is 13408-1992 - selection, installation & maint of ex equipments

8/17/2019 Is 13408-1992 - Selection, Installation & Maint of Ex Equipments

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IS 13408 (Part 1) : 1992

(Reaffirmed 2003)

Edition 1.1

(2003-10)

B U R E A U O F I N D I A N S T A N D A R D SMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

Price Group 12

© BIS 2003

Indian Standard

CODE OF PRACTICE FOR THE SELECTION,INSTALLATION AND MAINTENANCE OFELECTRICAL APPARATUS FOR USE IN

POTENTIALLY EXPLOSIVE ATMOSPHERES(OTHER THAN MINING APPLICATIONS OR

EXPLOSIVES PROCESSING AND

MANUFACTURE)PART 1 GENERAL RECOMMENDATIONS

(Incorporating Amendment No. 1)

UDC 621.31-213.34 : 006.76

L I C E N S E DT ORE L I A N C E I ND U S T RI E S L I MI T E D

- J AM NA GARRE F I NE RY ,

F ORI NT

E R NAL U S E AT T HI S L O C AT I O N O NL Y ,

S UP P L I E DBYB O OK S UP P L YB URE A U


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Electrical Apparatus for Explosive Atmospheres Sectional Committee, ET 22

CONTENTS

Section 1 General P AGE

1.2.

3.4.5.6.

SCOPEREFERENCE STANDARDS

DEFINITIONS AND E XPLANATION OF TERMSCLASSIFICATION OF H AZARDOUS A REAST YPES OF PROTECTIONLIGHT METALS AS CONSTRUCTIONAL M ATERIALS

11

1234

Section 2 Selection of Apparatus

7.8.

PROCEDURE FOR SELECTING ELECTRICAL A PPARATUSINTERCONNECTION OF A PPARATUS

46

Section 3 General Installation Recommendations

9.10.11.12.13.1415.16.17.18.19.20

GENERAL A CCESS FOR INSPECTIONPLANS AND RECORDSELECTRICAL R ATINGSINSTALLATION OF A PPARATUSINTEGRITY OF INSULATION A UTOMATIC O VERCURRENT PROTECTIONE ARTHING AND E ARTH F AULT PROTECTIONISOLATIONEMERGENCY SUPPLIESWIRING S YSTEMSPORTABLE AND TRANSPORTABLE A PPARATUS AND ITS CONNECTIONS

66666777777

10

Section 4 Inspection, Maintenance and Testing

21.22.23.24.25.26.27.

GENERALPERSONNELISOLATION OF A PPARATUSPRECAUTIONS CONCERNING THE USE OF IGNITING A GENCIESINITIAL AND PERIODIC INSPECTIONSINITIAL AND PERIODIC TESTINGM AINTENANCE RECOMMENDATIONS

10101010111212

Section 5 Properties of Flammable Liquids, Vapours and Gases

28.29.30.31.32.33.34.35.36.

GENERALMELTING POINT AND BOILING POINTRELATIVE V APOUR DENSITY FLASH POINTLIMITS OF FLAMMABILITY FLAMMABILITY R ANGEEFFECT OF ENVIRONMENTS WITH OTHER THAN NORMAL A TMOSPHERIC CONDITIONSIGNITION TEMPERATUREGENERAL CONSIDERATION

131313131313141414

Section 6 Marking of Apparatus37.38.39.40.

GENERALM ARKING REQUIREMENTS FOR A PPARATUSE XAMPLES OF M ARKING OF A PPARATUSGENERAL NOTES ON M ARKING

15151617

A NNEX A LIST OF REFERRED INDIAN STANDARDS A NNEX B RELEVANT INTERNATIONAL, EUROPEAN AND BRITISH STANDARDS AND DETAILS OF

THE N ATIONAL CERTIFYING A UTHORITY AND ITS CERTIFICATION STANDARDS A NNEX C FRICTIONAL SPARKING RISKS WITH LIGHT METALS AND THEIR A LLOYS A NNEX D D ATA FOR FLAMMABLE M ATERIALS A NNEX E C ALCULATION OF THE FLAMMABILITY LIMITS FOR A MIXTURE OF G ASES

18

19212235

( Continued on third cover )



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IS 13408 (Part 1) : 1992

1

Indian Standard

CODE OF PRACTICE FOR THE SELECTION,

INSTALLATION AND MAINTENANCE OFELECTRICAL APPARATUS FOR USE IN

POTENTIALLY EXPLOSIVE ATMOSPHERES(OTHER THAN MINING APPLICATIONS OR

EXPLOSIVES PROCESSING ANDMANUFACTURE)

PART 1 GENERAL RECOMMENDATIONS

Section 1 General

1 SCOPE

1.1 This code gives recommendations forselection, installation and maintenance of electrical apparatus for use in areas wherepotentially explosive materials are generated,processed, handled, stored or otherwiseencountered.

1.2 Flammable or potentially explosivematerials in this standard include gases,vapours, mists and solids.

1.3 This standard applies to the following:

a) thermal effects where those give rise todanger in flammable or potentiallyexplosive atmosphere;

b) protection against overcurrent whererelevant to circuits in flammable orpotentially explosive atmospheres;

c) methods for switching and isolation of electrical systems where relevant tocircuits in flammable or potentiallyexplosive atmospheres.

1.4 This code does not cover the following:

a) inherently explosive atmospheres, forexample, explosives manufacturing andprocessing;

b) underground mining application;

c) dust atmospheres;

d) areas where abnormal atmosphericconditions occur;

e) the effects of static electricity and highfrequency electromagnetic radiation.

NOTE — Detailed guidance on protection against risksdue to static electricity may be found in IS 7689 : 1989.

1.5 This standard is applicable to all new

permanent installations and, where resonably

practicable, to existing installations. Therecommendations for maintenance areapplicable to all installations.

1.6 The recommendations of the standard arealso applicable to those installations that aresemi-permanent or temporary but alternativesafety measures and procedures may need to beadopted to achieve the same level of safety asthat recommended in this standard.

NOTES

1 This standard does not give guidance on methods of protection against electric shock, the effects of lightning ( see IS 2809 : 1969 ), ignitions sources other than thoseassociated with electrical apparatus or toxic risks. Thetoxic risks associated with flammable materials apply toconcentrations that are usually very much less than thelower flammable limit.

2 Annex B contains, for information, a list of relevantinternational, European and British Standards,together with details of the national certifying authorityand its certification standards.

2 REFERENCES

The list of Indian Standards given in Annex A is a necessary adjunct to this standard.

3 DEFINITIONS AND EXPLANATION OFTERMS

3.0 For the purpose of this part and of the otherparts of the code generally, the following definitions shall apply.

NOTE — Further definitions that are particularlyrelevant to specific types of protection are included inthe appropriate parts of this code of practice.

3.1 Explosive Atmospheres

A mixture of flammable gas or vapour with airunder atmospheric conditions in which, afterignitions, combustion rapidly spreadsthroughout the unconsumed mixture.



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3.2 Mist

A free suspension in air of droplets of a liquidwhose vapour is flammable in free suspensionin air.

3.3 HazardThe presence, or the risk of the presence, of anexplosive atmosphere.

3.4 Hazardous Area

An area in which explosive atmospheres are, ormay be expected to be, present in quantitiessuch as to require special precautions for theconstruction and use of electrical apparatus.

3.5 Non-Hazardous Area

An area in which explosive atmospheres are notexpected to be present in such quantities that

special precautions for the construction and useof electrical apparatus are necessary.

3.4 Flash Point

The lowest temperature at which sufficientvapour is given off from a flammable materialto form an explosive atmosphere.

3.7 Ignition Temperature

The lowest temperature of a flammable gas orvapour at which ignition occurs when tested asdescribed in IS 7820 : 1975.

NOTE — Ignition temperature was formally known asautoignition or spontaneous ignition temperature.

3.8 Accessible Surface

A surface to which an explosive atmosphere hasaccess, and that is not explosion protected otherthan by temperature limitation.

3.9 Maximum Surface Temperature

The highest temperature attained underpractical conditions of operation within therating of the apparatus (and recognizedoverloads, and including defined faultconditions, if any, associated therewith) by anaccessible surface the exposure of which to an

explosive atmosphere may involve a risk.3.10 Temperature Class (T Class)

One of six values of temperature allocated toelectrical apparatus derived from a system of classification according to the maximumsurface temperature of the apparatus.

