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Area Classification Standards and Instrument Selection

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  • 7/31/2019 Area Classification Standards and Instrument Selection


    6.0 Classification of Hazardous Materials

    6.1 Hazardous Material Classification :

    The NEC (National Electrical Council) has classified all materials capable of forming an

    explosive atmosphere into three major classes:

    Class 1 - A Class 1 atmosphere hazard is an area consisting of gases and liquids, which

    have been further, divided into four groups.

    1. Group A (Acetylene)

    2. Group B (Hydrogen)

    3. Group C (Ethylene)

    4. Group D (Methane)

    Class 2 - A class 2 hazard covers 3 different groups of combustibles dusts based upon their


    1. Group E (Medal dust)

    2. Group F (Coal dust)3. Group G (Grain dust)

    Class 3 - A class 3 hazards covers location where combustible fibres or lying arepresents.

    No sub groups.

    6.2 Gas Grouping :

    All gases and vapours can be classified in to 4 major gas groups. The classification criteria

    are based upon laboratory tests to determine the maximum Experimental Safe Gap (MESG)and the Minimum Ignition Current (MIC). Gas group classification for typical gases will be :


    Group as per

    IS 2206


    IEC 79-1

    USA UL698

    Methane(mining use)

    I D

    Propane IIA D

    Ethylene IIB C

    Hydrogen IIC B

    Carbon IIC -

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    Acetylene IIC A

    Gases belonging to the IIC group are the most dangerous with the severity decreasingdown the scale to the IIA group. The performance and the testing requirements of electrical

    equipment is consequently more stringent for application in areas containing IIC gases thanin areas consisting IIA gases.

    6.3 Classification of Flammable Liquids :

    All flammable liquids vary in volatility and have a flash point below 930C and a vapour

    pressure not exceeding 2.81 kg/sq.m. These liquids can be divided into 3 classes:

    Class A - Flammable liquids have flash point below 230C. These liquids produce large

    volumes of vapour when released in appreciable quantities to the open.

    Class B - Flammable liquids having a flash point between 230C & 650C. These liquids areheavier and less volatile and have a flash point slightly below normal ambient temperature.At elevated temperatures, Class b liquids approach the characteristics of class A liquids in

    vapour release.

    Class C - Flammable liquids having flash point between 650C and 930C . These liquids have

    a low degree of hazards because the rate of release is nil at normal ambient temperature.These include a broad range from cleaning solvents to heavy oil in commercial grades.

    6.4 Properties of Flammable Materials :

    A complete knowledge of physical properties of flammable materials involved is essential forclassifying a hazardous area. Properties of primary interest from an ignition standpointareas below. Some of the characters have direct influence on the degree and extent of

    hazardous areas while the other affects the design of electrical equipment.

    Relative Density - Where a substantial volume of gas or vapour is released into the

    atmosphere from a localized source a relative density less than 1 i.e. lighter than air for thecombustible indicates that the gas or vapour will rise in a comparatively still atmosphere. A

    vapour density greater than 1 i.e. heavier than air indicates the gas or vapour will tend tosink and may thereby spread some distance horizontally and at low level. The latter affects

    will increase with compounds of greater relative vapour density.

    Flammable Limit -The lower the lower flammable limit larger may be the extent of

    hazardous area.

    Flash Point- A flammable atmosphere cannot exist if the flash points is significantly above

    the relevant maximum temperature of the flammable liquid. The lower the flash point largermay be the extent of hazardous area.

    Volatility - Boiling point can be used for comparing the volatility of flammable liquids. The

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    more volatile a liquid and lower its flash point the more closely it approximates a flammablegas.

    Ignition Temperature & Ignition Energy - ignition temperature and ignition energy of a

    flammable gas or vapour are taken into account in the design of electrical apparatus for thehazardous areas so that these do not present an ignition risk.

    6.5 Physical Properties of Hazardous Materials :

    The physical state for material (gas, liquid, or solid) has a definite relationship to thepotential danger posed by that material. A material that is dangerous in one state can be

    relatively innocuous in another. These are specific terms to describe the various physicalstate of material. As defined by American National Standards Institute, these are as follows.

    6.5.1 Dust :

    Dust is solid particles generated by handling, crushing, grinding, rapid, impact, detonation,and decrepitation of organic or inorganic material such as rock. Ore, metal, wood, andgrain.

    Dust does not tend to flocculate except under electrostatic forces; they do not diffuse in air,but settle under the influence of gravity.

