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Hazardous Location Equipment Classifications |
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 contact open and close; making and braking 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 | ||
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 I, DIVISION 2
These are Class I locations where ignitable concentrations of flammable gases, vapors or liquids are not likely to exist under normal operating conditions.
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 II, DIVISION 2
Where ignitable concentrations of ignitable fibres and flyings are not likely to exist under normal operating conditions
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 (nature 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 carbonaceious 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:
- 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 ½-inch or larger.
- Group C equipment shall be permitted for this atmosphere if such equipment is isolated in accordance with Section 501-5(a) by sealing all conduit ½-inch or larger.
- 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 (ASSICGA G2.1-1981).
- Petroleum Naphtha is saturated hydrocarbon mixture whose boiling range is 20° to 135°C. It is also known as benzine, ligroin, petroleum ether and naphtha.
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:
- Flammable and Combustible Liquids Code (NFPA 30-1990)
- Dry-cleaning Plants (NFPA 35-1987 [ANSI])
- Manufacture of Organic Coatings (NFPA 36-1988 [ANSI])
- Solvent Extraction Plants (NFPA 36-1988 [ANSI])
- Storage and Handling of Liquefied Petroleum Gases (NFPA 58-1992)
- Storage and Handling of Liquefied Petroleum Gases at Utility Gas Plants (NFPA 59-1992)
- Classification of Class I Hazardous Locations for Electrical Installations in Chemical Plants (NFPA 497A-1992 [ANSI])
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 200°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, 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 I the surrounding atmosphere. Enclosures shall meet dust penetration and temperature design tests, and aging of gaskets (if used).
There is no Zone classification for dusts, fibres or flyings at present in the NEC.
For further information on the classification of hazardous (classified) locations see:
NEC, NFPA 70 - National Electric Code, NFPA 70.
NFPA 30 - Flammable and Combustible Liquids Code.
NFPA 497 - Recommended Practice for the Classification of Flammable Liquids, Gases or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
NFPA 499 - Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
ANSI/API RP500 - Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2.
ANSI/API RP 505 - Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1 or Zone 2.
North American Hazardous (Classified) Locations
Apparatus selection according to Class 1
Apparatus for use in Class 1, Division 1
Explosion-proof.
Intrinsically safe.
Purged / pressurised (type X or Y).
Apparatus for use in Class 1, Division 2
Any Class I, Division 1 method.
Non-incendive.
Non-sparking device.
Purged / pressurised (type Z).
Hermetically sealed.
Oil immersion.
Apparatus for use in Class 1, Zone 0
Intrinsic safety AEx ia.
Class I, Division 1 intrinsically safe.
Apparatus for use in Class 1, Zone 1
Any Class I, Zone 0 method.
Any Class 1, Division 1 method.
Flameproof, AEx d.
Increased safety, AEx e.
Intrinsic safety, AEx ib.
Purged pressurised, AEx p.
Powder filling, AEx q.
Oil immersion, AEx o.
Encapsulation, AEx m.
Apparatus for use in Class 1, Zone 0
Any Class 1, Zone 0 or method.
Any Class 1, Division 1 or 2 method.
Type of protection AEx n.
Intrinsically safe equipment listed for use in Class I, Division 1, locations for the same gas or a permitted by Section 505-7(d) of the NEC, and with suitable temperature rating is permitted in Class I, Zone 0 locations.
Equipment approved for use in Class I, Division 1 or listed for use in Class I, Zone 0 locations for the same gas, or as permitted by Section 505-7(d) of the NEC, and with a suitable temperature rating is permitted in Class I, Zone 1 locations.
Equipment approved for use in Class I, Division 1 or Division 2 locations for the same gas, or as permitted by Section 505-7(d) of the NEC, and with a suitable temperature rating is permitted in Class I, Zone 2 locations.
Equipment listed as classified for use in Class I locations is not necessarily acceptable for Class II locations as it may not be dust-tight or operate at a safe temperature with a dust covering.
North American Hazardous (Classified) Locations
Apparatus selection according to the ignition temperature of the gas or vapor
The equipment must be selected so that its maximum surface temperature will not reach the ignition temperature of any gas or vapor that may be present.
