Hazardous Location

DOE-HDBK-1092-2004

In addition, door-interlock switches are required to prevent access to the equipment while the

circuits are energized. All of these accessories can add up to a considerable expenditure. For a

detailed description of purging/pressurizing systems see NFPA 496, Purged and Pressurized

Enclosures for Electrical Equipment in Hazardous Classified Locations.

5.3

HAZARDOUS LOCATIONS

Hazardous areas and locations are classified by group, class, and division. These classifications

are determined by the atmospheric mixtures of various gases, vapors, dust, and other materials

present. The intensity of the explosion that can occur depends on concentrations, temperatures,

and many other factors that are listed in NFPA codes.

Hazardous locations must be well understood by anyone designing, installing, working on, or

inspecting electrical equipment and wiring in such areas. Such locations carry a threat of

flammable or combustible gases, vapors, or dusts being present some or all of the time.

Information in this section will assist in classifying areas or locations with respect to hazardous

conditions, whether from atmospheric concentrations of hazardous gases, vapors, and deposits,

or from accumulations of readily ignitable materials.

This section covers the requirements for electrical equipment and wiring in locations that are

classified according to the properties of the flammable vapors, liquids, or gases or combustible

dusts that may be present and the likelihood that a flammable or combustible concentration is

present. The hazardous (classified) locations are assigned the following designations:

Class I Division 1

Class I Division 2

Class II Division 1

Class II Division 2.

Class I, Zone 0, Zone 1, Zone 2

Class III fibers and flyings are not covered in this section:

5.3.1

CLASS I

Class I locations are identified in the NEC as those in which flammable gases or vapors are or

may be present in the air in amounts sufficient to create explosive or ignitable mixtures. Gases

or vapors may be continuously or intermittently present. However, if a gas or vapor is present,

there is a potential that a flammable mixture will be present.

From an engineering standpoint, greater precautions are needed if a particular set of conditions

is likely to occur (e.g., the presence of a flammable mixture within the explosive range) than if it

is unlikely. This is the reason for dividing hazardous locations into two divisions.

5.3.1.1 DIVISION 1

NEC 500.5 defines Class I Division 1 hazardous locations as those in which:

5-17