TechnicalTechnical InformationHazardous Locations & Electric <strong>Heat</strong>er Applications (cont’d.)Class II — Dust 1 (NEC 500-3b)Groups E, F <strong>and</strong> G (Class II) — Combustibledusts are divided into Groups E, F <strong>and</strong> G.Classifi cation involves investigation <strong>and</strong> testingof the assembled enclosure including theclamped joints, clearances <strong>and</strong> shaft openings.The blanketing effect of layers of dust,the electrical conductivity <strong>and</strong> the ignitiontemperature of the dust are also evaluated.Group E Atmospheres contain metal dust,including aluminum, magnesium, their commercialalloys <strong>and</strong> other metals of similarlyhazardous characteristics having resistivityless than 10 5 Ohm-cm.Group F Atmospheres contain combustiblecarbonaceous dusts, charcoal, coal or otheratmospheres containing these dusts sensitizedby other hazardous materials <strong>and</strong> having resistivitygreater than 10 2 through 10 8 Ohm-cm.Group G Atmospheres contain combustibledusts such as fl our, grain, wood <strong>and</strong>chemicals having resistivity of 10 5 Ohm-cm,or greater.Class III — Fibers (NEC 500-7a) 1Atmospheres containing easily ignitable fi berssuch as rayon, cotton, fl ax, jute, hemp, kapok,excelsior <strong>and</strong> similar materials.Divisions in Hazardous LocationsThe NEC further sub-divides hazardous locationsinto Divisions (Div. 1 <strong>and</strong> 2). The requirementsfor Division 2 are less stringent than forDivision 1. The two divisions are discussed inthe following paragraphs.Division I LocationsClass I, Division 1 — NEC 500-5(a) is anarea where the hazard can exist under normaloperating conditions. Included are areas wherefl ammable or combustible liquids are transferredfrom one container to another, openvats, paint spray booths or any location whereignitable mixtures are used. Also included arelocations where a hazard is caused by frequentmaintenance, repair or equipment failure.Class II, Division 1 — NEC 500-6(a) is anarea where combustible dust is normally in theair in suffi cient quantities to produce ignitablemixtures or where mechanical failure orabnormal equipment operation might produceignitable mixtures. Locations also includeoperations where hazards exist because offrequent mechanical failure of machinery orequipment <strong>and</strong> where electrically conductivecombustible dusts (all Group E <strong>and</strong> someGroup F) are present in hazardous quantities.Class III, Division 1 — NEC 500-7(a) is anarea where easily ignitable fi bers or materialsproducing combustible fl yings are h<strong>and</strong>led,manufactured or used.Division 2 LocationsClass I, Division 2 — NEC 500-5(b) is an areawhere ignitable gases or vapors are h<strong>and</strong>led,processed or used, but which are normallyin closed containers or closed systems fromwhich they can only escape through accidentalrupture or breakdown of such containers orsystems.Class II, Division 2 — NEC 500-6(b) is anarea where combustible dust is not normallyin the air in suffi cient quantities to produceignitable mixtures or interfere with the operationof electrical equipment, or where dust ispresent as a result of infrequent malfunctioningof processing or h<strong>and</strong>ling equipment.Included are situations where combustibledust accumulations may interfere with the safedissipation of heat from electrical equipment.No electrically conductive dusts as defi nedin NEC 502-1, (last sentence) are included inClass II, Div. 2 atmospheres.Note — There is no Division 2 classifi cationfor Class II, Group E.Class III, Division 2 — NEC 500-7(b) is anarea where easily ignitable fi bers are stored orh<strong>and</strong>led.Class I — Adjacent DivisionsIn most indoor areas with adequate partitions,Div. 1 <strong>and</strong> 2 are self-contained areas. Withpartitions, a Div. 1 area may exist adjacent toa non-hazardous area. However, outdoors orin large indoor areas with few or no partitions,Class I, Div. 1 <strong>and</strong> Class 1, Div. 2 areasusually exist adjacent to each other. The Div. 1location being near the point of vapor release<strong>and</strong> Division 2 is at a given distance fromtherelease point of the fl