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Hazardous Areas Classification - North America
North American hazardous areas classification in classes, divisions and groups
Electrical devices used in hazardous areas need to be certified for use according the requirements specified
for the area.
In North America certification is provided by
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Factory Mutual - An approval agency primarily concerned with insurance underwriting.
Underwriters Laboratories - An independent, not-for-profit product safety testing and certification
organization.
Canadian Standards Association - A not-for-profit membership-based association serving business,
industry, government and consumers in Canada developing standards addressing public safety and
health.
The types of protection required depends on the risk involved in the area.
In general hazardous areas in North America are separated by classes, divisions, and groups to define the
level of safety required for equipment installed in these locations.
Classes
The classes defines the general nature of hazardous material in the surrounding atmosphere.
Class I
Hazardous because flammable gases or vapours are
present in the air in quantities sufficient to produce
explosive or ignitable mixtures.
Class II
Hazardous because combustible or conductive dusts are
present.
Class III
Hazardous because ignitable fibers or flyings are present,
but not likely to be in suspension in sufficient quantities
to produce ignitable mixtures. (Group classifications are
not applied to this class.)
Divisions
The division defines the probability of hazardous material being present in an ignitable concentration in the
surrounding atmosphere.
Division 1
The substance referred to by class is present during
normal conditions.
Division 2
The substance referred to by class is present only in
abnormal conditions, such as a container failure or
system breakdown.
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Groups
The group defines the hazardous material in the surrounding atmosphere.
Group A
Acetylene
Group B
Hydrogen, fuel and combustible process gases containing
more than 30% hydrogen by volume or gases of
equivalent hazard such as butadiene, ethylene, oxide,
propylene oxide and acrolein.
Group C
Ethyl and ethylene or gases of equivalent hazard.
Group D
Gasoline, acetone, ammonia, benzene, butane,
cyclopropane, ethanol, hexane, methanol, methane,
natural gas, naphtha, propane or gases of equivalent
hazard.
Group E
Combustible metal dusts, including aluminum,
magnesium and their commercial alloys or other
combustible dusts whose particle size, abrasiveness and
conductivity present similar hazards in connection with
electrical equipment.
Group F
Carbonaceous dusts, coal black, charcoal, coal or coke
dusts that have more than 8% total entrapped volatiles or
dusts that have been sesitized by other material so they
present an explosion hazard.
Group G
Flour dust, grain, wood, plastic and chemicals.
The specific hazardous materials within each group and their automatic ignition temperatures can be found in
Article 500 of the National Electrical Code and in NFPA 497.
Group A, B, C and D apply to class I locations. Group E, F and G apply to class II locations.
Temperature Code
A mixture of hazardous gases and air may ignite in contact with a hot surface. The condition for ignition
depends on several factors as surface area, temperature and concentration of gas.
Equipment approved receives a temperature code indicating the maximum surface temperature of the
equipment.
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Temperature Code
Maximum Surface Temperature
o
F
o
C
T1
842
450
T2
572
300
T2A
536
280
T2B
500
260
T2C
446
230
T2D
419
215
T3
392
200
T3A
356
180
T3B
329
165
T3C
320
160
T4
275
135
T4A
248
120
T5
212
100
T6
185
85
Equipment that not exceed a maximum surface temperature of 212 oF (104 oF ambient temperature) is not
required to be marked with a temperature code (NEC).
Recommended reading for this topic:
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National Electrical Code, NFPA 70, Chapter 5, Article 500
29 CFR 1910 Subpart S, Electrical 1910.307
NFPA 497, "Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous
Classified Locations"
NFPA Handbook, "Electrical Installations in Hazardous Locations, " by P. J. Schram and M. W. Earley
NFPA 70E, Chapter 5, "Hazardous (Classified) Locations"
NFPA 325, "Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids"
ANSI/UL 913, "Intrinsically Safe Apparatus"
NFPA 496, "Purged and Pressurized Enclosure for Electrical Equipment in Hazardous Locations."