relating to such a system shall beclearly stated in the accompanying documentation.Where a safety analysis of a combination of power supplies using non-linear outputs is carriedout then the interaction of the two circuits may cause a considerable increase in thedissipation in the regulating circuit components. This factor must be taken into account. It isrecommended to have only one power supply containing regulating semiconductors combinedwith linear and/or trapezoidal sources.

C.1 IntroductionThe installation rules in IEC 60079-14 permit the operator in control of a hazardous area tocombine several intrinsically safe circuits by interconnection. This also includes the casewhere several associated apparatus (that is, active in normal operation or only under faultconditions) are involved (see 12,2.5.2 of IEC 60079-14). Where this is done, it is not requiredto involve a testing station or an authorized engineer if a calculated or test-based proof of theintrinsic safety of the interconnection is carried out.The test-based proof shall be performed using the standard spark test apparatus according toIEC 60079-11 considering the safety factor 6f the combined electrical apparatus. In this case,certain fault conditions leading to the most unfavorable ignition conditions the worst caseapproach shall be taken into account, Thus this method of proof often meets with difficultiesin practice and is usually reserved for testing stations.

An assessment by calculation of the interconnection can be carried out easily at least forresistive circuits, if the electrical sources involved have a linear internal resistance as shownin Figure C.la). In this case, the ignition limit curves in IEC 60079-11 apply and the methoddescribed in IEC 60079-14, Annex A, or in Figure C.7 and Figure C.8 of this documentcan be used.

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lS/lEC 60079-25:2003

+---c5--J-- 12sU. RI 10 iFigure C.1 a) - Linear characteristics+-&--?-Q J-- L.,UQ . . .U. -. R 10 IFigure C.lb) - Trapezoidal characteristics

Figure C.lc) - Rectangular characteristics

Figure C.1 - Equivalent circuit and output characteristic of resistive circuitsThe first step is to evaluate the new maximum values of voltage and current resulting fromcombining the associated apparatus. If the associated apparatus are combined as shown inFigure C.2a), there is a series connection. The maximum open-circuit voltage values, (JO ofthe individual sub-assemblies are added and the maximum value of the short-circuit currents,l., of the sub-assemblies is taken. In an arrangement like that in Figure C.2C), there is aparallel connection. The short-circuit currents are added while the greatest value of the open-circuit voltage is taken.If the arrangement of the apparatus is not clearly defined with respect to the polarity (as inFigure C.2e)), then there may be a series or parallel connection depending on the faultcondition considered. In this case, voltage addition and current addition must be assumed forboth, but separately. The most,unfavourable values have to be taken as a basis.

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lS/lEC 60079-25:2003+ +

+i..u.., nu&:-1. ..... . .. ........-... -.....-------- +l_ ..LJFigure C.2a) - Series connection+ +/[1 yIi::/-,10 ~N~[; w,:~...._-_..~ .--+-+ -uo, %2......................i. / +[..................................... . . -

6+

LJ-../2 u. . .. .. .. . . .. --, Iwith voltage additionL...2=u.. ... . ..\ . . . I

L-_lFigure C.2b) - Series connection with voltage addition and possibly current addition

+ +uwFigure C.2C) - Parallel connection with current addition

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lS/lEC 60079-25:2003

UA

.. . ----- ,-, --- ../

n.... ... *I

Figure C.2d) - Parallel connection with current and possibly voltage addltlon

u

Iw. . . /\ .

Figure C.2e) - Series or parallel connection with current and voltage additionFigure C.2 - Current andlor voltage addition for interconnections

After determining the new maximum values of current and voltage, the intrinsic safety of thecombined circuit shall be checked by means of the ignition limit curves given in IEC 60079-11,taking account of the safety factor for the resistive circuit, and the new maximum permissiblevalues of external inductance LO and capacitance CO shall be determined. Here, however, theprocedure introduced in IEC 60079-14, Annex A, shows a weakness, caused by the following:

the maximum permissible inductances are valid only for a maximum voltage of 24 V;the occurrence of both inductance and capacitance is not taken into account

