HAZARDOUS LOCATION INFORMATION
Classes, Divisions and Equipment
Hazardous locations are those areas where a potential for explosion and fire exist because of flammable gases, vapors or finely pulverized dusts in the atmosphere, or because of the presence of easily ignitable fibers or flyings. Hazardous locations may result from the normal processing of certain volatile chemicals, gases, grains, etc., or they may result from accidental failure of storage systems for these materials. It is also possible that a hazardous location will be created when volatile solvents or fluids, used in a normal maintenance routine, vaporize to form an explosive atmosphere.
Regardless of the cause of a hazardous location, it is necessary that every precaution be taken to guard against ignition of the atmosphere. Certainly, no open flames would be permitted in these locations. But what about other sources of ignition?
Electrical Sources of Ignition
A source of ignition is simply the energy required to touch off an explosion in a hazardous location atmosphere.
Electrical equipment can be a source of this ignition energy. The normal operation of switches, circuit breakers, motor starters, contractors and plugs and receptacles release this energy in the form of arcs and sparks as contacts open and close, making and breaking circuits.
Electrical equipment such as lighting fixtures and motors are classified as "heat producing," and they will become a source of ignition if they reach a surface temperature which exceeds the ignition temperature of the particular gas, vapor or dust in the atmosphere.
It is also possible that an abnormality or failure in an electrical system could provide a source of ignition. A loose termination in a splice box or a loose lamp in a socket can be the source of both arcing and heat. The failure of insulation from cuts, nicks or aging can also act as an ignition source, again from sparking, arcing and heat.
Hazardous Locations and the National Electrical Code
The National Electrical Code treats installations in hazardous locations in articles 500 through 517.
Each hazardous location can be classified by the definitions in the NEC. Following are interpretations of these classifications.
| Class 1 | Locations | Equipment |
| Class 2 | Locations | Equipment |
| Class 3 | Locations | Equipment |
| Atmospheres | Group A | Group B |
| Group C | Group D | |
| Group E | Group F | |
| Group G | ||
| NEMA Standards | ||
Additional Chemical Classification Information
Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures.
CLASS I, DIVISION 1
These are Class I locations where the hazardous atmosphere is expected to be present during normal operations. It may be present continuously, intermittently, periodically or during normal repair or maintenance operations. Division 1 locations are also those locations where a breakdown in the operation of processing equipment results in the release of hazardous vapors and the simultaneous failure of electrical equipment.
Class II locations are those that are hazardous because of the presence of combustible dust.
CLASS II, DIVISION 1
These are Class II locations where combustible dust may be in suspension in the air under normal conditions in sufficient quantities to produce explosive or ignitable mixtures. This may occur continuously, intermittently or periodically. Division 1 locations also exist where failure or malfunction of machinery or equipment might cause a hazardous location to exist while providing a source of ignition with the simultaneous failure of electrical equipment. Included also are locations in which combustible dust of an electrically conductive nature may be present.
Class III locations are those that are hazardous because of the presence of easily ignitable fibers or flyings, but in which the fibers or flyings are not likely to be in suspension in the air in quantities sufficient to produce ignitable mixtures.
Class III, Division 1
These are locations in which easily ignitable fibers or materials producing combustible flyings are handled, manufactured or used.
Class III, Division 2
These locations are where easily ignitable fibers are stored or handled.
HAZARDOUS LOCATION EQUIPMENT
Devices for Class I locations are housed in enclosures which are designed strong enough to contain an explosion if the hazardous vapors enter the enclosure and are ignited. These enclosures then cool and vent the products of combustion in such a way that the surrounding atmosphere is not ignited.
Heat-producing equipment for hazardous locations, such as lighting fixtures, must not only contain the explosion and vent the cooled products of combustion but must also be designed to operate with surface temperatures below the ignition temperatures of the hazardous atmosphere.
Since the different vapors and gases making up hazardous atmospheres have varying properties, they have been placed in groups based on common flame propagation characteristics and explosion pressures. These groups are designated A, B, C and D, and the equipment selected must be suitable for the group of the specific hazardous gas or vapor, with regard to flame propagation, explosion pressures and operating temperatures.
Reference to the National Electrical Code will indicate that most of the equipment used for Class I Division 2 applications is the same as that used for Division 1 applications. However, in certain instances, standard location equipment may be used for some of the Class I Division 2 applications if the appropriate restrictions are followed. The National Electrical Code contains the specific rules.
The enclosures used to house devices in Class II locations are designed to seal out dust. Contact between the hazardous atmosphere and the source of ignition has been eliminated and no explosion can occur within the enclosure.
