A Class II location is defined in NEC Section 500-6 as an area where combustible dust presents a fire or explosion hazard. Class II locations are divided into two divisions based on the normal presence or absence of dust.

5.3.2.1CLASS II DIVISION 1

A Class 11 Division 1 location is one:

1. In which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitable mixtures;

2. Where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitable mixtures to be produced and might also provide a source of ignition through simultaneous failure of electrical equipment, operation of protective devices, or other causes; or

3. In which combustible dusts of an electrically conductive nature may be present in hazardous quantities. (See Table 5-6.)

5.3.2.2CLASS 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;

Table 5-6. Summary of typical combustible dust hazardous atmospheres.'

a. Chart from Crouse-Hinds ECM Code Digest, 1990.

b. NEMA Enclosures Type 9 shall be used for Class 11 Groups E, F, or G.

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).