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lS/lEC 60079-25:2003If proceeding on the basis of open-circuit voltages and short-circuit currents only, the safetyfactor obtained really decreases from the desired value of 1,5 to approximately 1,0 in thevoltage range above 20 V. This seems to be acceptable, since the interconnection accordingto IEC 60079-14 can only meet category nib generally, even if all the individual apparatusmeets category is. However, in the case of low voltages, the safety factor can dropconsiderably below the value of 1,0. Such an approach is thus not effective with regard tosafety,If one or more active sources within one circuit have non-linear characteristics, evaluating onthe basis of no-load voltages and short-circuit currents only cannot accomplish the originalintention.In practice, sources with trapezoidal shape (see Figure C.lb)) are used and rectangularoutput characteristics (see Figure C.1 c)) occur often if electronic current-limiting devices areused. For such circuits, the ignition limit curves in IEC 60079-11 cannot be used. Thisstandard therefore describes a method that allows the safety evaluation of the combination ofnetworks including non-linear circuits by means of diagrams. A new computer-aided sparkignition model allows the desired safety factor to be achieved for both non-linear sources andthe coincidence of inductance and capacitance in the circuit.The procedure introduced here is applicable for zone 1 and for explosion groups IIC and IIB.It should be emphasized that an instrument for interconnection is being proposed here; usingit for defining intrinsic safety parameters of individual circuits or apparatus makes sense onlyin the case of simple rectangular or linear circuits.

C.2 Basic types of non-linear circuitsC.2.1 ParametersWhilst assessing the intrinsic safety of active circuits, it is necessary to know the internalresistance and the source voltage. In the simplest case, the source can be characterized bytwo (constant) electrical values, either by the voltage (JO and the internal resistance Ri orby U. and the short-circuit current 10 (see Figure C.la)). U. often is determined by zenerdiodes. U. and 10 are maximum values that can occur under the fault conditions defined inIEC 60079-11. In the case of Figure C.la), the characteristic is linear, Unfortunately, inpractice, only a few circuits can be represented in this simple way.A battery, for example, fitted with an external current limiting resistor has no constant internalresistance. Likewise, the source voltage changes as a function of the degree of charge.In order to study the behaviour of such practical circuits, they are represented by their simplerequivalent circuits that must obviously not be less capable of causing ignition than the actualcircuit. In the above case of a battery, one would take the maximum open circuit as U. andthe external resistance as Ri as in Figure C.la), This equivalent circuit has a linearcharacteristic.Non-linear circuits can also be reduced, usually to the two basic types shown in Figures C.lb)and C.lc). The source with trapezoidal characteristic (Figure C.lb)) consists of a voltagesource, a resistance and additional voltage limiting components (for example, zener diodes)at the output terminals. The rectangular characteristic of Figure C.1 c) has the current limitedby an electronic current regulator.


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lS/lEC 60079-25:2003If one considers the output power of the different networks, it becomes obvious that differentignition limit values shall apply, since the igniting spark is also a load and its matching to thesource feeding it shall be taken into account. The maximum available power from the sourceshown in Figure C.la) is

P = /4 U. x 10maxand for the trapezoidal characteristic (Figure C.lb)):P = /4 UQ x 10max (for U.> ?4 x UQ), orPmax = UOX (UQ - UO)IR (for U.< % x UQ).The trapezoidal characteristic of Figure C. 1b) becomes the rectangular characteristic ofFigure C.lc) as UQ tends to infinity.Here:Pmax = U. x Io.For the complete electrical description of a source, two parameters are needed for the linearand rectangular characteristics and three parameters for the trapezoidal characteristic(Table C.1 ).

Table C. 1- Parameters necessary to describe the output characteristic[ Characteristic I Parameters necessary 1Linesr, Figure C.la) UO, 10 or UO, RTrapezoidal, Figure Cl b) UO, UQ, R or UO,R, 10 or UO, (JQ, 10

C.2.2

Rectangular, Figure C.lc) Uo,1~

Information given in the certificatesBecause, according to 12.2.1 or 12.3 of IEC 60079-14, apparatus with active intrinsically safecircuits shall be certified, it can be assumed that, for the individual apparatus which are to becombined with their intrinsically safe circuits, there is always a test certificate available whichcontains the corresponding electrical parameters.The first step in any safety-oriented assessment shall be the determination of the type ofcharacteristic and associated electrical parameters of the individual circuits. Since the circuitarrangements and the internal construction of the apparatus are not normally known to theuser or operator, they will have to trust the electrical data given in the test certificate.The values given usually are as follows: open-circuit voltage (here named Uo) and short-circuit current (here named l.) and normally the maximum available power Po. It is oftenpossible to conclude information about the type of characteristic from these values.Example (maximum values):[J = 12,5 V10 = O,IAP. = 313mW