    Dust is term used to describe airborne solid particles that range in size from 0.1 to 25micron. Dusts above 5 micron in size usually will not remain airborne long enough to

    present an inhalation problem.

    Dust may enter in air from various sources. It may be dispersed when a dusty material ishandled, such as when lead oxide is dumped into a mixture or a product is dusted with talc.When solid materials are reduced to small sizes in processes such as grinding, crushing,

    blasting, shaking, and drilling, the mechanical action of the grinding or shaking devicesupplies a source of energy to disperse the dust formed.

    6.5.2 Fumes :

    Fumes are generated by condensation from the gaseous state, generally after volatilization

    from molten metals. This physical change is often accompanied by a chemical reaction suchas oxidation. Fumes flocculate and some times coalesce.

    A fume is formed when a volatilized solid, such as metal, condenses in cool air. The solidparticles that make up a fume are extremely fine, usually less than 1 micron. In mostcases, the hot material reacts with the air to form an oxide. Examples are lead oxide fumesfrom smelting and iron oxide fumes from arc welding. A fume can also be formed when a

    material such as magnesium metal is burned or when welding or gas cutting is done ongalvanized metal. Metal fumes can be generated from soldering too. Gases and vapours are

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    not fumes, even if newspaper columnist often call them that.

    6.5.3 Smoke :

    Smoke is carbon or soot particles less than 0.1 micron in size, which results from theincomplete combustion of carbonaceous material such as coal or oil. Smoke generally

    contains droplets as well as dry particles. Tobacco for instance produces a wet smokecomposed of minute tarry droplets. The size of the particles contained in tobacco smoke is

    about 0.25 micron.

    6.5.4 Aerosols :

    Aerosols are liquid droplets (mist or fog) or solid particles dispersed in air (fumes dust andsmoke) that are of fine enough particle size to remain so dispersed for a period of time.

    6.5.5 Mists :

    Mists are suspended liquid droplets generated by condensation from gaseous to liquid state

    or by breaking up a liquid into dispersed state such as by splashing, foaming or atomising.A mist is formed when finely divided liquid is suspended in atmosphere.

    Examples are oil mist produced during cutting and grinding operation, acids mists fromelectroplating, paint, spray mist from spraying operations and condensation of water vapourto form fog or rain.

    6.5.6 Gases :

    Gases are normally formless liquids that occupy a space and can be changed to liquid orsolid state only by combined effects of increased or decreased temperature. Gases can

    diffuse into atmosphere. Examples are welding gases. Internal combustion engine exhaustgas and air.

    6.5.7 Vapours :

    Vapours are gaseous form of a substance which are normally in solid or liquid state( at

    room temperature and pressure) . The vapour can be changed to solid or liquid state eitherby increasing the pressure or decreasing the temperature. Evaporation is process by which

    the liquid is changed into Vapour State and mixed with surrounding atmosphere. Solvents

    with low boiling points will volatilise readily.

    In addition to the definition concerning the concentration of matter, other terms used todescribe degree of exposed are

    ppm - parts of vapour or gas per million parts ofcontaminated air by volume.

    Mppcf - Million of particles of a particulate per cubic foot of

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    Mg/cubic metre - milligrams of a substance per cubic metre

    of air. Microgram / litre - microgram of a substance per litre ofsolution.

    6.6 Density of Gases and Liquids :

    The density of gas in relation to density of air is called vapour density. It is calculated asfollows :

    Vapour density = weight of gas or vapour / Weight of equalvolume of air

    Relative density of a liquid is termed specific gravity. It is calculated as follows :

    Specific gravity = Weight of liquid/ Weight of equal volume

    of water.

    6.6.1 Vapour Density :

    Hazards of Material Lighter than Air - If vapour density is less than one, material islighter than air and will rise and dissipate in air if unconfined. Material could collect in high

    spots such as celling if confined. One example is hydrogen or helium.

    Hazards of Material Heavier than Air - If favour density is greater than one, material is

    greater than air and will sink. Material will linger near the grounds and collect in low spots.Examples are carbon dioxide, chlorine, gasoline vapours and trichloroethylene.