ammable liquid. Wherethe spread of fl ammable vapors <strong>and</strong> gases isnot contained by adequate partitions, the areadesignated as Class I, Div. 2 serves as a “transitionzone” between the hazardous <strong>and</strong> nonhazardousarea. Div. 1 is the hazardous areawhere fl ammable gases or vapors are releasedfrom the liquid. Div. 2 is the area further awayfrom the point of release, where the gases orvapors are not normally of suffi cient concentrationto produce an ignitable mixture.Class I & II — Temperature RatingsOriginally, equipment in each group had onemaximum temperature rating. The maximumfor Groups A, B <strong>and</strong> D was 280°C (536°F) <strong>and</strong>Group C was 180°C (356°F). Recognizing thatchemicals <strong>and</strong> gases have different ignitiontemperatures, NEC revised the temperatureratings accordingly. <strong>Heat</strong> producing equipmentmust now be identifi ed by Class, Group, Division<strong>and</strong> “T” rating. The “T” rating shall notexceed the ignition temperature of the specifi cgas, vapor or dust present. Values for “T” ratingsfor Class I <strong>and</strong> II equipment are shown inthe table below:T-Ratings for Class I <strong>and</strong> IIMaximum Temperature Identifi cationDegrees (°C) Degrees (°F) “T” Number450 842 T1300 572 T2280 536 T2A260 500 T2B230 446 T2C215 419 T2D200 392 T3180 356 T3A165 329 T3B160 320 T3C135 275 T4120 248 T4A100 212 T585 185 T6Note 1 — For a complete list defi ning propertiesof fl ammable liquids, gases, solids ordusts, refer to the latest edition of NFPA 325,NFPA 497 or NFPA 499.TECHNICAL215
TechnicalTechnical InformationHazardous Locations & Electric <strong>Heat</strong>er Applications (cont’d.)CENELEC (& IEC) Zone Classifi cation SystemIntroduced to North America in 1996, the EuropeanCENELEC (<strong>and</strong> IEC) system of classifi -cation of hazardous locations is also permittedto apply to installations in the U.S. <strong>and</strong> Canadaas an alternative in Class I Locations, <strong>and</strong> isnow part of the NEC (Article 505) <strong>and</strong> CE Code(Section 18).Class I, Zone 0 - A location in which explosivegas atmospheres are present continuously orfor long periods of time.Class I, Zone 1 - A location in which explosivegas atmospheres are likely to exist in normaloperation or may exist frequently because ofrepairs, maintenance operations, <strong>and</strong> leakageor where equipment breakdowns could releasegases or vapors <strong>and</strong> also cause simultaneousfailure of electrical equipment in a mode tocause the electrical equipment to become asource of ignition.Class I, Zone 2 - A location in which explosivegas atmospheres are not likely to occur innormal operation <strong>and</strong>, if they do occur, willexist for a short time only; or where volatilefl ammable liquids, flammable gas, or fl ammablevapors are h<strong>and</strong>led, processed, orused, but are normally confi ned within closedcontainers or systems from which they canescape only as a result of accidental rupture orbreakdown of the containers or system, or as aresult of abnormal operation of the equipmentwith which the liquids or gases are h<strong>and</strong>led,processed, or used; or where ignitable concentrationsof fl ammable gases or vapors arenormally prevented by adequate ventilation,but which may occur as a result of failure orabnormal operation of the ventilation system.Class I GroupsGroup I - Atmospheres containing explosivegas in underground coal mines. Electricalapparatus that is intended for use in undergroundmines.Group IIC - Atmospheres containing acetylene,hydrogen (H2), or gases of equivalent hazard.Group IIB - Atmospheres containing acetaldehyde,ethylene, or gases or vapors ofequivalent hazard.Group IIA - Atmospheres containing acetone,ammonia, ethyl alcohol, gasoline, methane,propane, or gases or vapors of equivalenthazard. Note: There is potential for confusionbetween the NEC/CE <strong>and</strong> IEC gas classifi cationsystems since the Group letters are reversed216<strong>and</strong> even combined. Care should also be takento avoid confusing Group II <strong>and</strong> Class II, sinceboth use Roman numerals. An unintendedresult of specifying the IEC gas