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lS/lEC 60079-25:2003Because P. is one-quarter of the product of open-circuit voltage and short-circuit current, itcan be deduced that in this example a linear characteristic (Figure C. la)) is effective.Example (maximum values):U. = 20,5 V10 = 35 mAP. = 718 mWHere P. is the product of the open-circuit voltage and the short-circuit current, and hence arectangular characteristic is given (Figure C.lc)).In certain cases, the values for power, current and voltage do not correspond with the abovebecause the power rating is specified for the stationary case (heating effect of componentsconnected subsequently) and the current or voltage values for the dynamic case (sparkignition) are given. In situations where there is a doubt, it is essential to verify whichcharacteristic to take as the basis for the interconnection with respect to spark ignition.In the case of a trapezoidal characteristic, the information in the test certificate is often notsufficient to determine the characteristic. The third parameter is missing (see Table C. 1),either UQ or R.When R is given as the additional parameter, there is the least danger of confusion. ThereforeR will generally be given in the test certificates. The parameter UQ (Figure C.lb)) can then bederived from UQ = 10 x R.In most cases, the test certificate will also give the characteristic shape of any non-linearcircuits.An example may look as follows.Maximum values (trapezoidal characteristic):V. = 13,7 v10 = 105 mAR= 438 QP. = 10IOmWThe characteristic represented is shown in Figure C.3a); Figure C.3b) shows the safetyequivalent circuit.Calculation is as follows:UQ = Iox R=46V andP. (UQ - UO) x UJR = 1010 mW

21


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lS/lEC 60079-25:2003UA

4SV L. . . . . ..13,7 v .

*o 105rn41

Figure C.3 a) - Output characteristics43e6-1 I+ T T o

Figure C.3 b) - Equivalent circuitFigure C.3 - Output characteristic and equivalent circuit of a sourcewith trapezoidal characteristic

In this way, the data needed for the interconnection can be obtained from the informationgiven in the test certificate. If there is no data in the older certificates, the values shall beobtained from the manufacturer of the apparatus or from the testing station.In designing intrinsically safe circuits, an attempt should be made always to keep theinterconnections and number of combined sub-assemblies low. This objective cannot alwaysbe achieved in practice, because it is also necessary to consider fault conditions. This meansthat some apparatus which are not acting in normal operation as sources have to be regardedas sources in the case of failures.The passive inputs of devices, for example, measurement transducers, plotters etc, can, fromthe safety point of view, also act as active sources. Therefore the maximum values indicatedin the certificates shall be referred to. As a result, the operational characteristics of a circuitmay deviate substantially from the safety characteristic. The values given in the certificatesfor open circuit voltage UO and short-circuit current 10 for the circuit concerned are stated onlyfor transient conditions in some cases. On the other hand, the power value applies for steady-state conditions which have to be considered for the temperature rise of connectedcomponents.

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lS/iEC 60079-25:2003

C.3 Interconnection of intrinsically safe circuits with more than one sourceC.3.1 Determination of a resultant output characteristicIt is assumed that the output characteristics of the circuits making up the combination, andwhich are to be regarded as sources, are known (see C.2). It is then necessary to ascertainfrom the type of interconnection whether, in normal operation and under fault conditions, it isnecessary to consider the voltage sum, the current sum, or both current and voltage sums,If the combined sources are connected in series and are not bonded, for example, to earth(Figure C.2a)), then, irrespectively of the polarity of the sources, voltage addition only ispossible. The resultant output characteristic is conveniently found by graphical addition. Thusfor each current value, the voltages of the individual sources are added. The dotted-line curvein Figure C.2 shows the resultant characteristics in the different cases.In the series circuit shown in Figure C.2b), where there is a common connection of bothvoltage sources at the load, current addition can be excluded only if the polarity of bothsources in the direction shown here is fixed with respect to safety (for example, for certainsafety barriers). With sources which can change the polarity operationally or under faultconditions, both voltage and current addition shall be considered (see Figure C.2e)).In the parallel arrangement of Figure C.2C), current addition is only possible if, with bipolarsources, two poles are connected in each case, Voltage addition is not possible in this caseand the resultant characteristic is generated by graphical addition of individual current values.If only one pole of each source is connected to that of the other (Figure C.2d)), then voltageaddition can be excluded only if the polarity of the sources as shown here is fixed regardingall circumstances (for example, with safety barriers). Otherwise, both voltage and currentaddition shall be considered (see Figure C.2e)).If several circuits are connected to a circuitry in which arbitrary interconnections shall beassumed (Figure C.2e)), then, depending on the fault conditions considered, a parallel orseries connection may be set up, so that both current and voltage addition, shall beconsidered. Because both cases are not possible at the same time, the resultantcharacteristic for current addition and that for voltage addition shall be constructedseparately. This procedure is necessary also in all cases of doubt for the circuits in FiguresC.2b) and C.2d) as well as with circuits with more than two conductors. The result so obtainedwill always be on the safe side.C.3.2 Safety assessment of the interconnection and determinationof the maximum permissible capacitance and inductanceWhen the resultant characteristic for the combination circuit has been determined as detailedin C.3.1 above, the next step is analysis of the intrinsic safety. For this purpose, the diagramsgiven in Figures C.7 and C.8 are to be used, They show the permissible limit curve for linearsource characteristics (dotted limit curve) and for rectangular characteristics (solid limitcurve), with a given inductance and the new maximum values of current and voltage in thecombined circuit. Further, curves are given to determine the highest permissible externalcapacitance for bcth cases. Table C.2 gives an overview.

