Cold Weather Catalog - Chromalox Precision Heat and Control

TechnicalTechnical InformationHazardous Locations & Electric Heater Applications (cont’d.)Class II — Dust 1 (NEC 500-3b)Groups E, F and G (Class II) — Combustibledusts are divided into Groups E, F and G.Classifi cation involves investigation and testingof the assembled enclosure including theclamped joints, clearances and shaft openings.The blanketing effect of layers of dust,the electrical conductivity and the ignitiontemperature of the dust are also evaluated.Group E Atmospheres contain metal dust,including aluminum, magnesium, their commercialalloys and 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 and having resistivitygreater than 10 2 through 10 8 Ohm-cm.Group G Atmospheres contain combustibledusts such as fl our, grain, wood andchemicals 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 and similar materials.Divisions in Hazardous LocationsThe NEC further sub-divides hazardous locationsinto Divisions (Div. 1 and 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 and where electrically conductivecombustible dusts (all Group E and 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 handled,manufactured or used.Division 2 LocationsClass I, Division 2 — NEC 500-5(b) is an areawhere ignitable gases or vapors are handled,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 handling 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 orhandled.Class I — Adjacent DivisionsIn most indoor areas with adequate partitions,Div. 1 and 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 and Class 1, Div. 2 areasusually exist adjacent to each other. The Div. 1location being near the point of vapor releaseand Division 2 is at a given distance fromtherelease point of the fl ammable liquid. Wherethe spread of fl ammable vapors and gases isnot contained by adequate partitions, the areadesignated as Class I, Div. 2 serves as a “transitionzone” between the hazardous and 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 and D was 280°C (536°F) andGroup C was 180°C (356°F). Recognizing thatchemicals and gases have different ignitiontemperatures, NEC revised the temperatureratings accordingly. Heat producing equipmentmust now be identifi ed by Class, Group, Divisionand “T” rating. The “T” rating shall notexceed the ignition temperature of the specifi cgas, vapor or dust present. Values for “T” ratingsfor Class I and II equipment are shown inthe table below:T-Ratings for Class I and 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