3.11 Apparatus Group and Sub-Group

Group or sub-group assigned to apparatusdepending on its suitability for use with specificgases.

An apparatus group or sub-group may be used

with flammable materials of a lesser risk

sub-group subject only to considerations of temperature and chemical compatibility.

3.12 Protection

3.12.1 Type of Protection

The measures applied in the construction of apparatus or part of apparatus to preventignition of surrounding explosive atmosphereby such apparatus.

NOTE — Formerly it was common for an individualitem of apparatus to employ one type of protection only;increasingly apparatus may now employ two or moretypes of protection. Thus a rotating machine mayincorporate a motor carcase in type of protection ‘d’(flameproof enclosure) and a terminal box in type of protection ‘e’ (increased safety). It has become commonusage to refer to ‘explosion-protected’ apparatus ratherthan to any one type of protection ( see also Section 2 ).

3.12.2 Enclosure Protection

The measures applied to the enclosures of apparatus to provide degrees of protection for :

a) persons against contact with live ormoving parts inside the enclosure andprotection for the apparatus againstingress of solid foreign bodies;

b) the apparatus against the ingress of liquids.

3.12.3 Electrical Protection

The measures applied to circuits to control theeffects of overload and overcurrent.

4 CLASSIFICATION OF HAZARDOUS AREAS

This code of practice is based on the concept of dealing with the risk of fire and explosion byarea classification. This concept recognizes thediffering degrees of probability with whichconcentrations of flammable gas or vapour mayarise in installations in terms of both thefrequency of occurrence and the probableduration of existence on each occasion.

The detailed considerations that should betaken into account in area classification aredescribed in IS 5572 (Part 1) : 1978. Forcompleteness, the definitions appropriate toarea classification are repeated here.

Zone 0 Zone in which an explosive gas-airmixture is continuously present, orpresent for long periods.

Zone 1 Zone in which an explosive gas-airmixture is likely to occur in normaloperation.

Zone 2 Zone in which an explosive gas-airmixture is not likely to occur innormal operation, and if it occurs it

will exist only for a short time.



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It should be noted that this area classificationdeals only with risks due to flammable gasesand vapours and, by implication, flammablemists. In a non-hazardous area normalelectrical techniques apply.

When the hazardous areas of a plant have beenclassified, the remainder will be defined asnon-hazardous.

5 TYPES OF PROTECTION

There are at present eight accepted types of protection for electrical apparatus for use inhazardous areas. A description of each type isgiven in Table 1. The relevant equipment

specifications, where they exist, should bereferred to for precise definitions of the type of protection.

Table 1 Types of Protection

Type ofProtection

Title Description Reference toIndian Standards

‘d’ Flameproof A method of protection where the enclosure for electricalapparatus will withstand an internal explosion of theflammable gas or vapour (for which it is designed) which mayenter it, without suffering damage and withoutcommunicating the internal flammation to the external

flammable gas or vapour for which it is designed, through any joints or structural openings in the enclosure.

IS 2148 : 1981

‘i’ Intrinsicallysafe apparatusor system

A protection technique based upon the restriction of electricalenergy within apparatus and in the interconnecting wiring,exposed to a potentially explosive atmosphere, to a levelbelow that which can cause ignition by either sparking orheating effects. Because of the method by which intrinsicsafety is achieved it is necessary that not only the electricalapparatus exposed to the potentially explosive atmospherebut also other (associated) electrical apparatus with which itis interconnected is suitably constructed.

IS 5780 : 1980

‘p’ Pressurizationcontinuousdilution andpressurised

rooms

A method of protection using the pressure of a protective gasto prevent the ingress of an external flammable atmosphereto a space which may contain a source of ignition and, wherenecessary, using continuous dilution of an atmosphere within

a space which contains a source of emission of gas which mayform an explosive atmosphere.

IS 7389 (Part 1) : 1976

‘e’ Increasedsafety

A method of protection by which additional measures areapplied to an electrical apparatus to give increased securityagainst the possibility of excessive temperatures and of theoccurrence of arcs and sparks during the service life of theapparatus. It applies only to an electrical apparatus, no partsof which produce sparks or arcs or exceed the limiting temperature in normal service.

IS 6381 : 1972

‘n’ Type ofprotection n

A type of protection applied to an electrical apparatus suchthat, in normal operation, it is not capable of igniting asurrounding explosive atmosphere, and a fault capable of causing ignition is not likely to occur.

IS 8289 : 1976

‘s’ Special

protection

A concept for those types of electrical apparatus that, by their

nature, do not comply with the constructional or otherrequirements specified for apparatus with established typesof protection, but which nevertheless can be shown, wherenecessary by test, to be suitable for use in hazardous areas inprescribed zones.

‘o’ Oil-immersed

A method of protection where electrical apparatus is madesafe by oil-immersion in the sense that flammable gases orvapours above the oil or outside the enclosure will not beignited.

IS 7693 : 1975

‘q’ Sand-filled A method of protection where the enclosure of electricalapparatus is filled with a mass of powdery material such that,if an arc occurs, the arc will not be liable to ignite the outerflammable atmosphere.

IS 7724 : 1975

NOTE — Other types of protection are under consideration internationally. These may include type of protection ‘h’

hermetically sealed and type of protection ‘m’ encapsulation.



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6 LIGHT METALS AS CONSTRUCTIONALMATERIALS

6.1 Particular consideration should be given tothe location of an apparatus that incorporateslight metals in the construction of its enclosure.

The propensity of such materials to give rise tosparking that is incentive under conditions of frictional contact has been well established.Reference should be made to Annex C forfurther detailed guidance.

Section 2 Selection of Apparatus

7 PROCEDURE FOR SELECTINGELECTRICAL APPARATUS

7.1 General

Hazardous area apparatus should be selectedfor use in accordance with each of the following criteria as appropriate:

a) classification of area;

b) temperature classification;

c) apparatus sub-grouping;

d) environmental conditions.

Selection procedures according to the abovecriteria are recommended in 7.2 to 7.5.

Special considerations may be required for anon-hazardous area apparatus that isassociated with hazardous area apparatus ( seeappropriate Indian Standards for intrinsicsafety and increased safety apparatus,respectively ).

7.2 Selection According to Classificationof Area

Reference should be to IS 5572 (Part 1) : 1978for details of area classification which results inthe division of the hazardous area into one ormore zones.

7.2.1 Having established the zones, the types of protection to be applied to the electricalapparatus should be selected in accordancewith Tables 2A and 2B.

Table 2A Selection of Apparatus According to Zone of Risk

Zone Type of Protection Reference to IS

0 ‘ia’ IS 5780 : 1980

1 Any explosion protectionsuitable for Zone 0 and

‘d’

‘ib’‘o’ ( see Note 1 )

‘p’ ( see Table 2B )

‘q’

‘s’

IS 2148 : 1981

IS 5780 : 1980IS 7693 : 1975

IS 7389 (Part 1) : 1976

IS 7724 : 1975

—

2 Any explosion protectionsuitable for Zones 0 or 1 and

‘e’ ( see Note 2 )

‘n’ ( see Note 2 )

‘p’ ( see Table 2B )

IS 6381 : 1972

IS 8289 : 1976

IS 7389 (Part 1) : 1976

NOTES

1 Oil-immersed apparatus may be used only in case its security will not be impaired by tilting or vibration of theapparatus.

2 For outdoor installations, the apparatus with type of protection ‘e’ and ‘n’ should be used with enclosures providing atleast the following degree of protection in accordance with IS 4691 : 1985:

a) IP 55 where there are uninsulated conducting parts internally, and

b) IP 44 for insulated parts.

3 A substantial saving in cost may be achieved by using apparatus with type of protection ‘n’ in Zone 2 areas.

Table 2B Minimum Actions of Failure of Protective Gas For Type of Protection ‘p’

( Clause 7.2.1 )

AreaClassification

Enclosure Does not ContainIgnition-Capable Apparatus

Enclosure ContainsIgnition-Capable Apparatus

Zone 1

Zone 2

Alarm

No action required

Alarm and switch off

Alarm



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7.3 Selection According to TemperatureClassification

When selecting apparatus according totemperature classification, the maximumsurface temperature of the T class of theapparatus (or, where the apparatus is marked

with a particular maximum surfacetemperature, that temperature) should notexceed the ignition temperature of the gases orvapours involved.