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lS/lEC 60079-25:2003Table C.2 - Assignment of diagrams to apparatus groups and inductances

Figure Oroup Permissible inductance LoFigure C.7a) 0,15 mHFigure C.7b) 0,5 mHFigure C.7C) Ilc 1 mHFigure C.7d) 2 mHFigure C.7e) 5 mHFigure C.8S) 0,15 mHFigure C.8b) 0,5 mHFigure C.8C) IIB 1 mHFigure C.8d) 2 mHFigure C.8e) 5 mH

To assess the intrinsic safety, first select the explosion group and then the total inductancerequired for the combination. If only small inductances (that is no lumped inductances, onlyshort cable lengths) are concerned, then the diagram with the lowest inductance should beselected (i.e. Figure C.7a) for Group IIC and Figure C.8a) for Group IIB).The resultant output characteristic is then in the diagram concerned. If, in accordance withC.3. 1 current and voltage additions are considered, then both resultant characteristics shallbe plotted.It is now possible to determine directly whether the combination of sources together with theinductance for that diagram and the selected explosion group is intrinsically safe. Theresultant sum characteristic shall not intersect the limit curve for the rectangular source in thediagram at any point. In addition, the point in the diagram defined by the maximum voltageand the maximum current of the sum characteristic shall be below the curve for the linearsource.The maximum permissible capacitance of the resulting circuit is found as the lowest valuafrom the two Co limit curve families, being the highest Co value that is not intersected by theresultant output characteristic for the linear limit and for the rectangular limit. If a higherpermissible capacitance Co is required for the purpose of an application, then this can beobtained by starting with a diagram for a lower inductance. The same approach can also beused where the resultant output characteristic intersects the curve for the inductive limit of thelinear or rectangular source. If, even for the smallest inductance value in the diagrams(0,15 mH), the relevant limit curve(s) is exceeded in the IIC diagram, then tha use of the IIBdiagrams is recommended. If these limits are also exceeded, then the combination is notintrinsically safe for explosion Group 119 either.C.3.3 Supplementary comments about the procedure using output characteristicsThe procedure described above in C.3. 1 and C.3.2 for the safety assessment of inter-connections of intrinsically safe circuits is based on fundamental research work and modelcalculations. The actual calculation method gives results differing from those in former report.

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lS/lEC 60079-25:2003In future, somewhat larger capacitances are permissible in the small voltage range. For highervoltages the difference can be up to a factor of 3. In contrast to the diagrams in a formerrepoti, the limit curve for the purely resistive circuit is omitted in Figures C.7 and C.8; but it isinherently established through the inductive limits. Further, the limit curves for linear sourceswere inserted here. Apart from this, tne graphic process remains the same in general,

The graphic method is based upon a reduction of the actual source characteristic inabstracted linear as well as rectangular sources and comparison with the associated limitcurves. Only in the case where the actual source characteristic is either linear or rectangularcan the safety factor be derived from the diagram with a guarantee to be exactly 1,5. In someof the more complex sources, it may be of benefit to construct an enveloping linear orrectangular characteristic and the safety facto~ is preserved. If both limit criteria are made useof, the actual safety factor can be slightly smaller (always greater than 1 however). This is aresult of the reduction of the actual circuit conditions used in this simple graphic method.General expert opinion indicates that this is acceptable when considering zone 1 installations,When using the diagrams given in Figures C.7 and C.8, the interaction of inductance andcapacitance (mixed circuit) is always covered. The procedure should be used also for thecombination of purely linear circuits (output characteristic in accordance with Figure C.la).The method specified does not distinguish between lumped inductances or capacitances andthose derived from distributed cable parameters. When cables with transmission times of upto 10 KS occur, then the current view is that there is no need for this difference. Thecalculation based on concentrated elements lies on the safe side and does not, in contrast toearlier calculation methods, cause severe limitation in practice.The advantage of this procedure is that all information relating to safety data can be takenfrom a single diagram. Nevertheless, an additional comparison of the maximum open-circuitvoltage with the maximum capacitance according to Table A.2 in IEC 60079-11 should bemade, because in certain cases the procedure described here gives a higher permissiblecapacitance. The values shall then be taken from IEC 60079-11 because misunderstandingscan arise otherwise.The values obtained for the maximum permissible external inductance and capacitance arethose for the total combination, that is the inductances and capacitances of all the individualdevices, which are effective at the external terminals, shall be considered.The calculation procedure used for the diagrams shows no significant systematic deviationsfrom the results obtained from the ignition tests during the research projects. It is known thatthe numerous experimental results have an uncertainty in the range of 10 A. The reason forthis is the test method and the spark test apparatus itself. The method presented here is notestimated to have greater deviations.