Temperature Class Of Electrical Apparatus
Maximum Surface Temperature Of Electrical Apparatus
Ignition Temperature Of Gas Or Vapor
T1
450°C
> 450°C
T2
300°C
> 300°C
T2A
280°C
> 280°C
T2B
260°C
> 260°C
T2C
230°C
> 230°C
T2D
215°C
> 215°C
T3
200°C
> 200°C
T3A
180°C
> 180°C
T3B
165°C
> 165°C
T3C
160°C
> 160°C
T4
135°C
> 135°C
T4A
120°C
> 120°C
T5
100°C
> 100°C
T6
85°C
> 85°C
Low ambient conditions require special consideration. Explosion proof or dust ignition proof equipment may not be suitable for use at temperatures lower than -25°C (-13°F) unless they are identified for low temperature service. Unless the equipment is marked otherwise, it is for use only in an ambient temperature range of -25°C (-13°F) to +40°C (+104°F).
Equipment that is approved for Class I and Class II should be marked with the maximum safe operating temperature.
For information regarding data for flammable gases and vapors, see NFPA 497 and NFPA 325.
Apparatus selection according to the ignition temperature of the dust
The equipment must be selected so that its maximum surface temperature will be less than the ignition temperature of the specific dust. For information regarding data for dusts, see NFPA 499.
Apparatus selection according to apparatus grouping
Equipment that is approved for Class I and Class II should be marked with the maximum safe operating temperature.
The grouping of Class I gases and vapors are classified into categories A, B, C and D.
Gas / Vapor Group
A (typical gas - acetylene).
B (typical gas - hydrogen).
C (typical gas - ethylene).
D (typical gas - propane).
The grouping of Class II dusts are classified into categories E, F, and G.
Dust Group
E (typical atmospheres containing combustible metal dusts).
F (typical atmospheres containing coal dusts).
G (typical atmospheres containing grain dusts).
North American Hazardous (Classified) Locations - Apparatus Construction Standards
ANSI / UL 1203 - Explosion-proof and Dust-ignition Proof Electrical Equipment for use in Hazardous (Classified) Locations.
ANSI / ISA - S12.12 - Non-incendive Electrical Equipment for use in Class I and II, Division 2 and Class III, Divisions 1 and 2 Hazardous (Classified) Locations.
ANSI / NFPA 496 - Standard for Purged and Pressurised Enclosures for Electrical Equipment.
ANSI / UL 913 - Intrinsically Safe Apparatus and Associated Apparatus for use in Class I, II and III, Division 1, Hazardous Locations.
ANSI / UL 698 - Industrial Control Equipment for use in Hazardous (Classified) Locations.
ANSI / UL 2225 - Metal-Clad Cables and Cable-Sealing Fittings for use in Hazardous (Classified) Locations.
UL 1604 - Electrical Equipment for use in Class I and II, Division 2 and Class III Hazardous (Classified) Locations.
ANSI / UL 2279 - Electrical Equipment for use in Class I, Zone 0, 1 and 2 Hazardous (Classified) Locations.
ISA S12.0.01 - Electrical Apparatus for use in Class I, Zone 0, 1 Hazardous (Classified) Locations, General Requirements.
ISA S12.22.01 - Electrical Apparatus for use in Class I, Zone 1 and 2 Hazardous (Classified) Locations, Type of Protection - Flameproof ‘d’
ISA S12.16.01 - Electrical Apparatus for use in Class I, Zone 1 and 2 Hazardous (Classified) Locations, Type of Protection - Increased Safety ‘e’.
Installation Standards and Codes
NEC, NFPA 70 - National Electrical Code (NEC).
USCG 46 CFR Parts 110 - 113 - Shipping, Sub-Chapter J, Electrical Engineering.
ANSI / API RP 14F - Recommended Practice for Design and Installation of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class I, Division 1 and Division 2 Locations
API RP 14FZ - Recommended Practice for Design and Installation of Electrical Systems for fixed and floating offshore petroleum facilities for Unclassified and Class I, Zone 0, Zone 1, and Zone 2 Locations.