groups, whichcombine the traditional Groups A <strong>and</strong> B intoGroup IIC, is that equipment approved for hydrogen(H2) would also have to be approvedfor acetylene. Since very little equipment isdesigned for acetylene, the wording as originallyadopted severely limits the availabilityof equipment for hydrogen applications. As aresult, NEC Section 505-7(d) now allows forequipment to be listed for a specifi c gas orvapor, specifi c mixtures of gases or vapors, orany specifi c combination of gases or vapors.One common example is equipment markedfor “IIB + H2”. At present, the NEC or CE Codedoes not recognize any CENELEC or IEC dustclassifi cations.Combustion PrinciplesThree basic conditions must be satisfi ed fora fi re or explosion to occur. First, a fl ammableliquid, vapor or combustible dust mustbe present in suffi cient quantity. Second,the fl ammable liquid, vapor or combustibledust must be mixed with air or oxygen in theproportions required to produce an explosivemixture. Finally, a source of energy must beapplied to the explosive mixture.In applying these principles, the quantity ofthe fl ammable liquid or vapor that may beliberated <strong>and</strong> its physical characteristics mustbe recognized. Vapors from fl ammable liquidsalso have a natural tendency to disperse intothe atmosphere, <strong>and</strong> rapidly become dilutedto concentrations below the lower explosionlimit, particularly when there is natural ormechanical ventilation. In order to have anexplosive gas atmosphere, the concentrationof the gas or vapor must be above the LowerExplosive Limit (LEL) but below the UpperExplosive Limit (UEL). The possibility that thegas concentration may be above the upperexplosion limit does not afford any degree ofsafety, as the concentration must fi rst passthrough the explosive range to reach the upperexplosion limit.Equipment Marking RequirementsElectrical equipment permitted for use inhazardous locations must be marked to showthe Class, Division (or Zone under NEC Article505 <strong>and</strong> CE Section 18), Group, <strong>and</strong> maximumsurface operating temperature or temperaturecode referenced to a 40˚C (104˚F) ambienttemperature (some exceptions apply). Notethat the maximum external temperature of theequipment shall not exceed the minimum ignitiontemperature of the atmosphere that theequipment is located in.Electrical equipment approved for operation atambient temperatures exceeding 40˚C shall bemarked with the maximum ambient temperaturefor which the equipment is approved,<strong>and</strong> the operating temperature or temperaturerange at that ambient temperature.Equipment not marked to indicate a division,or marked “Division 1” or “Div. 1”, is suitablefor both Division 1 <strong>and</strong> 2 locations. Equipmentmarked “Division 2” or “Div. 2” is suitable forDivision 2 locations only. Equipment that islisted for a Zone 0 location shall be permittedin a Zone 1 or Zone 2 location of the same gasor vapor. Equipment that is listed for a Zone 1location shall be permitted in a Zone 2 locationof the same gas or vapor.Explosion-Proof EnclosuresMaximum Surface Temperature CodesMaximum Surface Identifi cation NumberTemperature NEC/CE IEC˚C (˚F) T-Code T-Code450˚ C (842˚F) T1 T1300˚ C (572˚F) T2 T2280˚ C (536˚F) T2A260˚ C (500˚F) T2B230˚ C (446˚F) T2C215˚ C (419˚F) T2D200˚ C (392˚F) T3 T3180˚ C (356˚F) T3A165˚ C (329˚F) T3B160˚ C (320˚F) T3C135˚ C (275˚F) T4 T4120˚ C (248˚F) T4A100˚ C (212˚F) T5 T585˚ C (185˚F) T6 T6An enclosure which will withst<strong>and</strong> an internalexplosion of a gas or vapor without rupture <strong>and</strong>without causing the ignition of an external gasor vapor.Explosion-proof enclosures are not water-proof.They are designed to contain <strong>and</strong> dissipateexplosions but they are not water-proof.To prevent the ignition of an external explosiveatmosphere, the enclosure must not only bestrong enough to withst<strong>and</strong> the internal explosionpressure, but all of the openings (e.g.,cover joints, conduit or cable entries, operatingshafts, etc.) must be tight enough to cool thehot burning gases before they can come intocontact with the external atmosphere.
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