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Both national rating
agencies, as well as the American National Standards Institute adhere to
the same definitions of what contributes a hazardous area. These areas
are defined as Class I (combustible gas and liquids), Class II
(combustible dust), and Class III (combustible fibers). Class I is
subdivided into groups A (acetylene), B (Hydrogen and butadiene), C
(diethyl ether, ethylene, isoprene, and UDMH), and D (acetone, gasoline,
lacquer solvent, styrene, propane, and natural gas). Class II is divided
into Groups E (metal dust), F (carbon black, coal, and coke), and G
(flour, starch, and grain dusts).
All classes include two divisions. Division I covers electrical
equipment directly exposed in an explosion atmosphere of the material of
a specific group. Division II covers electrical equipment in an
explosive atmosphere only when accident or fallout occurs, or in a
properly vented direct exposure.
Qualification for a rating automatically qualifies the equipment for a
lower class and group. For example: Class I equipment can be used in
Class II and Class III applications with no restrictions.
An "explosion-proof" rating is given only to a single piece of equipment
for a specific class, division, and group. Equipment installation is the
sole responsibility of the end user, and the National Electrical Code
clearly defines the requirements of this installation. For example, a
piece of equipment can carry a Class I rating and qualify only for a
Class II rating after installation and inspection if the installation is
not up to the original rating requirements. The National Electrical Code
allows no modification of the rated equipment.
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CLASS |
A Class is specified with a Roman numeral I, II,
or III |
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Class I |
Gases |
Locations where flammable gases or vapors are, or
may be, present in the air quantities sufficient to produce
explosive or ignitable mixtures. |
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Class II |
Dusts |
Locations where the presence of combustible dust
present in a fire or explosion hazard. |
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Class III |
Fibers & Flyings |
Locations where easily ignitable fibers or
flyings are present but not likely to be suspended in the air in
quantities sufficient to produce ignitable mixtures. |
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DIVISION |
Divisions are defined with the numeral 1 or 2. |
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Division 1 |
The hazardous concentration of
flammable gases, vapors, or suspended combustible dusts are
present continuously, intermittently, or periodically under
normal operating conditions. |
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Division 2 |
Volatile, flammable liquids or
flammable gases present, but normally confined within closed
containers or systems from which they can escape only under
abnormal operating or fault conditions. Combustible dusts not
normally in suspension nor likely to be thrown into suspension |
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GROUP |
Groups are defined by letters. Groups A, B, C,
and D fall under Class I. Group A is the most easily ignitable,
Group D the least. Groups E, F, and G fall under Class II.
For more complete summary, refer to NFPA 497M "Classification of
Gases, Vapors, and Dusts for Electrical Equipment in Hazardous
(Classified) Locations".
Click
here to see Group Classifications of Gases, Vapors and Dusts |
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