C.4 Illustration of the procedure using output characteristicsby means of an exampleIn the example shown in Figure C.4, an analyser with an amplifier is located inside thehazardous area and supplied by an intrinsically safe power supply (l). The intrinsically safeamplifier output signal (O ...20 mA signal) is fed to a display (11) and a plotter (Ill).

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lS/lEC 60079-25:2003

1

I

2 ---+ 6

II

Iv3 EEx ibIIB

7,/

Current/voltage additioninterconnected circuit Ex ib IIBP. = 1,9 W, U. = 26,7 V, 10 = 264 mALo = 0,5 mH, Co = 400 nFKey1 control room 5 recorder operationally pessive maximum values:2 switch room 1 V, 31 mA, 10 mW Iinaar characteristic3 field (hazardous area) 6 power supply maximum values: Ex ib IIB 15,7 V,100 mA, 1,57 W, LO s 1 mH, Co s 650 nF electronic4 display operationally passive maximum values: current regulation rectangular characteristic12 V, 133 mA, 0,4 W linear characteristics 7 analyser with amplifier (intrinsically safe apparatus)

Figure C.4 - Example of an interconnectionThe analyser is an intrinsically safe apparatus; the power supply, the display and the plotterare associated apparatus within the meaning of IEC 60079-11. In normal operation, only themains supply is effective as an active source, whilst display and plotter are passive. Forsafety analysis however, the highest possible values are taken as a basis which are found inthe test certificates for the three devices when in a fault condition.The following information is available.1. Power supply

Output with type of protection Ex ib IIBMaximum valuesU. = 15,7 vJo =100mAP. = 1,57 wLo=lmHCO = 650 nFRectangular output characteristic (Figure C.lc))

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lS/lEC 60079-25:2003Il. Display

Input with type of protection Ex ib IICMaximum valuesUO= 2V10 =133mAP = 0 4 wLO = 1 8 mHco = 1,4 pFLinear output characteristic (Figure C. la))

III. PlotterInput with type of protection Ex ib IICMaximum valuesUo=lv = 31 mAP = 10 mWLO = 36 mHCO= 200 FFLinear output characteristic (Figure C,la))

With the circuit arrangement in Figure C.4, and depending on the fault conditions in theanalyser, voltages or currents can be added as in Figure C.2e). The individual characteristicsand the two sum characteristics for voltage and current addition are shown in Figure C.5.

Uv t3028,7

Fl

\ \ \

.(=\ \ \ 18,7 V\ AloomA

15,7 -- -- - -115- - /I 112- ,-1.10- - I .

II

1 .-. .1 1 D-OI 31 100 133 200 2s4300 I mA

Figure G.5 - Sum characteristics for the circuit as given in Figure C.4

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\lS/lEC 60079-25:2003Inorder tocheck theintrinsic safety, the two sum characteristics are drawn in Figure C.8b)(explosion Group IIB, L = 0,5 mH) (Figures C.6a) and C.6b)).The corner point at 18,7 V and IGL mA in the voltage addition curve obviously is the criticalpoint - it is nearest to the inductive limit of the rectangular source, but does not reach it.At this point the theoretically highest power of 1,9 W is resched.Since both resultant characteristics of the combination do not intersect the inductive limitcurves for the linear and rectangular sources in Figures C.6a) and C.6b), the safety test hascome out positively. For the maximum voltage (28,7 V) of the resultant characteristic in thepresent example, the maximum permissible capacitance of the combination from the family ofcurves in Fjgure C.6b) can be read off to be 400 nF. If Table A.2 of IEC 60079-11 is checkedfor the value 28,7 V Group IIB, the permissible value of capacitance is 618 nF - higher thanthe value of 400 nF established here.