7.3.1 In certain cases, temperatures higherthan that of the marked temperature class canbe permitted, for example, for smallcomponents such as transistors or resistors,provided it is proved by tests or otherexperimental evidence that there is no risk of direct or indirect flammation, deterioration ordeformation by such high temperatures.

7.3.2 A T class is assigned to apparatus on thebasis of temperature rise tests assuming that

the apparatus may be used in an environmentwith an ambient temperature not exceeding 40°C. In special cases, apparatus may bedesigned on request to operate in anenvironment with a temperature of more than40°C. In this case the designed maximumpermitted temperature of the environment willbe clearly marked on the apparatus. TheT class will have been assigned on the basis of temperature rise tests and the assumption thatthe apparatus may operate at temperatures upto the designed maximum permittedtemperature of the environment.

7.3.3 When apparatus, which has had a T classassigned assuming a 40°C maximum ambienttemperature, is to be used at a higherenvironmental temperature, for example,because it is mounted on a hot surface, themaximum surface temperature of theapparatus should not exceed the ignitiontemperature of the gases or vapours likely to bepresent. It is also important to check with themanufacturer on the suitability of theapparatus to operate at these highertemperatures. For example, when apparatus isto be mounted under pipe-lagging, it isvirtually unaffected by the air temperature, butmay fail due to heat from the pipe.

7.3.4 The permitted maximum surface tempe-

ratures classified according to IS 13346 : 1992are given in Table 3.

7.3.5 An example can be given of therelationship between T class and ignition

temperature. Cyclohexane has an ignitiontemperature of 259°C and, therefore, assuming a maximum environmental temperature of 40°C, apparatus with a temperature class T2(that is, 300°C) would not be suitable whereasapparatus with a temperature class of T3 (that

is 200°C) would be suitable.NOTE — In special cases, apparatus can be markedwith a particular temperature as well as one of the six Tclasses. For example, in the case cited above if theapparatus were marked with a temperature of 259°C, aswell as the temperature class of T2, it would still besuitable for use in areas where a potentially explosiveatmosphere of cyclohexane might form.

7.4 Selection According to Apparatus Sub-Grouping

In the new groupings, agreed internationally,Group I is reserved for apparatus for minessusceptible to methane (firedamp) and istherefore outside the scope of this standard.Group II is reserved for apparatus for use in all

other places where there may be potentiallyexplosive atmospheres and as appropriate issub-grouped IIA, IIB and IIC.

7.4.1 It should be noted that apparatussub-grouping is applied to the technique of prevention of flame transmission andlimitation of energy. Apparatus with type of protection ‘s’ may be subject to sub-grouping where it used one or more of these techniques.The protective features of other types of protection apply equally to all industrial gases,subject only to temperature classification;apparatus sub-grouping is not thereforenecessary ( see also Section 6 ).

7.4.2 Apparatus may be protected by one ormore types of protection which may requireapparatus sub-grouping. Such apparatus willbe marked as described in 40.4. In thesecircumstances, the apparatus should beselected according to the apparatus sub-groupto which the flammable materials that may bepresent are allocated.

7.4.3 Apparatus should be used only withmaterials allocated to the appropriateapparatus sub-group, as indicated in Table 6( see Annex D ), whose ignition temperaturesare not less than the maximum temperature of the T class certified for the apparatus. Apparatus certified for a particular sub-groupmay also be used with materials allocated to alower risk sub-group, subject again toconsiderations of temperature classification.

7.4.4 It will sometimes be found necessary touse electrical apparatus in applications wherematerials may be present that have not yetbeen allocated to a sub-group. In thesecircumstances, expert advice should beobtained on the sub-group allocation of thematerials in question.

NOTE — In IS 2148 : 1981 and IS 9570: 1980 flammablegases and vapours are grouped or classified,respectively, according to the experimental data forlimiting safe gaps or igniting currents measured under

precisely specified conditions. In the case of IS 9570 : 1980 many materials are classified according

Table 3 Relationship Between T Class andMaximum Surface Temperature

T Class Maximum Surface Temperature

°C

T1

T2

T3

T4

T5

T6

450

300

200

135

100

85



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to their chemical similarity with gases and vapours thatare already classified on the basis of experimental data.Neither method of grouping takes into account the needfor surface temperature classification, since ignitiontemperatures generally are not related to othercombustion characteristics. It is therefore inaccurate toassume that any particular apparatus complying withthe design requirements for a group of gases could beused safely with all materials allocated to that group.For this reason, instead of grouping gases, an apparatusitself is now grouped according to design criteriaspecified in the appropriate Indian Standards such asIS 6381 : 1972 and IS 8289 : 1976.

7.5 Selection According to EnvironmentalConditions

Apparatus and its component parts should beconstructed so as to guard against electricaland mechanical failure in the intendedconditions of use. The integrity of someelectrical apparatus may be effected when

required to operate under temperature orpressure conditions outside those for which theapparatus has been constructed. In suchconditions further advice should be sought.

7.5.1 Particular attention should be given to the

need for protection against the weather, theingress of liquids and particulate matter,corrosion, the effect of solvents and the effect of heat from adjacent plant ( see also 27.3 to 27.5 ).

8 INTERCONNECTION OF APPARATUS

The safety of individual pieces of apparatusmay be influenced by any other apparatus withwhich it is interconnected. For such systemconsiderations, reference should be made toappropriate standards covering individualprotection concepts.

Section 3 General Installation Recommendations

9 GENERAL

9.1 In addition to any recommendations theremay be for installation in non-hazardous areas,further recommendations for installations inhazardous areas as a consequence of the natureof the environment and that are generallyapplicable to all types of protection aredescribed in 10 to 20.

9.2 The explosion protection of someinstallations, for example, those using electricsurface heating, pressurization ‘p’, etc, isdependent on the overall design of the finishedsystem. In such cases, the appropriate design

codes or standards should be used for guidanceto carry out the installation ( see B-3 ).

9.3 Alterations to apparatus or systems mayinvalidate any certificate or otherdocumentation relating to that apparatus orsystem. Such alterations should be made onlywith the agreement of the occupier inconsultation with the manufacturer whereappropriate.

9.4 There may be special requirements listed inthe certification documents ( see 40.2 ) that willaffect the method of installation. Installers andoccupiers should satisfy themselves that such

conditions are properly met.10 ACCESS FOR INSPECTION

Installations should be designed and theapparatus and materials installed with a viewto providing ease of access for inspection andmaintenance.

11 PLANS AND RECORDS

For each site a responsible person shouldmaintain and make available plans or recordsof the following items:

a) the classification and extent of hazardousareas together with the other information

as recommended in IS 5572 (Part 1) : 1978;

b) records sufficient to enable the explosionprotected equipment to be maintained inaccordance with its type of protection; and

c) the type, route and details of undergroundcables.

12 ELECTRICAL RATINGS

12.1 Electrical apparatus and materials shouldbe installed, used and maintained within theirelectrical ratings for power, voltage, current,frequency, duty and such other characteristicswhere non-compliance might jeopardize thesafety of the installation.

12.2 In respect of electrical apparatus fromoverseas suppliers, because of the differing nature of the supply networks, special careshould be taken that:

a) the voltage and frequency rating areappropriate to the supply system on whichthe apparatus is to be used; and

b) the temperature classification has beenestablished for the correct voltage,frequency, etc.

13 INSTALLATION OF APPARATUS

13.1 All apparatus should be installed with dueregard to the possibility of external mechanical

damage affecting the type of protection of theapparatus. Where equipment is to be installedin areas of high mechanical risk, additionalmeasures such as the provision of guards forlight transmitting parts, may be necessary.However, additional measures should notimpair the integrity of the type of protection.

13.2 Special installation conditions apply toany type of certified explosion protectedapparatus where the certificate number has asuffix marking of ‘B’ or ‘X’. The certificationdocuments should be studied to ascertain theconditions of installation. The manufacturersshould also have provided clear installation

instructions.



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14 INTEGRITY OF INSULATION

Care should be taken during installation tomaintain the degree of electrical insulationprovided by the construction of the apparatusso that the possibility of accidental sparking orarching is avoided.

15 AUTOMATIC OVERCURRENTPROTECTION

In general, all circuits and apparatus inhazardous areas should be provided withautomatic means of disconnection in the eventof overcurrent (short circuit and overload)conditions. However, apparatus with particulartypes of protection may have additional and/oralternative requirements, and reference shouldbe made to the recommendations in theappropriate Indian Standard. Suitableprotection should be fitted where there is adanger that three-phase apparatus may be

subjected to excess current during single-phaseoperation.

