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DOE-HDBK-1092-2004
5.3.2.2 CLASS II DIVISION 2
A Class II Division 2 location is one where:
1. Combustible dust is not normally in the air in quantities sufficient to produce explosive or
ignitable mixtures;
2. Dust accumulations are normally insufficient to interfere with the normal operation of
electrical equipment or other apparatus, but where combustible dust may be suspended in
the air as a result of infrequent malfunctioning of handling or processing equipment; and
3. Combustible dust accumulations on, in, or in the vicinity of the electrical equipment may be
sufficient to interfere with the safe dissipation of heat from electrical equipment or may be
ignitable by abnormal operation or failure of electrical equipment. (See Table 5-6.)
5.3.3
GROUPS
Until publication of the 1937 edition of the NEC, Class I hazardous locations were not
subdivided; a flammable gas or vapor was classified as presenting a single degree of hazard. It
was recognized, however, that the degrees of hazard varied with the substance and that
equipment suitable for use where gasoline was handled was not necessarily suitable for use
where hydrogen or acetylene was handled.
The difficulty of manufacturing equipment and enclosures for use in hydrogen atmospheres was
also recognized, as was the expense of the equipment. It was not logical from an engineering
standpoint, for example, to require in gasoline stations use of explosion-proof equipment that
was also suitable for use in hydrogen atmospheres. Not only would this unnecessarily increase
the cost of the electrical installation in one of the most common types of hazardous locations,
but it would also make some types of equipment unavailable. Even today, there are no listed
motors or generators suitable for use in Group A or B atmospheres.
5.3.4
IGNITION TEMPERATURE
Ignition temperature of a substance, whether solid, liquid, or gaseous, is the minimum
temperature required to initiate or cause self-sustained combustion independently of the heating
or heated element.
Ignition temperatures observed under one set of conditions may be changed substantially by a
change of conditions. For this reason, ignition temperatures should be viewed only as
approximations: Ignition temperatures under one set of conditions may be changed substantially
by a change of conditions. Some of the variables known to affect ignition temperatures are
percentage composition of the vapor or gas-air mixture; shape and size of the space where the
ignition occurs; rate and duration of heating; kind and temperature of the ignition source,
catalytic or other effect of materials that may be present; and oxygen concentration. Another
variable is the many differences in methods and conditions of testing ignition temperature (size
and shape of containers, method of heating, and ignition source).
5.3.5
FLAMMABLE (EXPLOSION) LIMITS
As mentioned in Section 5.3.1.1, in the case of gases or vapors that form flammable mixtures
with oxygen, there is a minimum concentration of gas or vapor in air or oxygen below which
5-23


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