As in Class 1 equipment, heat-producing equipment must be designed to operate below the ignition temperature of the hazardous atmosphere. However, in Class II equipment, additional consideration must be given to the heat buildup which may result from the layer of dust which will settle on the equipment.
Dusts have also been placed in Groups designated E, F and G, based on their particular hazardous characteristics. It is important to select equipment suitable for the specific hazardous group.
Class III locations require equipment which is designed to prevent the entrance of fibers and flyings, prevent the escape of sparks or burning material and operate at a temperature below the point of combustion.
Acetylene
Acrolein (inhibited) (2)
Butadiene (1)
Ethylene oxide (2)
Formaldehyde (gas)
Hydrogen
Manufactured gases containing more than 30% hydrogen (by volume)
Propylene oxide (2)
Propyl nitrate
|
Acetaldehyde |
hydrogen sulfide |
|
Allyl alcohol |
isobutyridehyde |
|
Butyl mercaptan |
isopropyl glycidyl ether |
|
n-butyraldehyde |
methylacetylene |
|
carbon monoxide |
methylacetylene-propadiene (stablized) |
|
crotonidehyde |
methyl ether |
|
dicyclopentadiene |
methyl formal |
|
diethyl ether |
methyl mercaptan |
|
diethylamine |
monomethyl hydrazine |
|
di-isopropylamine |
morpholine |
|
dimethylamine |
nitroethane |
|
1, 4-dioxane |
nitromethane |
|
di-n-propylamine |
2-nitropropane |
|
epichlorohydrin |
propionaldehyde |
|
ethylene |
n-propyl ether |
|
ethylenimine |
tetrahydrofuran |
|
ethyl mercaptan |
triethylamine |
|
n-ethyl morpholine |
unsymmetrical dimethyl hydrazine (UDMH 1, 1-dimethyl hydrazine) |
|
hydrogen cyanide |
valeraldehyde |
|
hydrogen selenide |
|
|
Acetic acid (glacial) |
isopropylamine |
|
Acetone |
isopropyl ether |
|
Acetonitrile |
liquified petroleum gas |
|
Acrylonitrile |
mesityl oxide |
|
Allyl chloride |
methane (natural gas) |
|
Ammonia (3) |
methanol (methyl alcohol) |
|
n-amyl acetate |
methyl acetate |
|
benzene |
methyl acrylate |
|
butane |
methylamine |
|
1-butanol (butyl alcohol) |
methycyclohexane |
|
2-butanol (secondary butyle alcohol) |
methyl ethyl ketone |
|
n-butyl acetate |
methyl formate |
|
sec-butyl acetate |
methyl isobutyl ketone |
|
butylamine |
methyl isocyanate |
|
butylene |
methoyl methacrylate |
|
chlorobenzene |
2-methyl-1 –propanol (isobutyl alcohol) |
|
chloroprene |
2-methyl-2 –propanol (tertiary buthyl alcohol) |
|
cyclohexane |
naphtha (petroleum) |
|
cyclohexane |
nonane |
|
cyclopropene |
nonene |
|
1, 1-dichloroethane |
octane |
|
1, 2-dichloroethylene |
octene |
|
1, 3-dichloropropene |
pentane |
|
di-isobutylene |
1-pentanol (amyl alcohol) |
|
ethane |
2-pentanone |
|
ethanol (ethyl alcohol) |
1-pentene |
|
ethyl acetate |
petroleum naphtha (4) |
|
ethyl acrylate (inhibited) |
propane |
|
ethylamine |
1-propanol (propyl alcohol) |
|
ethyl benzene |
2-propanol (isopropyl alcohol) |
|
ethyl chloride |
n-propyl acetate |
|
ethylenediamine (anhydrous) |
propylene |
|
ethylene dichloride |
propylene dichloride |
|
ethylene glcol monomethyl ether |
propylene oxide (2) |
|
ethyl formate |
pyridine |
|
gasoline |
styrene |
|
heptaine |
toluene |
|
heptene |
tripropylamine |
|
hexane |
turpentine |
|
2-hexanone |
vinyl acetate |
|
hexenes |
vinyl chloride |
|
isoamyl acetate |
vinylidene chloride |
|
isoamyl alcohol |
xylenes |
|
isobutyl acrylate |
|
|
isoprene |
|
|
isopropyl acetate |
Atmosphere containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment.
(FPN): Certain metal dusts may have characteristics that require safeguards beyond those required for atmospheres containing the large type dusts of aluminum, magnesium, and their commercial alloys. For example, zirconium, thorium, and uranium dusts have extremely low ignition temperatures [as low as 20°C (68°F)] and minimum ignition energies lower than any material classified in any of the Class I or Class II Groups.