16 EARTHING AND EARTH FAULTPROTECTION

16.1 Earthing of Power Systems

Generally earthing of power distributionsystems should be in accordance with therecommendations of IS 3043 : 1987.

In typical industrial systems, for example415 V and 440 V, earth fault protection may beafforded by the overcurrent protective devices.However, where the earth loop impedance is toohigh for that method to be adopted, it will be

necessary to provide earth fault protection byother means such as residual current devices.Smaller rating systems do not require residualcurrent devices.

In higher voltage systems, for example, 3.3 kV and 6.6 kV, apparatus should have earth faultprotection in addition to overcurrent protection.

If environmental conditions could lead toaccelerated degradation of earth loop paths,anti-corrosive protection should be provided orthe earth loop impedance values decreasedaccordingly.

16.2 Earthing of Intrinsically Safe

Electrical Systems/Apparatus ‘i’Detailed recommendations will be covered laterin a separate standard.

16.3 Lightning Protection

Recommendations for lightning protection aregiven in IS 2309 : 1969.

16.4 Electrostatic Phenomena

Recommendations for the avoidance of ignitionrisks due to static electricity are given inIS 7689 : 1989.

16.5 Cathodic Protection

Recommendations for cathodic protection

systems are given in IS 8062 (Part 1) : 1976.

16.6 Protective Multiple Earthing (PME)

Where the power supply is directly from asystem that is protected by PME, specialprecautions may be required within thehazardous area and specialist advice should besought.

16.7 Interconnection of Earthing Systems

The power, lightning, and static earthing systems, where they exist in the same area,should be effectively connected together toensure as far as possible that all metal work ina particular area is at the same potential underall conditions.

17 ISOLATION

17.1 At a suitable point or points outside thehazardous area there should be single ormultiple means of isolating mains supplies tothe hazardous area.

17.2 A means of isolation of each circuit orgroup of circuits, to include isolation of theneutral, should be provided. Where the meansof isolation is located inside the hazardous areait should be provided with an appropriate typeof protection.

17.3 Labelling should be provided immediatelyadjacent to each means of isolation to permitrapid identification of the circuit or group of circuits thereby controlled. There should beeffective measures to prevent the restoration of supply to the apparatus whilst the risk of exposing live conductors to an explosiveatmosphere continues.

18 EMERGENCY SUPPLIES

Special precautions should be taken in theprovision of emergency supplies to electricalapparatus (for example emergency lighting,critical instruments, valves) that may berequired to operate during periods of powerfailure.

19 WIRING SYSTEMS

19.1 General

The types of wiring that in principle may beused for installations in hazardous areas aredescribed in this clause. The detailed

recommendations for permitted types of cableand their accessories are described in IS 5571 :1979.

For a Zone 0 installation, cabling will be eitherpart of an ‘ia’ systems or as recommended fortype of protection ‘s’ for Zone 0.

Types of wiring and systems that should beused in Zone 1 and 2 areas are:

a) cables drawn into screwed, solid drawn orseam welded steel conduit; and

b) cables that are otherwise suitablyprotected against mechanical damage.

NOTE — The sheath of a metal sheathed cable should

not be used as the neutral conductor.



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Examples of the types of cable which are inaccordance with the recommendations of (b) are:

1) thermoplastic or elastomer insulatedscreened or armoured cable without a loadsheath and with polyvinylchloride (PVC)polychroprene (PCP) or similar sheath

overall;2) cables enclosed in a seamless aluminium

sheath with or without armour, with anouter protective sheath ( see also Annex C );

3) mineral insulated metal sheathed cable( see also Annex C );

4) thermoplastic or elastomer insulatedflexible cable or cord with a flexiblemetallic screen or armour and a PVC, PCPor similar sheath overall; and

5) thermoplastic insulated cable withsemi-rigid sheath.

For type of protection ‘1’ (intrinsic safety) andfor apparatus used in Zone 2 only, other cablesmay be used.

19.2 General InstallationRecommendations for Wiring Systems

19.2.1 The wiring entry to the apparatusshould maintain the integrity of the type of protection of the apparatus.

19.2.2 Unused cable entries in electricalapparatus should be closed with plugs suitablefor the type of protection of the apparatus.

19.2.3 Where accessories (for example, a junction box) are used for the interconnection of cable, only those accessories that are suitablefor the zone concerned should be used.

19.2.4 Electrical continuity between metallicenclosures and conduit, armour or cablesheaths and armour, or across any joints in thecoduit or armour, should be maintained by theintegrity of the joint itself. If external bonding is in certain circumstances necessary, it shouldpreferably be connected directly across the joint.

19.2.5 Where circuits traverse a hazardousarea in passing from one non-hazardous area toanother, the wiring in the hazardous areashould be in accordance with therecommendations of this code.

19.2.6 Except for trace heating, fortuitouscontact between conduit or the metallicarmouring/sheathing of cables and pipe work orequipment containing flammable gases, vapouror liquids should be avoided. The insulationprovided by a non-metallic outer sheath on acable will usually be sufficient to avoid this.

19.2.7 Wiring should be installed, so far as ispracticable, in positions that will prevent itsbeing exposed to mechanical damages and(with the exception of trace heating cables) tothe effects of heat and to corrosive or solventagencies. Where exposure of this nature isunavoidable, appropriate protective measuresshould be taken.

19.2.8 All apparatus connections and wiring whose functions are not immediately apparentshould be clearly marked to enable theirrelationship to any associated apparatus,connections and wiring to be clearly recognized.

19.2.9 Where cables or conduit pass through a

floor, wall, partition or ceiling that forms a gasor fire barrier, the hole provided for themshould be made good with cement or similarincombustible material to the full thickness of the floor, wall, partition or ceiling. Alter-natively, cable glands or cable transits may beused for this purpose.

19.2.10 Where trunking, ducts, pipes ortrenches are used to accommodate cables,precautions should be taken to prevent thepassage of flammable gases, vapours or liquidsfrom one area to another and to prevent thecollection of flammable gases, vapours orliquids in trenches. Such precautions mayinvolve the sealing of trunking, ducts and pipes

and the adequate ventilation or sand filling of trenches.

19.2.11 For terminal connections to fixedapparatus that may be required to be moved asmall distance (for example, motors on slidsrails), cables arranged to permit the necessarymovement without detriment to the cable, orone of the types of cable suitable fortransportable apparatus may be used. If flexible conduit is used, it and its fittingsshould be so constructed and assembled thatdamage to the cable consequent upon its use isavoided. Adequate earthing or bonding shouldbe maintained other than by means of the

flexible conduit.19.2.12 Where an overhead line provides poweror telecommunication services to apparatus ina hazardous area it should be terminated in anon-hazardous area and the service continuedby cable. Suitable surge protection apparatusshould be fitted at or near the terminal point.The armouring or metal sheathing of the cableshould be electrically continuous, and the endadjacent to the point of connection with theover-head line should be bonded to the earthelectrode of the surge protection apparatus. Inaddition the armouring or metal sheathing should be independently earthed as near aspossible to the lighting protective system, if

any, associated with the hazardous area ( seeIS 2309 : 1969).

19.3 Environmental Factors AffectingChoice of Wiring System

19.3.1 Insulating materials may be attacked bycertain chemicals, and where contact may besufficiently long that deterioration could resultfrom such exposure, then consideration shouldbe given to the use of a protective sheath.

19.3.2 Where cables may be subjected toexcessive flexing, consideration should be givento the avoidance of fracture, for example, whenusing mineral insulated cable, by forming thecable into a loop close to the point of

termination.



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19.3.3 Where electrolytic corrosion of metalsheath may result from contact with walls orother surfaces to which the cable is attached, itshould be kept clear of such surfaces or coveredwith a protective sheath.

19.4 Conduit Systems

Where used, screwed heavy gauge steelconduit, solid drawn or seam welded, should bein accordance with the requirements of IS 9537(Part 2) : 1981.

Such screwed steel conduit systems aresatisfactory for many situations but should notbe used where vibration may cause fracture orloosening of joints or where excessive stressmay be imposed as a result of its rigidity of where corrosion or excessive internal con-densation of moisture is likely to occur. In suchcircumstances, flexible conduit of very heavymechanical strength should be used.

