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CLASSIFICATION GUIDE
What is a Hazardous Location?
Hazardous locations are areas where a potential for explosion and/or fires exists due to flammable gases, vapors or finely pulverized dusts in the atmosphere. Easily ignitable fibers or flyings in the atmosphere can also cause a location to become hazardous. The normal processing of certain volatile chemicals, gases, grains, etc. or accidental failure of storage systems for these materials may produce a hazardous location. It is also possible that a hazardous location can result from volatile solvents or fluids, used in a normal maintenance routine, that vaporize to form an explosive atmosphere. Whatever the cause of a hazardous location, it is necessary to take precautions to guard against ignition of the atmosphere.
Hazardous Locations and the National Electric Code
The National Electric Code treats installations in hazardous locations in articles 500 through 516. Hazardous locations are classified by NEC definitions. The following are interpretations of these classifications and applications:
CLASS I LOCATIONS
Class I locations are those in which inflammable gases or vapors are or may be present in sufficient quantities to produce
explosive or flammable mixtures.
Class I, Division 1
Class I, Division 1 locations are where hazardous atmosphere may be present during normal operations. It may be present
continuously, intermittently, periodically or during normal repair or maintenance operations, or those areas where a breakdown in processing equipment releases hazardous vapors with the simultaneous failure of electrical equipment.
Class I, Division 2
Class I, Division 2 locations are those in which volatile flammable liquids or gases are handled, processed or used. Normally they will be confined within closed containers or in closed systems from which they can escape only in the case of rupture or deterioration of the containers or systems.
CLASS II LOCATIONS
Class II locations are those that are hazardous because of the presence of combustible dust.
Class II, Division 1
Class II, Division 1 locations include areas where combustible dust may be in suspension in the air under normal conditions in sufficient quantities to produce explosive or ignitable mixtures (Dust may be emitted into the air continuously, intermittently or periodically), or where failure or malfunction of equipment might cause a hazardous location to exist and provide an ignition source with the simultaneous failure of electrical equipment. Included also are locations in which combustible dust of an electrically conductive nature may be present.
Class II, Division 2
Class II, Division 2 locations are those in which combustible dust will not normally be in suspension nor will normal operations put dust in suspension, but where accumulation of dust may interfere with heat dissipation from electrical equipment or where accumulations near electrical equipment may be ignited.
CLASS III LOCATIONS
Class III locations are those considered hazardous due to the presence of easily ignitable fibers or flyings, which are in quantities sufficient to produce ignitable mixtures.
Class III, Division 1
Locations in which easily ignitable fibers or materials producing combustible flyings are handled, manufactured or used.
Class III, Division 2
Locations where easily ignitable fibers are stored or handled.
Equipment for Hazardous Locations
Equipment for Class I Locations
Devices for Class I locations should be housed in enclosures strong enough to contain an explosion if hazardous vapors
entering the enclosure are ignited: These enclosures then cool and vent the combustion, ensuring that the surrounding atmosphere is not ignited. Heat producing equipment in hazardous locations, such as lighting fixtures, must contain the explosion and vent the cooled products of combustion. They must also operate with surface temperatures below ignition temperatures of the hazardous atmosphere. Since the different vapors and gases making up the hazardous atmosphere have varying properties, they have been placed in groups based on a common range of ignition temperatures and explosion pressures. These groups are designated A, B, C and D. The equipment selected must be suitable for the specific
hazardous gas or vapor.
Equipment for Class II Locations
Enclosures used to house devices in Class II locations must be designed to seal out dust. Contact between the hazardous
atmosphere and the source of ignition is eliminated and no explosion can occur within the enclosure. As in Class I, heat
producing equipment must operate below the ignition temperature of the hazardous atmosphere. However, in Class II equipment, design and consideration must make allowances for heat buildup, resulting from dust settling on the equipment. Dusts have also been placed in Groups designated E, F, and G based on particular hazardous characteristics. Again it is important to select specific hazardous group equipment. Flexibility, similar to Class I, Division 2 applications, is also permitted in the selection of equipment and wiring methods for Class II, Division 2 applications.
Equipment for Class III Locations
Class III locations require equipment which will restrict the entrance of fibers and flyings, prevent the escape of sparks of burning materials and operate at a temperature below the combustion point.