    6.6.2 Specific Gravity :

    Hazards of Materials Lighter Than (but insoluble in) - If specific gravity is less than

    one, material will float on water. If the material enters the environment it will sink in porousmaterials such as soil or sand until it encounters water i.e. ground water, where it will then


    Hazards of Material is Heavier than Water - If specific gravity is greater than one,

    material will sink in water. This is true of materials like carbon disulphide, chlorinatedsolvents and Polly chlorinated biphenyl. If the materials enters the environment it will sink

    in porous materials such as soil or sand, then sink unaffected by water until it reaches anon porous surface.

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    7.0 Classification of Hazardous Areas

    All hazardous areas can be classified into 3 zones according to probability of there being an

    explosive gas air mixture present.

    Zones Description RemarksZone-0 Areas, where flammable

    atmosphere iscontinuously present or

    is present for longperiods

    Application only where hazards will exist

    continuously. In the petroleum industry such acondition is rarely encountered except in confined

    spaces such as the vapor spaces closed processvessel, storage tanks inside of containers of

    reactors. Any arc or spark would almost certainlylead to fire or explosion. any electrical apparatusmust afford a degree of protection as near aspracticable to absolute. it is recommended to avoid

    installing electrical equipment in zone 0 areas to

    the extent possible.Zone-1 Areas, where flammable

    atmosphere is likely to

    occur periodically oroccasionally during

    normal operation.

    In zone I, the hazard is likely to occur at any timerequiring fullest practicable application of

    measures. Typical e.g. areas surrounding zone 0,areas surrounding drain, discharge equipment.

    Zone-2 Areas, where flammable

    operation is not likely tooccur in normaloperation and if it doesit is only for short


    Application to areas where hazards is unlikely and

    may be caused only by the highly improbable andsimultaneous occurrence of an arc or sparktogether with an hazardous atmosphere arising outof failure of conditions of control. It presupposes

    that any abnormal occurrence is rapidly dispersedso that possible contact with electrical apparatus is

    of minimum duration. Typical e.g. areassurrounding flanged gaskets, areas surrounding

    zone 0 or zone 1.

    This area classification deals only with risk due to combustible gases and vapours andcombustible mists. It does not deals with dusts since these material can be quiescent for

    long periods of time until they are disturbed into suspension by a suitable mechanism.

    Europe and British follow IEC and CENELEC standards which classifies zone-0, 1 & 2.

    America & Canada follow North American standards, which classifies div. 1 & 2. IndianStandard follows IEC standard.

    Div.1 covers Zones - 0 & 1. Div. 2 covers Zone - 2.

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    7.1 Different Methods of Confining Zones :

    10) Protection by Closed Circuit Operation :

    When this method is used it is the safest way to limit risk the explosive atmosphere isconfirmed to the interior of one or several stocking receptacles. The electric equipment can

    be easily be installed outside. The workshop-classified zone 1 became zone 2 by installing aclosed circuit process : using closed tank, supplying regents via piping and emptying the

    finished product also via piping

    Before Protection After Protection

    20) Protection by Over Pressure :

    The entrance of inflammable gases or vapors into an enclosure containing ordinaryelectrical equipment or any other ignition source, is prevented by maintaining, in this

    enclosure, a gas pressure (usually air) superior to the pressure out side (e.g. control room).The control position of a workshop in zone 1 can be derated to zone 2 by installing an over-

    pressured cubicle.

    Before Protection After Protection

    30) Protection by Under Pressure :

    The diffusion of inflammable gases or vapors outside an enclosure is prevented by the

    maintenance therein of a pressure less than that of the surrounding zone. This surroundingzone, which is not hazardous, can also contain without risk conventional electrical

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    equipment or any other ignition sources.

    40) Protection by Inert Gas :

    By adding an inert gas, the oxygen content of the air is an enclosure is lowered to such aweak value that the atmosphere would no longer be explosive, whatever the concentration

    of inflammable gases or vapors. The inert gases generally used in nitrogen, but carbondioxide is used in some occasions. The workshop becomes zones 2 by introducing an inert

    gas preventing the formation of an explosive atmosphere.

    50) Protection by Continuous Dilution :

    The inflammable gas and vapor content is lowered to a value below that of the lowerexplosivity limit (in general < 25% of the lower explosivity limit) by adding a protective

    which is introduced to the desired zone permanently. The protective gas is ,in general, air.

    60) Protection by Suction at Source :

    The creation of localized depression by means of ducting diverts the inflammable gases andvapors to a zone carrying no risk of explosion when in operation. This type of protection is

    essentially used in workshops where there are few discharge points. Inside the suction zonethe electrical equipment should be of safety type. A zone 1 classified workshop becomes

    zone 1(in part) and zone 2 for the remainder by installing a suction device at source.