Where a run of conduit, irrespective of size,

passes from a hazardous area to anon-hazardous area, a stopper box or sealing device should be inserted at the hazardous areaboundary or, failing this, on the side remotefrom the hazardous area.

NOTE — The use of a sealing device is essential to themaintenance of certain types of protection.

Elbows of the solid types may be used for theimmediate connection of conduit to apparatus.

Surface mounted conduit should be supportedby spacing saddles.

All joints in an assembly of conduit should beprepared before or after assembly so that the

development of rust is inhibited and earthcontinuity and bonding is maintained.

All bends in conduit should be machine-madeand rounded so as to facilitate drawing in of cables.

For additional requirements for conduitsystems that are relevant to particular types of protection, appropriate standards may bereferred to.

19.5 Cable Systems

19.5.1 General

Conductors may be copper or aluminium butplain aluminium should not be used for

conductors with cross-sectional areas less than16 mm2 ( see also C-7 for precautions on the useof aluminium in Ex ‘d’ flameproof enclosures).

Cable runs should, where practicable, beuninterrupted. Where discontinuities cannot beavoided, the joint should be encapsulated ormade in a compound-filled joint box or beprotected in accordance with the requirementsof the zone.

Certain types of protection require specifictypes of cable glands. Compression type glandswould serve the purpose as the cables would beterminated in a terminal box. Direct entrywithout a terminal box shall be treated as a

special case.

Attention is drawn to the need to preventtransmission of flammable liquids or gasesfrom a hazardous to a non-hazardous areathrough the interstices of cables terminating inapparatus (for example pressure switches orcanned pumps) into which such fluids areintroduced. Under fault conditions (forexample, diaphragm or can failure) the fluidmay be released inside the apparatus underconsiderable pressure against whose effects itmay be difficult to seal the cable interstices. Apparatus of this kind should be deliberatelyvented to atmosphere, but if this is not possible(for example, a type ‘d’ enclosure) a specialsealing joint or length of mineral insulatedmetal sheathed cable should be introduced inthe cable run.

All cables should be provided with adequatemechanical protection. Cables should beadequately supported throughout their length,care being taken to avoid excessive pressure

when cleats are used. Horizontal cables may becarried on support or cable trays or throughprotective troughs or tubes. Rising cablesshould be clipped, cleated or otherwise attachedto suitable supports that provide adequatemechanical protection and support.

Where single core metal sheathed or armouredcables are used, precautions should be taken toavoid dangerous sheath voltages or currents.

19.5.2 PVC Covered and/or Insulated Cables

Attention is drawn to the fact that as thetemperature decreases, PVC becomesincreasingly stiff and brittle with the resultthat, if PVC cables are bent too sharply or arestruck at temperatures of about 0°C or lower,there is a risk for most PVC cables of shattering the PVC. Therefore, it is advisable to store PVCcable at temperatures above 0°C for at least24 h prior to installation ( see 19.3.1 ).

19.5.3 Paper Insulated Cables

Where paper insulated armoured cables areused, and particularly where such cables maybe exposed to high temperature, preferenceshould be given to non-draining cable. In thecase of other types of paper insulated armouredcables, vertical runs should be avoided.

Cable boxes should, if necessary, be arranged

for sealing the cable insulation and should beprovided, as necessary, with adequate meansfor ensuring the earth continuity of metallicsheath and/or armour.

Adequate arrangements should be made toensure that cable boxes can be completely filledwith the necessary compound or insulating oilaccording to their design,

19.5.4 Mineral Insulated Metal SheathedCables

Cable terminations should incorporate a sealfor the cable insulation and, where necessary,means of ensuring adequate earth continuity.

NOTE — Certain types of protection require specific

types of seal.



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Seals in which compound is used should not beexposed to temperature that may, in the courseof normal working, affect the satisfactoryfunctioning of such compound.

Where there is a risk of mineral insulatedcables being exposed to excessive voltages suchas inductive surges, surge suppression shouldbe fitted. Where surge suppression devices areinstalled in hazardous areas, they should besuitably exposition protected.

19.5.5 Aluminium Sheathed Cables

Aluminium sheathed cables, unless sheathwith a protective covering, should not beinstalled in contact with walls or floors.Consideration should be given to the avoidanceof frictional contact with such cables ( see Annex C ).

20 PORTABLE AND TRANSPORTABLE APPARATUS AND ITS CONNECTIONS

Apparatus other than fixed apparatus shouldhave a type of protection appropriate to thezone(s) of use. During use such apparatusshould not be transferred from a zone of lowerrisk to a zone of higher risk unless it is suitablyprotected for the higher risk. Additionally, theapparatus group and T class should be

appropriate to all of the gases and vapours inwhich the apparatus may be used.

NOTE — Ordinary industrial portable or transportableapparatus, welding equipment, etc, should not be usedin a hazardous area unless its use is undertaken undera controlled procedure and the specific location has beenassessed to ensure that potentially flammable gas or

vapour is absent.

Should plugs and sockets be in a hazardousarea, they should be suitable for use in theparticular zone of risk and should havemechanical and/or electrical interlocking toprevent danger during insertion or removal of the plug.

In many cases the type of cable to be used willbe stated on the apparatus certificate. In caseswhere this is not so, the cable should besuitable for rough usage and other adverseenvironmental conditions (for example,elevated temperatures, presence of solvents). Itshould also be suitable for the circuit protective

arrangements (for example, where earthmonitoring is used, the necessary number of conductors should be included). Where theapparatus needs to be earthed, the cable mayinclude an angular earthed flexible metallicscreen in addition to the earth conductor.

Section 4 Inspection, Maintenance and Testing

21 GENERAL

21.1 To minimize the risk of ignition of anexplosive atmosphere by electrical apparatus,efficient inspection, maintenance and testing of the apparatus, systems and installations are

essential. It should be noted that correctfunctional operation does not itself indicateconformity with the recommendations for thesafe use of apparatus. The general recommen-dations for inspection, maintenance and testing that are applicable to all types of protection aredescribed in 22 to 27.

22 PERSONNEL

22.1 The inspection, testing, maintenance,replacement and repair of apparatus, systemsand installations should be carried out only bypersons whose training includes instruction onthe various types of protection involved. Appropriate refresher training should be givenfrom time to time.

23 ISOLATION OF APPARATUS

23.1 No apparatus should be opened in ahazardous area other than apparatus with typeof protection ‘i’ (intrinsic safety) and non-incendive apparatus until it has beendisconnected from its source of supply ( see 17 )and effective measures, such as the locking of the disconnector in the open position or fuseremoval, have been taken to prevent its being made alive before re-assembly. Particularattention should be paid in the case of anapparatus that may be live even after it has

been disconnected from a source of supply.Where heavy rotating machinery is involved,the back e.m.f. of such plant should beconsidered and precautions will usually need tobe taken to ensure that the apparatus, or anyapparatus associated with it, is not opened

until the rotating plant is stationary. Mostpower capacitors are fitted with dischargeresistors and it should be noted that these takea finite time to bring the terminal voltage to aharmless value.

23.2 Where for purpose of electrical testing it isessential to restore the supply before theapparatus is re-assembled, then this workwould be under a controlled procedure and thespecific location assessed to ensure thatpotentially flammable gas or vapour is absent.For the testing of intrinsically safe electricalapparatus and systems see IS 5780 : 1980.

24 PRECAUTIONS CONCERNING THE

USE OF IGNITING AGENCIES24.1 No operation involving the use of an openflame or other source of ignition should beattempted in a hazardous area until theconditions have been made safe by the controlof the flammable material that may give rise tothe risk. Such operations should be undertakenonly on the issue of a gas free certificate,confirming that adequate control measureshave been taken and that tests have been madeand will be repeated at sufficiently frequentintervals to ensure that safe conditions aremaintained.



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25 INITIAL AND PERIODICINSPECTIONS

25.1 All electrical apparatus, systems andinstallations should be inspected prior tocommissioning to check that the selection andinstallation is appropriate and in accordancewith IS 5571 : 1979. Examples of some of theitems which should be included in an initialinspection are given in Table 4.

25.2 Following any replacement, repair, modifi-cation or adjustment, the items concernedshould be inspected, to check that theappropriate recommendations of the code havebeen maintained.