Groups- Gases
Flammable gases and vapors are divided in four large groups, Group A, B, C and D. The classification involves determination of maximum explosion pressure and maximum safe clearances between parts of a clamped joint in an enclosure. Certain vapors explode more violently when ignited, thus creating higher explosive pressure and heat. Group A gas (Acetylene creates the highest pressure during an explosion, thus is most difficult to control. Group B is next in creating higher pressure, then Group C and last Group D. It is necessary, therefore, that equipment be approved not only for class but also for the specific group of the gas or vapor that will be present
Group "A" Atmospheres-- Acetylene
Group "B" Atmospheres--acrolein (inhibited)(2), arsine, butadiene (1), ethylene oxide (2), hydrogen, manufactured gases containing more than 30% hydrogen (by volume), propylene oxide (2), propylnitrate
Group "C" Atmospheres--acetaldehyde, amyl alcohol, n- butyraldehyde, carbon monoxide, crotonaldehyde, diethyl ether, diethylamine, epichlorohydrin, ethylene, ethylenimine, ethyl mercaptan, hydrogen cyanide, hydrogen sulfide, 2-nitropropane, tetrahydrofuran, unsymmetrical dimethyl hydrazine (UDMH 1, 1-dimethyl hydrazine)
Group "D" Atmospheres--acetic acid (glacial), acetone, acrylonitrile, ammonia (3), benzene, butane, 1-butanol (butyl alcohol), 2-butanol (secondary butyl alcohol), n-butyl acetate, isobutyl acetate, di-isobutylene, ethane, ethanol (ethyl alcohol), ethyl acetate, ethyl acrylate (inhibited), ethylene diamine (anhydrous), ethylene dichloride, gasoline, hepatanes, hexanes, isoprene isopropyl ether, mesityl oxide, methane (natural gas). Methanol (methyl alcohol), methyl ethyl ketone, methyl isobutyl ketone, 2-methyl-1 propanol (isobutyl alcohol), 2-methyl-2-propanol (tertiary butyl alcohol), petroleum naptha (4), pyridine, octanes, pentanes, 1-pentanol (amyl alcohol), propane, 1-propanol (propyl alcohol), 2-propanol (isopropyl alcohol), propylene, styrene, toluene, vinyl acetate, vinyl chloride, xylenes.
Groups- Dusts
The explosion characteristics of air mixtures of dusts vary with the material involved. The classification involves the tightness of the joints of the assembly and shaft openings to prevent the entrance of dust in the dust-ignition proof enclosure, the blanketing effect of layers of dust on the equipment that may cause overheating, and the ignition temperature of the dust. It is necessary, therefore, that the equipment be approved not only for the class, but also for the specific group of dust that will be present. Combustible dusts are divided into three groups, Group E, F and G.
Group "E" Atmospheres-- contain metal dust, including aluminum, magnesium, and their commercial alloys, and other metals of similarly hazardous characteristics having resistivity of 102 OHM-centimeter or less.
Group "F" Atmospheres--contain carbon black, charcoal, coal or coke dusts which have more than 8 percent total volatile material or atmospheres containing these dusts sensitized by other materials so that they present an explosion hazard and
having resistivity greater than 102 OHM-centimeter.
Group "G" Atmospheres-- containing flour, starch, grain or combustible plastics or chemical dusts having resistivity 108 OHM-centimeter or greater.
(1) Group D equipment shall be permitted for this atmosphere if such equipment is isolated in accordance with Section 501-5(a) by sealing all conduit 1/2 inch size or larger.
(2) Group C equipment shall be permitted for this atmosphere if such equipment is isolated in with Section 501-5(a) by sealing all conduit 1/2 inch size or larger.
(3) For Classification of areas involving ammonia atmosphere see Safety Code for Mechanical Refrigeration (ANSI/ASHRAE 15-1978) and Safety Requirements for the Storage and Handling of Anhydrous Ammonia (ANSI/CGA G2 1-1972).
(4) A saturated hydrocarbon mixture boiling in the range of 20-135°C (68-72°F). Also known by the synonyms benzine, ligroin, petroleum ether, or naphta. For complete listing, see “Classification of Gases, Vapors and busts for electrical equipment in hazardous (classified) locations. NFPA 497-1991”.