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lS/lEC 60079-25:2003

40aonl=

35

Wrll= 25lpF 202~ 155JlF10@

105

0 0 100 200 300

Key1 inductive limit for rectangular source2 inductive limit for Iinaar source

Figure C.6a) - Current addition40

monF 35U. v 30

5oorrF 25lpF 20211F 15

1050

400 50010 mA

I .[ I { I I I 1 1 I I 1 I 1 1 I I [ I 1 I 1 I1.1 1 1 1 1 I 1 1 1 1 I 1 1 I I 1 I 1 1 1 I----- 1 1

0Key

100 200 mo 400 xro10 MAindu tive limit for rectangular sourceinductiva limit for linear source

Figure C.6b) - Voltage additionFigure C.6 - Current and/or voltage addition for the example given in Figure C.4

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lS/lEC 60079-25:2003The resultant values for the combination are as follows:Explosion Group IIBMaximum valuesUO= 28,7 V10 = 264 mAPO=1,9WLO = 0,5 mHco = 400 nFBecause, in the present example, the associated apparatus (power supply, display andplotter) have no effective inductance or capacitance values at the intrinsically safeinputs/outputs, the maximum values for capacitance and inductance may be used for theintrinsically safe apparatus (analyser) and for the interconnection cables.

C.5 SummaryIn the design and construction of measuring and process plant in the chemical andpetrochemical industries, it is frequently necessary to combine several certified pieces ofapparatus with intrinsically safe circuits.The installation rules of IEC 60079-14 permit the designer, constructor or operator of anelectric installation in a hazardous area to handle such combinations at his own responsibilityif a calculated or measured proof of the safety of the interconnection is carried out. Sincethe operator has, generally, no facility for a measured proof (the required equipment isnot available to the operator), the operator is left with a suitable calculation procedure.IEC 60079-14 has up to now provided only a procedure that can be used exclusively forsources with purely linear internal resistance and even this does not always result in safeconfigurations. In practice however, sources with non-linear characteristic occur frequently,and up till now the combination of these were only possible with the support of a testingstation.A method was therefore developed which permits the safety assessment of the combination ofnetworks with linear and non-linear circuits to be performed by means of diagrams. Theprocedure described here is applicable to explosion Groups IIB and IIC and for hazardousarea zone 1.The basic part of the procedure is the graphical summation of the output characteristics of theintrinsically safe sources involved. The resultant characteristics are then plotted in a suitablediagram from which the intrinsic safety of the resistive, inductive, capacitive and combinedcircuits can be assessed (that is with a simultaneous inductive and capacitive load).A significant advantage of this procedure is that all information and boundary conditionsrelating to the safety data can be taken from just one diagram. The required safety factorof 1,5 is already incorporated into the diagrams.

C.6 DiagramsThe diagram in Figure C.9 is included so that it may be used for copying onto a transparency.The self-calculated diagrams for voltage sum or current sum then can be drawn and laidupon the different limit diagrams (common scale versions) for assessment. On the followingpages the limit diagrams according to Table C.2 are given both in a common scale and inoptimized scale.