25.3 If at any time there is a change in the areaclassification or in the characteristic of theflammable material used in the area, or if anyapparatus is moved from one location toanother, a check should be made to ensure thatthe type of protection, apparatus group andT class, where appropriate, are suitable for therevised conditions.25.4 The extent, complexity and frequency of inspection of apparatus, systems andinstallations when in use should be determinedby those whose training and experience willenable them to recognize any potential hazardsand who are familiar with the local environ-mental conditions and use.

25.5 The frequency of inspection depends onthe type of equipment, the factors governing itsdeterioration, and the finding of previousinspections. Examples of factors which canaffect the deterioration of apparatus orotherwise lead to an unsafe condition are givenin Table 5.

25.6 It is, therefore, recommended that oncethe apparatus has been taken into use, initialintervals between inspections should be of areasonably short duration and that a system beestablished to enable subsequent inspectionintervals to be reviewed and modified in thelight of operational experience. Whereinspection intervals and methods are alreadyestablished and are found by experience to besatisfactory for similar apparatus andenvironments, these may continue to be used.It is recommended that an inspection recordsystem be operated so that the review of inspection frequencies is effective.

25.7 When large number of similar items suchas luminaires, junction boxes, etc, are installed,it may be feasible to carry out inspections on asample basis provided that the degree of sampling in addition to the inspectionfrequency is subjected to review.

Table 4 Example of Schedule for Initial Inspection

( Clause 25.1)

Initial Inspection Items

Apparatus appropriate for the area classification

Correct temperature classification Appropriate apparatus group or sub-groupCorrect circuit identificationMaintenance of integrity of enclosureCable entries and stoppers, etc, are completeElectrical connections are tightSatisfactory earthing Correct rating of apparatus and componentsDamage to installation

Adequate environmental protection (e.g. against weather, mechanical damage)No unauthorized modifications

NOTE — Items are not listed in any order of priority. Where integrity of type of protection is dependent on electricalprotection, this should be checked initially.

Table 5 Example of Schedule for Subsequent Inspections

( Clause 25.5 )

Factors Affecting Integrity of Apparatus, Systems and InstallationsCorrosion of enclosures, fixings, cable entries, etcUndue accumulation of dust and dirtLoose electrical connections including those for earthing Loose fixings, glands, conduit, stoppers, etcCondition of enclosure gaskets and fasteningsLeakage of oil or compoundCondition of bearingsInadvertent contact between rotating and fixed partsIntegrity of guardsIncorrect lamp ratings or typeUndue vibrationMalfunction of relays and protective devicesUnauthorized modifications or adjustmentsInappropriate maintenance, e.g. not in accordance with manufacturer’s recommendations

NOTE — Items are not listed in any order of priority. Where integrity of type of protection is dependent on electricalprotection, this should be checked initially.



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26 INITIAL AND PERIODIC TESTING

26.1 All electrical apparatus, systems andinstallations should be tested prior tocommissioning and these tests should includethe following:

a) insulation resistance measurement;

b) earth electrode resistance measurement;c) earth loop impedance measurement; andNOTE — It may be advantageous to make the earth loopimpedance measurement with both a substantial currentand with an intrinsically safe tester to enable themeasurement to be made with the intrinsically safetester alone for subsequent tests and to allow thoseresults to be compared with the initial test results. Itshould be noted that tests made with an intrinsically safetester may not necessarily identify certain badconnections because of the low test current.

d) operation and setting of protective devices.

The results of all tests should be recorded andincluded in the record of inspections describedin 25.

26.2 The testing of apparatus, systems andinstallations when in use should be carried outby those whose experience will enable them torecognize the potential hazards and who arefamiliar with local environmental conditionsand use.

26.3 Test procedures should be designed tocause the minimum disturbance to theinstallation or system.

26.4 Diagnostic testing and recording of resultsof such functions as time, operating voltage,circuit resistance, insulation resistance, etc,should be used where possible to show trends inelectrical condition and to aid the determinationof both the nature and frequency of subsequent

tests and inspections.26.5 The frequency of testing depends on thetype of equipment or system, the factorsgoverning its deterioration and in particular thefinds of previous tests. Factors which may affectthe deterioration of apparatus are identified inTable 6 and as these should be taken intoaccount in determining the frequency of inspection, the frequency of testing will not begreater than, and in fact may be less than, thefrequency of inspection for the relevantapparatus or system. The frequency of testing should be established and reviewed in a mannersimilar to that recommended for the inspection.

26.6 Testing in continuously operating plantand on similar apparatus or systems may beundertaken on a sample basis subject to reviewsimilar to that recommended for inspectionsconducted on a sample basis.

26.7 Tests should be made in such a way thatthe safety devices used in low energy or lowvoltage apparatus and circuits are not subject todamage by excess voltage. For testing intrinsically safe electrical systems ‘i’ seeIS 5780 : 1980.

26.8 Test should be undertaken under acontrolled procedure to ensure that the specificlocation has either been assessed for the absence

of a flammable gas or vapour or that the testmethod is non-incendive.

27 MAINTENANCE RECOMMENDATIONS

27.1 Alterations to Apparatus

The general condition of all apparatus should benoted periodically as recommended in 25 andappropriate remedial measures should be takenwhere necessary. Care should be taken,

however, to maintain the integrity of the type of protection provided for the apparatus; this mayrequire consultation with the originalmanufacturer. Replacement parts should be inaccordance with the conditions of certification.No alteration that might invalidate thecertificate or other document relating to thesafety of apparatus should be made to anyapparatus without appropriate approval.

27.2 Maintenance of Flexible Cables

Flexible cables, flexible conduits and theirterminations should be inspected at regularintervals and should be replaced if found to bedamaged or defective.

27.3 Precautions Against CorrosionMetallic enclosures of apparatus should wherenecessary, be treated with an appropriateprotective coating as a precaution againstcorrosion, the frequency and nature of suchtreatment being determined by theenvironmental conditions.

NOTE — For special precautions concerning the use of certain protective materials (e.g. on the flange faces of Ex‘d’ flameproof apparatus) reference should be made toIS 2148 : 1981.

27.4 Cleanliness

All parts of installations should be kept cleanand free from accumulations of dust anddeleterious substances.

27.5 Precautions Against ExcessiveTemperature Rise

Apparatus should be kept of free from dustdeposits of such a nature as could causeexcessive rise in temperature.

When replacing lamps in luminaires the correctrating and type should be used for excessivetemperature rise may result.

The etching, painting or screening of lighttransmitting parts may lead to excessivetemperatures.

27.6 Withdrawal from Service

Should it be necessary for maintenancepurposes to withdraw apparatus, etc, fromservice, the exposed conductors should beterminated in an appropriate certifiedenclosure; alternatively, the cable may beprotected adequately by insulating the ends of the conductors and isolating the cable from allsources of power supply. Should the apparatusbe permanently withdrawn from service,associated wiring should be removed or,alternatively, otherwise correctly terminated inan appropriate certified enclosure.

27.7 Fastenings and Tools

Where special bolts and other fastenings or

special tools are required, these items should beavailable and should be used.



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Section 5 Properties of Flammable Liquids, Vapours and Gases

28 GENERAL

28.1 Available data applicable only to the use of electrical apparatus in hazardous areas aregiven in Table 6 ( see Annex D ) for thoseflammable gases and vapours that have been

allocated to apparatus groups. The physicalproperties of these materials that have to beconsidered when the degree of risk appropriateto a particular application or installation isbeing assessed are defined and discussed in 29to 35 in the same order as that in which theyare given in Table 6 [ see also IS 5572(Part 1) : 1978 ].

28.2 Properties of the materials given inTable 6 are generally for materials in the pureform and may be different if there areimpurities or where there are mixtures of materials. In such cases expert advice shouldbe sought. For descriptions of the concepts of temperature classification and apparatusgrouping, see 7.3 and 7.4.

29 MELTING POINT AND BOILINGPOINT

29.1 The melting and boiling points have theirusual meanings, and the data listed in Table 6apply at standard atmospheric pressure.

30 RELATIVE VAPOUR DENSITY

30.1 The relative vapour density of a materialis the mass of a given volume of the material inits gaseous or vapour form compared with themass of an equal volume of dry air at the sametemperature and pressure. It is often calculated

as the ratio of the relative molecular mass of the material to the average relative molecularmass of air (the value of the latter being approximately 29).

31 FLASH POINT

31.1 General

The flash point of a material is the minimumtemperature at which it gives off sufficientvapour to form a flammable mixture with airnear the surface of the material or within theapparatus used for flash point determination.