Atmospheres containing combustible carbonaceous dusts, including carbon black, charcoal, coal, or coke dusts that have more than 8 percent total entrapped volatiles, or dusts that have been sensitized by other materials so that they present an explosion hazard.
(FPN): See ASTM D 3175-89, Standard Test Method for Volatile Material in the Analysis Sample for Coal and Coke.
Atmospheres containing combustible dusts not included in Group E or F, including flour, grain, wood, plastic and chemicals.
(FPN No. 1): For additional information on group classification of Class II materials, see Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations, NFPA 497M-1991.
Notes:
Additional Chemical Classification Information
Certain chemicals may have characteristics that require safeguards beyond those required for any of the defined groups. Carbon disulfide is one of the chemicals because of its low ignition temperature, 100°C (212°F), and the small joint clearance to arrest its flame propagation.
For additional information from NFPA reference:
Certain metal dusts may have characteristics that require safeguards beyond those required for atmospheres containing the dusts of aluminum, magnesium, and their commercial alloys. For example, zirconium, thorium, and uranium dusts have extremely low ignition temperatures-as low as 20°C (68°F), and minimum ignition energies lower than any material classified in any of the Class I or Class II Groups.
Certain dusts may require additional precautions due to chemical phenomena that can result in the generation of ignitable gases. See National Electrical Safety Code (ANSI C2-1993). Section 127A-Coal Handling Areas.
Combustible dusts which are electrically non-conductive include dusts produced in the handling and processing of grain and grain products, pulverized sugar and cocoa, dried egg and milk powders, pulverized spices, starch and pastes, potato and woodflour, oil meal from beans and seed, dried hay, and other organic materials which may produce combustible dusts when processed or handled. Dusts containing magnesium or aluminum are particularly hazardous and the use of extreme caution will be necessary to avoid ignition and explosion.
Dust that is carbonized or excessively dry is highly susceptible to spontaneous ignition.
Equipment and wiring of the type defined in Article 100 as explosion-proof shall not be required and shall not be acceptable in Class II locations unless approved for such locations.
Where Class II, Group E dusts are present in hazardous quantities, there are only Division 1 locations.
NEMA 3 ENCLOSURES
Type 3 enclosures are intended for outdoor use primarily to provide a degree of protection against windblown dust, rain and sleet, and to be undamaged by the formation of ice on the enclosure. They shall meet rain, external icing, dust and rust-resistance design tests. They are not intended to provide protection against conditions such as internal condensation or internal icing.
NEMA 3R ENCLOSURES
Type 3R enclosures are intended for outdoor use primarily to provide a degree of protection against falling rain, and to be undamaged by the formation of ice on the enclosure. They shall meet rain entry, external icing, and rust-resistance design tests. They are not intended to provide protection against conditions such as dust, internal condensation or internal icing.
NEMA 4X ENCLOSURES
Type 4X enclosures are intended for indoor and outdoor use primarily to provide a degree of protection against corrosion, windblown dust and rain, splashing water and hose-directed water, and to be undamaged by the formation of ice on the enclosure. They shall meet hosedown, external icing, and corrosion-resistance design tests. They are not intended to provide protection against conditions such as internal condensation or internal icing.
NEMA 6 ENCLOSURES
Type 6 enclosures are intended for indoor or outdoor use primarily to provide a degree of protection against the entry of water during temporary submersion at a limited depth; and to be undamaged by the formation of ice on the enclosure. They shall meet submersions, external icing, and rust-resistance design tests. They are not intended to provide protection against conditions such as internal condensation, internal icing, or corrosive environments
NEMA 7 ENCLOSURES
Type 7 enclosures are for indoor use in locations classified as Class I, Groups A, B, C, or D, as defined in the National Electrical Code.
Type 7 enclosures shall be capable of withstanding the pressures resulting from an internal explosion of specified gases, and contain such an explosion sufficiently that an explosive gas-air mixture existing in the atmosphere surrounding the enclosure will not be ignited. Enclosed heat-generating devices shall not cause external surface to reach temperatures capable of igniting explosive gas-air mixtures in the surrounding atmosphere. Enclosures shall meet explosion, hydrostatic, and temperature design tests.
NEMA 9 ENCLOSURES
Type 9 enclosures are intended for indoor use in locations classified as Class II, Groups E, F or G, as defined in the National Electrical Code.
Type 9 enclosures shall be capable of preventing the entrance of dust. Enclosed heat generating devices shall not cause external surfaces to reach temperatures capable of igniting or discoloring dust on the enclosure or igniting dust-air mixtures in the surrounding atmosphere. Enclosures shall meet dust penetration and temperature design tests, and aging of gaskets (if used).