    Before Protection After Protection

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    70) Protection by General Suction :

    The inflammable gas and vapor content is maintained at a level lower than that of the lower

    level of explosivity in a large zone by a general suction device. This type of protection is

    very delicate to operate because hazardous concentration of inflammable gases and vaporsgenerally remain at their point of discharge and in dead zones.

    Before Protection After Protection

    What should be done in order to prepare for a possible failure in one of the types

    of protection?

    These types of protection use equipment that is very often very complex (inertia,

    overpressure, suction, devices) which can be possibly malfunction.

    In this case the room will revert to its initial Classification.

    It is therefore necessary, for each device, to define supplementary safety measures to betaken to guarantee, in the case of failure, the safety of the workers.

    Example: the disruption of energy sources (such as electricity water, inert gas) cancreate failure in the mode of protection.

    It is therefore necessary to supply the room from the start, to provide for built-inemergency circuit to ensure total safety.

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    Selection of Electrical Equipment

    10.1 General :
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    The selection of electrical apparatus for explosive gas atmosphere is based on the generalprinciple that the likelihood of the simultaneous presence of a hazardous atmosphere and

    source of ignition of reduced to an acceptable low level.

    When the electrical apparatus does not have an internal release of flammable materials,

    the classifications of the hazardous area surrounding the electrical equipment decide theselection of an adequate type of protection.

    When the electrical apparatus has an internal release flammable material and actualhazard is a combination of the external and internal hazards and both must be taken intoaccount when selecting an adequate type of protection.

    The selected apparatus will normally have a recognized type of protection and should be

    also be selected for the appropriate temperature class and the appropriate apparatusgroup applicable. Where gases of different degrees of hazards exist in the same area the

    ype of protection appropriates for the highest degree of hazards shall be applicable.

    The selected electrical apparatus shall be adequately protected against corrosive andsolvent agencies and against water ingress and thermal and mechanical stresses asdetermined by the environmental conditions. These construction requirements should

    ensure that the protection against explosion is not reduced when the apparatus is used inthe specified conditions of service.

    Particular consideration shall be given to the location of apparatus which incorporatesaluminum or light alloys in the construction of its enclosures. These have been outlined inIS : 5571 1979 reference to which should be made.

    As electrical apparatus for flammable gas atmospheres require special safety maintenance

    after they are installed, the selection of electrical apparatus shall be made with fullconsiderations to the facility and frequency of inspection and maintenance, preparation for

    spares and repairing materials and the extent of allowable interruption of power supplyduring maintenance work.

    It is advisable that the selection of electrical apparatus after full consideration has beengiven not only to the initial cost of installation but also to the expected life and expenses

    for operation and maintenance of apparatus.

    Unless otherwise specified for particular equipment, the guidance provided in table 2 shall

    be followed in selection of equipment for hazardous areas. However any equipment whichin the opinion of the authorized inspector offers a degree of safety not less than affordedby the equipment specified may be accepted as an alternative.

    The necessary for equipment with flameproof enclosures or other enclosures maysometimes be eliminated by the adoption of special design, such as pressurizedequipment.

    10.2 Selection Procedure :

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    10.2.1 Logical Steps :

    A number of logical steps are involved in the proper selection of electrical equipment for

    hazardous areas.

    1. Clearly identify the gas/ vapor, its gas group andtemperature class.2. Define the are zone 0,1, or 2.

    3. Select the type of protection select the equipmentkeeping in mind other gas group and the temperature rating.4. For the given type of protection select the equipmentkeeping in mind the gas group and temperature rating.

    5. Depending on the weather the equipment is to beinstalled indoor & outdoor, ensure the correct IP protection for

    the equipment.

    10.2.2 Necessary Information :

    In order that electrical apparatus may be selected in hazardous areas, the followinginformation is necessary :

    The classification of the area, that is-the zone

    The ignition temperature of the gas or vapor involved, or

    the lowest values of ignition temperature if more than onecombustible material is present.

    This will permit determination of the temperature classification

    required for the apparatus, or the upper limit temperature forany unprotected surface according to IS: 8239-1976

    The characteristics of the gas or vapor involved in relationto :

    1. Ignition current or minimum ignition energy in the case ofinstallations of intrinsically safe apparatus, or

    2. Safe gap data in the case of installations for flameproofenclosures.

    Apparatus certified to the constructional and design requirements for a particular groupmay also be used with compounds of lesser risk and which will be allocated therefore to a

    lesser group, subject again to consideration of temperature classification and chemicalcompatibility.