31.1.1 Flash point data are normally associatedwith materials in the liquid phase. There are a

few materials, however, that give off sufficientvapour in the solid phase to form flammablemixtures with air. For those materials andthose that sublimate (i.e. pass from solid tovapour without the normal intermediate liquidphase), flash point data will be associated withthe materials in their solid form.

31.1.2 The test apparatus used for themeasurement of flash point is normally one of two types, of which there are several variants.These are called generally open cup and closedcup flash point testers. For most liquids theflash point determined by the closed cupmethod is slightly lower (in the region of 5% to10% when measured in °C) than that

determined by the open cup method. Flash

points measured by the more sensitive closedcup method are, therefore, normally used andare given in Table 6.

31.2 Materials having High Flash Points

Some materials have such high flash pointsthat they do not form flammable mixtures withair at normal ambient temperatures, evenwhen exposed to the sun in tropical locations.These should not be discounted as ignitionhazards, however, since exposure to a suitablyhot surface or use of the material at a tem-perature above its flash point may create aflammable mixture locally, which may beignited by the same hot surface or analternative ignition source. It is thereforenecessary to consider the limitation of surfacetemperatures even when materials of highflash point are being processed.

31.2.1 It should be noted also that materials

having high flash points may be used inprocesses involving high temperatures andpossibly high pressures. The normal oraccidental release to the atmosphere of compounds under such conditions may presentlocal explosion risks that would not normally beassociated with high flash point materials.Materials having high flash points can formflammable mixtures with air at subatmosphericpressure.

31.3 Classification of Flash Points

In some industries, it has been foundconvenient to group materials into prescribedranges of flammability according to their flash

points to facilitate safe handling. In certainapplications, legislation specifies the limits forthese ranges.

32 LIMITS OF FLAMMABILITY

32.1 All combustible gases and vapours arecharacterized by flammable limits betweenwhich the gas or vapour mixed with air iscapable of sustaining the propagation of flame.

32.2 The limits are called the lower flammablelimit (LFL) and the upper or flammable limit(UFL) and are usually expressed aspercentages of the material mixed with air byvolume. They are also sometimes, expressed as

milligrams of material per litre of air. Whereappropriate, both sets of data are included inTable 6.

33 FLAMMABILITY RANGE

33.1 The range of gas or vapour mixtures withair between the flammable limits over whichthe gas/air mixture is continuously flammableis called the flammability range. Gas/airmixture outside this range are, therefore,non-flammable under normal atmosphericconditions. Concentrations above the UFL infree atmospheric conditions cannot becontrolled and further dilution with air willproduce mixtures within the flammability

range.



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34 EFFECT OF ENVIRONMENTS WITHOTHER THAN NORMAL ATMOSPHERICCONDITIONS

34.1 It should be noted that the data given inTable 6 apply only to mixtures of flammablegases and vapours with air under normalconditions of atmospheric temperature and

pressure or at suitably elevated temperatures if the flash point of the vapour is above thenormal ambient temperature. Caution shouldbe exercised therefore in assessing theexplosivity of gas or vapour with air underenvironmental conditions that are other thannormal. It is possible here to give only generalguidance on the influence of changes intemperature, pressure and oxygen content of the mixture.

34.2 Generally, the effect of increasedtemperature of pressure is to lower the LFLand to raise the UFL. Reduction in temperatureor pressure has the opposite effect.

34.3 An increase in oxygen content of a gasmixture, compared with a mixture of theflammable gas or vapour with air only, haslittle or no effect on the LFL but generallyresults in an increase UFL. The increase in theupper limit depends on the degree of oxygenenrichment and may be substantial. Thus, theeffect generally of an increase in oxygencontent is to broaden the flammability range.

35 IGNITION TEMPERATURE

35.1 The ignition temperature of a material isthe minimum temperature under prescribedtest conditions at which the material will igniteand sustain combustion when mixed with air at

normal pressure, without initiation of ignitionby spark or flame.

35.2 The ignition temperature, formerly knownas the auto-ignition or spontaneous ignitiontemperature, should be clearly distinguishedfrom the flash point. In the latter case, ignitionis initiated by a small flame simply todetermine that a flammable mixture exists. Inthe former, ignition is a consequence of chemical reactions initiated on account of thetemperature of the local environment and maytherefore in practice be a result of thetemperature of hot surfaces adjacent to theflammable atmosphere.

35.3 The majority of ignition temperatureslisted in Table 6 have been determined with thetest apparatus and procedure described inIS 7820 : 1975 or with equivalent testapparatus. Where a determination by a methodother than that in IS 7820 : 1975 provides theonly data available, the classification may beregarded as provisional only and has been, forguidance, enclosed in parentheses in Table 6.

35.4 The direct result of established ignitiontemperatures is to limit the surfacetemperatures of electrical apparatus inhazardous areas so that these do not present anignition risk. Formerly permitted surfacetemperatures were limited to a certain

proportion of the measured ignitiontemperature (commonly 80%) to provide a

factor of safety. It is now generally accepted,however, that the sensitivity of the recognizedtest methods is such that the temperatures of unprotected surfaces of electrical apparatusmay safely be allowed to rise to the ignitiontemperature of the gas or vapour that presentsthe explosion risk. Where more than one

flammable material may be present in aparticular application, the surface temperatureshould be limited to the lowest value of theignition temperatures of the combustiblesconcerned or the ignition temperature of theparticular mixture as determined by test.However where there is a possibility of catalyticinteraction between the components or wheremixtures of hydrogen, moisture orhydrocarbons with carbon monoxide occur, thesurface temperature may need to be less thanthe lowest ignition temperature of theindividual components.

35.5 It should be noted that the value forignition temperature is dependent on the

method chosen for its measurement. Inparticular, factor such as the geometry,dimensions and materials of the test apparatusinfluence the measured ignition temperature.Care should be exercised, therefore, if theignition temperature data contained in thisstandard are applied to equipment other thanelectrical apparatus.

36 GENERAL CONSIDERATIONS

36.1 Relation Between Ignition Temperatureand Maximum Surface Temperature

The vapour given off from a flammable liquidwill form a flammable mixture with air,provided the temperature of the liquid is at or

above its flash point. The flammable mixturemay then be ignited by one of severa1 means: aflame, a suitable frictional spark, an electricalspark of sufficient energy or a hot surface. If, onthe other hand, the local ambient temperatureand that of the electrical apparatus, etc, arebelow the flashpoint, the vapour will eventuallycondense to a mist of liquid droplets and spreadas such both through the atmosphere and overthe surfaces of the apparatus. It is in the latterrespect that adequate resistance to chemicalattack may be particularly important.

36.1.1 For ignition by a hot surface, the surfacetemperature has generally to be greater than

the ignition temperature of the flammablematerial. Therefore, to ensure that ignition byhot surfaces does not occur, it is necessary thatthe temperature of all unprotected surfacesexposed to the gas or the vapour/air mixtureshould not be greater than the ignitiontemperature. This has led to the concept of temperature classification described in 7.3.

NOTE — Surfaces that are catalytically active canignite vapours at temperatures lower than the normalignition temperature.

36.2 Mixtures of Materials

Single-component flammable materials are notoften encountered in practice. Most frequently,

mixtures of two or more materials are present,in ratios that may vary between prescribed



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limits. Consideration has then to be given tothe characteristics required for electricalapparatus in the light of the properties of eachindividual component present. Often this willimpose no difficulty since, by the nature of theprocess, the various materials will possesssimilar chemical properties and, often, similar

combustion properties.36.2.1 There are occasions, however, when thisis not the case. The materials may be of different gas classifications or have widelydifferent ignition temperatures. In these cases,it is possible to give only the most general of rules for guidance. In general it should beassumed that, at some time during the processor the life of the plant, the component in themixture having the most demanding of thecharacteristics being considered (e.g. the gasclassification, the flammable limits, the flashpoint or ignition temperature) will be presentas the largest proportion of the mixture, and

the electrical installation should be designedaccordingly.

36.2.2 However, this can impose limitationsthat may be severe, and further considerationof the relative rates and quantities of thematerials used in the process and the degree of control thereof may be required. Somerelaxation may then be possible, but expertadvice should always be sought in thesecircumstances.