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40u. v50nF

35m nF

100 nF ~

25150 nF

2WrIF 20

300 nF

500 nF ,5

1 ILF

102 PF

5pF10 pF

lS/lEC 60079-25 :2003

14LLLLa4 I 1--t-r

00 100

Key1 inductive limit for rectangular source

203 300 400 50010 MA

2 inductive limit forhnearaource

Figure C.7a) - Diagram for 0,15 mH31


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lS/lEC 60079-25:2003

I50 nF

70 v70 nF

100 nF

150 nF200 nF300 nFS00 nF

1 pF

2 BF5 PF

10 BF

40

35

a

25

20

15

10

5

0

.I I

I I II I 1 I I I I I I0 160

Key1 inductive limit for rectangular source2 inductive limit for l inear source

200 300 400 50(Ie MA

Figure C.7a) - Diagram for 0,15 mH @or?tifwed

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lS/lEC 60079-25:200340

U. v50 nF

s

70 nF

3

100 nF 25

150 nF20

200 nF

Wnl=

5oonF 15

lM 10

2 pF

5pF 510 pF

o

Key

o 50 1(XI 150 200 250 30010 mA

1 inductive limit for rectangular source2 inductive limit for linear source

Figure C.7b) - Diagram for 0,5 mH


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ISIIEC

40WInF

Uov=70 nF

30

100 nF ~

150 nF 20200 nF3oonF500nF 15

t ILF 102@=5pF 510 pF

o

I

+ =. - Ii I\ -t.\ -1- . I 1

\20 . I1 PF 1 ~ 1 ~ RI l\ \15 .~ ~

2 IIF ~-l.- -1 1 NWl-+- [I -r-l y, 110 . I Is II I I 7


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lS/lEC 60079-25:2003Annex D

(normative)Verification of inductive parameters

Figure D.1 illustrates the system being analysed,I?i is the inherent resistance of the inductive coil. If the coil resistance is supplemented by anadditional resistor, then that resistor shall meet the criteria of an infallible resistor.RO is the output resistance of the linear source of power, that is UOIIO.If Li is less than LO, then the permitted inductance of the cable may be taken, as thedifference betweenthe two values and the system is acceptable.If LilRi is less than the permitted LOIROof the power source, then the system is acceptable andthe permitted L/R ratio of the cable remains LOIRO,NOTE Where a power supply uses the lowest value of current-limiting resistor permitted by Table A.1 ofIEC 60079-11, then there is no permitted inductance for a cable without taking into consideration the cebleresistance, and Lo equals zero.If the inductive apparatus does not meet either of these two requirements, then a moreextensive analysis should be undertaken as follows.Determine the current, which flows through the inductance. In the circuit illustrated, this isI = UOI(RO+ Ri).Multiply this current by 1,5 and use the inductive curves in IEC 60079-11 appropriate to therequired apparatus group to determine the maximum permitted inductance LmaX.If .Lmax is less than the inductance of the coil Li, then the circuit is not acceptable.If LmaX is greater than Li then the permitted cable inductance LC is the smaller of the twovalues (LmaX - Li) or LO.If required, maximum inductance to resistance ratio of the cable (Lc/RC), which may beconnected in the system, shall be calculated using the following formula. This formula takesaccount of a 1,5 factor of safety on current and shall not be used where Ci for the outputterminals of the apparatus exceeds 1 YO of Co.

wheree is the minimum spark-test apparatus ignition energy in microjoules, and is for

Group I apparatus: 525 KJ, Group 11A apparatus: 320 wJ, Group 119 apparatus: 160 I.LJ, Group IIC apparatus: 40 PJ;

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lS/lEC 60079-25:2003R is the total circuit resistance (RO + Ri), in ohms;UO is the maximum open circuit voltage, in volts;L is the total circuit inductance (Li + internal inductance of power source) in henries;The permitted LC/RC ratio of the system cable is whichever is the smaller of this calculatedvalue and the LOIRO ratio of the source of power.NOTE In determining the temperature classification of such an inductor, the assumption is made that the coilresistance fails to the value permitting maximum power transfer.

HAZAROOUS AREA

2

NON HAZARDOUS AREA

1I1II r

qIIP,

Ri

L,

U.10=RO+Ri

1I11II1I1II

10LdRO

U.

10

RO

P.

I II1IIII u,2-L;I Ii 2101I P, .2P.1

Key1 associated apparatus2 parameters of inductance

Figure D.1 - Typical inductive circuit

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lS/lEC 60079-25:2003Annex E(informative)

A possible format for descriptive systems drawingsand installation drawings

This annex is intended to Illustrate the information that is considered desirable in preparingdescriptive system drawings and installation drawings. It is not intended to promote aparticular format for these drawings or suggest that other methods of storing the informationcannot be equally effective. The example illustrated was deliberately chosen because of itscomplexity and illustrates almost all facets of system design. The majority of applications aremuch simpler than this and comprise a single transmitter and interface.The block diagram contains all the information necessary to confirm the status of the systen,,and to make possible the analysis described in Annexes A and B. The note on the RTDconfirms that it is simple apparatus and that its temperature classification is determined by thelocal process temperature. The failure to comply with the 500 V insulation test means that it isregarded as being earthed at the point and hence relies on the galvanic isolation within thetransmitter to satisfy the requirement of the circuit being earthed at one point only.The transmitter is certified apparatus and has safety parameters specified for both the RTDinput connections and the 4 mA to 20 mA output connections. The input capacitancemarginally changes the permitted cable capacitance, and the permitted ambient temperaturerange ensures that the transmitter is suitable for plant mounting in most locations.The galvanically isolated interface has well-defined output parameters which are used todetermine the permitted cable parameters. The restrictive cable parameter is the 80 nF cablecapacitance, which is highlighted in the note under the document number. The alternative,parameter in Group IIB is given since this might be more relevant to a particular application.The installation drawing is intended to convert the descriptive system drawing to therequirements of a patitcular installation. The assumption is made that the installing technicianrequires the information necessary to create an installation, which has already been correctlydesigned. The technician would only need access to the descriptive system drawing if he hadsome reason to doubt the adequacy of the installation. The installation drawing adds thejunction box, which is simple apparatus and specifies the particular cables and glands to beused. In this case they are agreed company standards complying with the relevantrequirements. The temperature classification of the RTD is clarified and specific instructionson the bonding of the cable screens are given. The level of information on this drawing shouldbe adequate to permit subsequent inspections to be carried out.It is important to reiterate that this annex illustrates only one method of presenting thisinformation. The essential requirement is that the descriptive system document contains allthe information, which enables an adequately safe system to be created. The installationdocu~ent should contain the necessary information to enable a particular embodiment of thatsystem to be safely instailed in a specific location.

