Cold Weather Catalog - Chromalox Precision Heat and Control

TechnicalTechnical InformationHazardous Locations & Electric Heater Applications (cont’d.)CENELEC (& IEC) Zone Classifi cation SystemIntroduced to North America in 1996, the EuropeanCENELEC (and IEC) system of classifi -cation of hazardous locations is also permittedto apply to installations in the U.S. and Canadaas an alternative in Class I Locations, and isnow part of the NEC (Article 505) and 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, and leakageor where equipment breakdowns could releasegases or vapors and 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 and, if they do occur, willexist for a short time only; or where volatilefl ammable liquids, flammable gas, or fl ammablevapors are handled, 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 handled,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 and IEC gas classifi cationsystems since the Group letters are reversed216and even combined. Care should also be takento avoid confusing Group II and Class II, sinceboth use Roman numerals. An unintendedresult of specifying the IEC gas groups, whichcombine the traditional Groups A and 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 and its physical characteristics mustbe recognized. Vapors from fl ammable liquidsalso have a natural tendency to disperse intothe atmosphere, and 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 and CE Section 18), Group, and 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,and 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 and 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 withstand an internalexplosion of a gas or vapor without rupture andwithout causing the ignition of an external gasor vapor.Explosion-proof enclosures are not water-proof.They are designed to contain and dissipateexplosions but they are not water-proof.To prevent the ignition of an external explosiveatmosphere, the enclosure must not only bestrong enough to withstand 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.