    Similar electrical apparatus which is designed so that may be used with certain flammablematerials in a particular zone may be used with flammable material in zone of lesser riskwithout restriction provided it is determined that the flammable material likely to bepresent are compatible with the following characteristics of the apparatus :

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    1. The apparatus grouping (where this is applicable)2. The temperature classification

    3. Chemical compatibility

    10.3 Selection of Type of Protection :

    This selection of type protection of the equipment for different zone of hazardous areasshall be made in accordance with tables 2 and 3 as applicable.

    10.3.1 Types of Protection for Different Hazardous Areas (Table-2) :

    Zone Type of Protection Description

    Zone-0 No electrical equipment

    should be allowed; when thisis not practicable ,

    intrinsically apparatuscategories ia and ib and

    associated circuit should beemployed type of protection'I'

    Intrinsic safety (Ex-I) IS : 5780-1980 A

    circuit or a part of circuit intrinsically safewhen any spark or thermal effect produced

    normally (i.e., by breaking or closing thecircuit) or accidentally (for e.g. by short

    circuit or earth fault) is incapable, underprescribed test conditions, of causing ignitionof a prescribed gas or vapor.

    An intrinsically safe apparatus is one in whichall electrical circuits are intrinsically safe. It isplaced in one of the following categories:

    a. Category (Ex- ia) - Apparatus in thiscategory is incapable of causing ignition in

    normal operation, or with a single fault, orwith any combination of two faults applied

    with a specified safety factor for current and/or voltage.

    b. Category (Ex-ib) - Apparatus in thiscategory is incapable of causing ignition innormal operation, or with any single faultapplied with a specified safety factor for

    current and / or voltage.

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    Zone-1 a. Type of protection

    adequate for zone 0

    Flameproof Enclosure (Ex-d)

    b. Flameproof equipment

    Type of protection d.

    An enclosure for electrical apparatus that will

    withstand when the covers or any otheraccess doors are properly secured, an

    internal explosion of the flammable gas orvapor which may originate inside theenclosure, without suffering damage andwithout communicating the internal

    flammation to the external flammable gas orvapor for which it is designed, through any

    joints or structural openings in theenclosure(IS : 2148-1981)

    c. Pressurised enclosure-Typeof protection 'p'

    Pressurized enclosure( Ex -P)(IS: 7389-1976).

    An enclosure for electrical apparatus in whichthe entry of flammable gas or vapor is

    prevented by maintaining the air(or othernon flammable gas)within the enclosure at apressure above that of the externalatmosphere

    This type of protection has the followingcategories:

    [Ex-P(1)]- Pressurization with air and alarm

    in the case of less of air pressure

    [Ex-P(2)]-Pressurization with ir andautomatic switching off from electric supply

    in case of loss of air pressure

    Ex-P(3)]- pressurization with inert gas and

    alarm in case of loss of inert gas pressure

    [Ex-P(4)]-Pressurization with inert gas and

    automatic switching off from electric supplyin case of inert gas pressure

    [Ex-P(5)]-Dilution with air and alarm in caseof loss of air supply

    [Ex-P(6)]-Dilution with air and automaticswitching off from electric supply in case ofloss of air supply

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    d. Sand filled apparatus Type

    of protection 'q'

    Sand filled apparatus (Ex-q)IS: 7724-1975

    Electrical apparatus which has all its liveparts entirely embedded in a mass of

    powdery material in such a way that underthe condition of use for which the apparatus

    has been designed, no arc occurs within theouter explosive atmosphere either by thetransmission of flame or by the overheatingof the walls of the enclosure.

    e. Oil immersed apparatus-type of protection 'o'

    Oil immersed apparatus (Ex-o) IS: 7693-1975

    f. Such other apparatus asmay be specifically certified

    or assessed for use in zone1areas-Type of protection 's'

    Electrical apparatus in which all parts onwhich arc may occur in normal service are

    immersed in oil to a sufficient depth toprevent ignition of an explosive gas mixture

    that may be present above the surface of the

    oil and all live parts on which arc do notoccur in normal service are either immersedin oil or protected by some other recognized


    Zone-2 a. type of protection

    applicable for Zone 0 or Zone1

    Non sparking apparatus (Ex-n)IS:8289-1976

    b. Non-sparking apparatus-Type of protection 'n'

    Apparatus, which in normal operation is notcapable of igniting a surrounding explosive

    atmosphere and a fault capable of causingignition, is not likely to occur.

    c. Increased safety apparatus- Type of protection 'e'

    Increased safety apparatus (Ex-e) IS : 6381-1972.