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lS/lEC 60079-25:2003Annex F

(informative)Surge protection of an intrinsically safe circuit

F.1 GeneralThis annex illustrates a possible technique for protecting an intrinsically safe circuit from thesurges induced by a nearby lightning strike. This type of protection is only applied when a riskanalysis of the probability of a lightning strike and the consequences of such an event showsit to be necessary. The example is intended to demonstrate the necessary analysis; it is notthe only possible solution.

F.2 Installation to be protectedFigure F. 1 illustrates a typical installation where the neutral is directly connected to an earthmat, Other bonding techniques are equally acceptable. The temperature-sensing elementpenetrates the Faraday cage of a storage tank containing a flammable material. The sensingelement resistance is converted to 4 mA to 20 mA by a converter with internal isolation. Thiscurrent is then fed into the computer-input network via a galvanic isolator. The combination ofisolator, converter and the sensing element needs to be anaiysed as being an intrinsicallysafe system and is the system analysed in Annex E.

F.3 Lightning induced surgesOne possible scenario is lightning striking the tank at point X and the resultant current beingdispersed via the foundations of the tank and the equipotential bonding, of the installation.A transient voltage (typically 60 kV) would be developed between the tank top (X) and thebonding point of the computer O volt (Y). The transient voltage would cause breakdowns ofthe galvanic isolator and the converter isolation and could create a side flash within thevapour space of the tank with a high probability of an explosion.

F.4 Preventive measuresA surge suppresser can be mounted on the tank to protect the transmitter segregation thuspreventing a potential difference within the tank. The surge suppresser is bonded to the tankto preserve the Faraday cage. The multi-element surge suppresser restricts the voltageexcursion (60 V) to a level which can readily be absorbed by the transmitter isolation.A second surge arrester is necessary to prevent the galvanic isolator and computer inputcircuits being damaged. This surge suppresser would normally be mounted in the safe areaand connected as indicated. The resultant common-mode surge on the isolator would notoverstress the isolation within the galvanic isolator.

The system is not intrinsically safe during the transient voltage but the high currents andvoltages are removed from the highest hazard location within the tank and are present in therelatively secure location of the interconnecting cables.

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lS/lEC 60079-25:2003The system is indirectly earthed (bonded) at two places and during the transient period thecirculating current flowing is incendive. However, in normal operation the indirect earths arenon conducting and require a relatively high voltage (120 V) between the bonding connectionsof the surge suppression networks for any significant current to flow. Such a voltage shouldnot exist for any significant time and hence the circuits are adequately safe.

F.5 Supporting documentationThe descriptive system document should be modified to include the surge suppressionnetworks fitted. Their effect in normal operation needs to be analysed taking into account theirrelevant characteristics, which may include small values of capacitance and inductance.The indirect earthing in two places should be recorded and analysed and an argument ofacceptability presented.

F.6 Further protectionWhere lightning is recognized as a significant problem, consideration should be given to fittingsurge suppression to the mains supply to the instrumentation system. Mains borne surgescould damage the galvanic isolators from the power supply or signal connections. Somedegree of immunity is implicit in the normal requirements of compliance with EMC standardsbut this is not adequate against most lightning induced surges.Similarly, the other possible invasion route along network interconnections requires somedegree of surge protection.

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lS/lEC 60079-25:2003

Key1 converter2 surge suppressor3 bonding strap4 mains supply

JM1-5 galvanic isolator 9 mains filter suppressor6 equipotential bond 10 tank shell7 signal suppressor 11 instrument housing6 data link

Figure F.1 - Surge protection requirements of an instrument loop

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Bureau of Indian StandardsBIS is a statutory institution established under the Bureau of /ndian Standards Act, 1986 to promoteharmonious development of the activities of standardization, marking and quality certification of goodsand attending to connected matters in the country.Copyright61S has the copyright of all its publications. No part of these publications may be reproduced in anyform without the prior permission in writing of BIS. This does not preclude the free use, in course ofimplementing 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 StandardsAmendments are issued to standards as the need arises on the basis of comments. Standards arealso reviewed periodically; a standard along with amendments is reaffirmed when such reviewindicates that no changes are needed; if the review indicates that changes are needed, it is taken upfor revision. Users of Indian Standards should ascertain that they are in possession of the latestamendments or edition by referring to the latest issue of BIS Catalogue and Standards: MonthlyAdditions,This Indian Standard has been developed from Dot: No. ETD 22 (5729).

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iec 60079-25 intrinsically safe systems

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