Class I Equipment
Equipment for use in Class I hazardous locations, as defined in the NEC (National
Electrical Code), is tested with respect to acceptability of operation in the presence of
flammable and explosive mixtures of specific vapors and gases with air. For purposes of
area classification for Divisions 1 and 2, such mixtures have been grouped on the basis of
their characteristics, as follows:
Class I, Group A — Atmospheres containing acetylene.
Class I, Group B — Atmospheres containing acrolein, butadiene, ethylene oxide,
propylene oxide, hydrogen, or fuel and combustible process gases containing more than
30 percent hydrogen by volume.
Class I, Group C — Atmospheres containing ethyl ether, ethylene, or gases or vapors of
equivalent hazard.
Class I, Group D — Atmospheres containing acetone, ammonia, benzene, butane,
cyclopropane, ethanol, gasoline, hexane, methane, methanol, naphtha, propane, or gases
or vapors of equivalent hazard.
Class I Temperature Considerations
The marked operating temperature of the equipment is based on either the maximum
external temperature or internal temperature of the equipment, depending on the
protection method used.
For Class I, Division 1 equipment, in general, the operating temperature is the maximum
temperature of external surfaces of the equipment. For Class I, Division 2 equipment, in
general, the operating temperature is the maximum temperature of all parts of the
equipment, including internal parts, that may be exposed to the flammable material.
Equipment is required to be marked with the operating temperature or operating
temperature code if the maximum operating temperature is more than 100°C (212°F).
This temperature marking shall not exceed the ignition temperature of the specific gas or
vapor to be encountered.
Class I Equipment in Class II Locations
Equipment Listed or Classified for use in Class I locations is not necessarily acceptable
for Class II locations as it may not be dusttight or operate at a safe temperature when
blanketed with dust.
Class II Equipment
Dust-ignition-proof equipment for use in Class II hazardous locations, as defined in the
NEC, is tested with respect to acceptability of operation in the presence of combustible
dusts in air. For purposes of area classification, the NEC groups combustible dust-air
mixtures as follows:
Class II, Group E — Atmospheres containing combustible metal dusts, including
aluminum, magnesium, and their commercial alloys, or other combustible dusts whose
particle size, abrasiveness, and conductivity present an equivalent hazard.
Class II, Group F — Atmospheres containing carbon black, charcoal, coal or coke dusts
which have more than 8 percent total volatile material (carbon black per ASTM D1620,
charcoal, coal and coke dusts per ASTM D271) or atmospheres containing these dusts
sensitized by other materials so that they present an explosion hazard.
Class II, Group G — Atmospheres containing combustible dusts not included in Group
E or F, including flour, grain, wood, plastic and chemicals.
Division 1: Location where a flammability or combustible atmosphere is present
under normal operating conditions.
Division 2: Location where a flammability or combustible atmosphere is present only
under abnormal conditions.
)
