Class II Division 2

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

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