    A method of protection in which measuresadditional to those adopted in ordinaryindustrial practice are applied, so as to giveincreased security against the possibility of

    excessive temperature and the occurrence ofarc or sparks in electrical apparatus which

    does not produce arcs or sparks in normalservice.

    d. Hermetically sealed

    apparatus - types ofprotection 's'

    A method of protection in which the

    equipment is so designed and constructedthat the external inflammable gases orvapors cannot enter the enclosures andhence prevents a source of ignition like anarc or spark from coming into contact with

    the inflammable gas or vapor

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    Note-1 :special protection category is reserved for those types of protection that cannotbe classified as belonging wholly to any one of the above types. This may be that a

    combination of several types of protection is incorporated within one piece of apparatus.

    Note-2 : For outdoor installation, the apparatus with type of protection e and n should

    be used with enclosures providing at least the following degrees of protection:

    IP 55 where there are uninsulated conducting parts

    internally, and IP 44 for insulated parts

    Note-3 : Oil immersed apparatus may be used only in cases its security will not beimpaired by titling or vibrating of the apparatus .

    Note-4 : For apparatus with type of protection p, e, n and where applicable ,q onlyarea classification and ignition temperature are required. However, where apparatus is

    protected by Ex-i or Ex-d in addition to one of these types of protection, it is necessary

    to determine the appropriate apparatus grouping according to IS:2148-1981 respectively

    10.3.2 Minimum Actions Required if Protective Gas Fail for Type of Protection P(Table -3) :

    Area Classification

    Enclosures Does Not

    Contain Ignition CapableApparatus

    Enclosures Contain

    Ignition CapableApparatus

    Zone-1 Alarm Alarm And Switch Off

    Zone-2 No Action Required Alarm

    10.3.3 Temperature Classes :

    Besides the danger of explosion caused by an electrical spark or arc, there is also adanger of ignition at a hot surface exposed to a flammable atmosphere. The maximum

    surface temperature of any unprotected surface of electrical equipment should not exceedthe ignition of the gas or vapour.

    Flammable gases and vapours fall into fairly well defined groups when classified withreference to their ignition temperature. To simplify the manufacture of apparatus

    therefore the permitted maximum surface temperature have been classified in IS:8239-

    1976 as follows :



    Maximum Surface


    T1 450

    T2 300

    T3 200

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    T4 135

    T5 100

    T6 85

    Note-1 : A reference ambient temperature of 40


    C is normally assumed when theapparatus is designed to operate within one of the temperature classes indicated above.For special applications the maximum surface temperature may be based on a reference

    ambient temperature other than 400C is used, the value will be clearly marked on theapparatus.

    Note-2 : It should be noted that for apparatus with type of protection d flameproofenclosures, the surface to be considered is the external surface. For apparatus with other

    types of explosion protection, internal surfaces are equally important if the explosive gasair mixture has access to them.

    10.3.4 Apparatus Groups :

    For the purpose of flammable enclosures and intrinsic safety, gases and vapors have beenclassified according to the groups and subgroups of apparatus required for use in the

    particular gas or vapor atmosphere. The groups of apparatus are:

    Group I For mining application

    Group II Application in other industry

    Group II apparatus is subdivided according to requirements appropriate to the nature of

    the flammable atmosphere for which the apparatus is intended. These subgroups with arepresentative gas ad the design parameters are as follows:



    MaximumExperimental Gap

    Minimum IgnitionCurrent Ratio

    IIA Propane > = 0.9 mm > 0.8

    IIB Ethylene 0.5 < IIB < 0.9 mm 0.45 < IIB < 0.8

    IIC Hydrogen < = 0.5 mm < 0.45

    Various gases and vapors, for which a particular group of enclosure is suitable are listed in

    IS : 9570-1980

    Note :For flameproof enclosures, gases and vapors are classified according to their

    maximum experiment safety, gases and vapors are safe classified according to the ratio oftheir minimum igniting currents(MIC) with that of laboratory methane.