36.2.3 Particular consideration should be givento those materials whose behaviour may beanomalous. It is known, for example, thatcarbon monoxide, with which Group IIA

apparatus may be safely used, may be added inconsiderable quantity to hydrogen without

altering the group of apparatus, namely GroupIIC, that has to be used with this lattercompound. Carbon monoxide also exhibitsunusual behaviour under other test conditions.For example, it has been shown that theaddition of moisture to mixtures of carbonmonoxide with air to the point of saturation

serves to change the gas classification for thismaterial from Group IIA to Group IIB. Thischange in gas classification is also observed if methane is added to carbon monoxide in theproportion 15 : 85 methane to carbon monoxide.

36.2.4 When the individual components of mixtures and their proportions of the totalmixture are precisely known or can besustained, it is often possible to calculate theresultant flammable limits for the mixture withair. Examples of this care described in Annex E. However, if the mixture ispredominantly carbon monoxide, expert adviceshould be sought.

36.3 Mists

The characteristics described in this standardapply to mixtures of gases and vapours onlywith air. The distinction to be drawn between agas and a vapour in this context is simply thatthe latter may be in contact with its liquidphase at normal temperature and pressure,whereas a gas cannot be liquefied under normalatmospheric conditions. In practice, mistsconsisting of clouds of condensed vapour canalso occur. In general, the characteristicsdescribed in this standard should be consideredapplicable to mists, since local ignition sourcesor hot surfaces generally may serve to restore

the condensed material to its vapour phase.

Section 6 Marking of Apparatus

37 GENERAL

37.1 It has been agreed internationally, thatthe letters ‘Ex’ should be used to indicateexplosion protection and that this symbolshould be accompanied by a lower case better toindicate the type of protection used. The lettercode is described in 5.

37.2 The marking requirements for the varioustypes of explosion protected apparatus shall bein accordance with IS 13346 : 1992.

38 MARKING REQUIREMENTS FOR APPARATUS

38.1 The general marking requirementsdescribed in this clause are intended to aid theuser in identifying the type of protection and toassist identification of the particular explosionrisk for which the apparatus is suitable.

38.2 The marking requirements include thefollowing :

a) Identification of the manufacturer. Themanufacturer may be identified by nameor trademark.

b) The manufacturer’s name or typedesignation for the apparatus.

c) The certifying authority mark and thecertificate number.

d) identification of the type of protection( see 39 ).

e) The apparatus sub-group, if applicable.The system of marking for indicating apparatus sub-grouping is described in 39.

f) The T class or maximum surfacetemperature. This requirement is

described in 7.3.

g) Any other, relevant information, will nor-mally include voltage and current ratings,under prescribed conditions wherenecessary, and such other information asmay be necessary for the satisfactoryoperation of the apparatus.

38.3 The marking includes, therefore, not onlythe manufacturer’s name and rating data withwhich all electrical apparatus should normallybe marked, but also sufficient additionalinformation to indicate the suitability of theapparatus for use in particular flammable

atmospheres. This is explained in 39 and 40.



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39 EXAMPLES OF MARKING OF APPARATUS

39.1 General

The following examples are included toillustrate the application and interpretation of

the marking requirements. They do notrepresent all the variations of marking that arepossible, nor is it intended to include in theseexamples all the marking requirements thatare usually specified. More detailed marking requirements that are relevant to particulartypes of protection are described in theappropriate standard and in the relevantapparatus standards.

39.2 Flameproof Enclosure ‘d’

The marking on apparatus with type of protection ‘d’ will normally include thefollowing:

a) The symbol for the type of protection ‘d’;b) The apparatus group and sub-group; and

c) The temperature classification.

For example, EExd IIB T5 indicates that theenclosure complies with Group IIBrequirements and that the apparatus has atemperature classification of T5.

39.3 Increased Safety Apparatus ‘e’

The marking on apparatus with type of protection ‘e’ will normally include thefollowing to indicate compliance with therequirements of IS 6381 : 1972:

a) The symbol for the type of protection ‘e’;b) The apparatus group; and


For example, EEx e II T3 indicates that theapparatus may be used safely with allflammable gases and vapours allocated toapparatus Group II, subject to a temperatureclassification of T3.

39.4 Intrinsic Safety ‘i’

39.4.1 Intrinsically Safe Apparatus

Intrinsically safe apparatus can be category ‘ia’or ‘ib’ and may be suitable for connection to

external circuits subject to certain possiblelimitations on their characteristics.

The marking on apparatus with type of protection ‘i’ will normally include the following to indicate compliance with the requirements of IS 5780 : 1990:

a) The symbol for the type of protection ‘i’and the apparatus category;

b) The apparatus group and sub-group; and


For example EEx ia IIC T5 indicates that theapparatus group is IIC and that the apparatushas a temperature classification of T5.

Additionally, there may be marking to indicatethe limiting characteristics of external circuits,e.g.

Lext = 12 µH, Cext = 1 200 pF, U max = 28 V.

The second mark indicates the limiting values

of inductance, capacitance and voltage of theexternal circuits which may be connected to theapparatus to maintain its safe use under theconditions indicated by the first mark.

39.4.2 Associated Electrical Apparatus

Associated electrical apparatus installed in anon-hazardous area can be category ‘ia’ or ‘ib’.This limitation in its use is denoted on themarking by enclosing the applicable symbols insquare brackets.

The marking on such apparatus includes thefollowing:

a) The symbols of the type of protection ‘i’and the apparatus category; and

b) The apparatus group and sub-group.

For example, (EEx ib) IIC indicates compliancewith IS 5780 : 1980. There is no requirement tomark an apparatus temperature classificationsince the apparatus is installed in anon-hazardous area.

39.4.3 Intrinsically Safe Electrical System ‘i’

Certified intrinsically safe electrical systems ‘i’which comply with all the requirements forIS 5780 : 1980 will be marked as described in40.4.1 and additionally will be marked in a

strategic position with the letters SYST.For intrinsically safe electrical systems ‘i’complying with the requirements of IS 5780 :1980 which have not been certified, marking which permits positive identification of thesystem and reference for the relevant systemdescription will normally be applied.

39.5 Type of Protection ‘n’

The marking in accordance with IS 8289 : 1976on this apparatus will normally include thefollowing to indicate compliance with therequirements of that standard:

a) The symbol of the type of protection ‘n’

and the apparatus category;b) The apparatus group; and


For example, Ex n II T3 indicates that theapparatus may be used safely in Zone 2 areaswith all flammable gases and vapours allocatedto apparatus Group II, subject to a temperatureclassification of T3.

39.6 Other Types of Protection

The types of protection that have not beenincluded in the examples in 39.2 to 39.5 willnormally be marked in accordance with similarrules.



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40 GENERAL NOTES ON MARKING

40.1 General

In 40.2 to 40.4 some of the finer details of marking requirements that have not been

indicated in the examples in 39.2 to 39.4 areexplained.

40.2 Certificate Number

The certificate number may refer to a singledesign or to a range of apparatus of similardesign. The same number may also be used formodifications to the original design.

Where the certificate number is followed by ‘X’,it indicates that special conditions for safe useapply, and therefore particular attentionshould be paid to certification documents beforeapparatus is selected.

The certificate number may also be followed by‘S’. This suffix indicates that the apparatus somarked is suitable for installation in a safearea only, but that it may form part of acertified system.

40.3 Ex component Approval

Component parts or sub-assemblies forming part of certified apparatus may be the subject of separate component certificates. Thisprocedure is called ‘component approval’. Approval Ex components will generally bemarked in accordance with the rules describedin 37 to 39, with the addition of the letter ‘U’

after the approval number, but will notnormally carry the testing house certificationmark.

40.4 Multiple Types of Protection

40.4.1 Increasingly, electrical apparatus for

hazardous areas may incorporate more thanone type of protection. The examples in 40.4.2and 40.4.3 serve to illustrate the marking requirements that will normally be applied.

40.4.2 A flameproof apparatus such as a motormay incorporate an increased safety terminalbox. In this case, where each type of protectionis readily and separately identifiable, eachenclosure will normally be marked accordingly.

40.4.3 An apparatus with type of protection ‘e’(increased safety) overall may incorporatesparking contacts protected by a flameproof enclosure (type of protection ‘d’). In this case,the types of protection that are used internallycannot be identified by external examinationother than by inspection of the marking plate.The external marking will indicate the overallprotection, which will determine theinstallation requirements, and may indicatethe additional protective features in asecondary manner. For example, an increasedsafety apparatus with internal flameproof enclosure having a T3 temperatureclassification would normally be marked asfollows:

EEx e d IIC T3.



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