Groups

DOE-HDBK-1092-98

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. (See NEC Section 500-3 for more information on groups).

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 propagation

of flame cannot occur on contact with a source of ignition. There is also a maximum concentration

of vapor or gas in air above which propagation of flame cannot occur. These boundary-line mixtures

5-24