Description, Features, and Test Criteria of Enclosures for Hazardous (Classified) Locations (PER NEMA 250)

DOE-HDBK-1092-2004

5.6

DESCRIPTIONS, FEATURES, AND TEST CRITERIA OF

ENCLOSURES FOR HAZARDOUS (CLASSIFIED) LOCATIONS

(PER NEMA 250)

Type 7 and 10 enclosures, when properly installed and maintained, are designed to contain an

internal explosion without causing an external hazard. Type 8 enclosures are designed to

prevent combustion through the use of oil-immersed equipment. Type 9 enclosures are

designed to prevent the ignition of combustible dust.

As mentioned earlier, hazardous locations (other than in mines) are classified according to the

flammability or combustibility of the materials that may be present and also according to the

likelihood that a flammable or combustible concentration is present. For definitions and

classifications, see the NEC, Article 500, and NFPA 497M, Classification of Gases, Vapors and

Dust for Electrical Equipment in Hazardous Classified Locations. Descriptions and tests in this

standards publication cover equipment that is suitable for installation in locations classified as

Division 1 or 2. In Division 2 locations, other types of protections and enclosures for

nonhazardous locations may be installed if the equipment does not constitute a source of

ignition under normal operating conditions. See the specific sections of Articles 501 through 503

of the NEC.

Intrinsically safe equipment (not capable of releasing sufficient electrical or thermal energy

under normal or abnormal conditions to cause ignition of specific hazardous atmospheres) may

be installed in any type of enclosure otherwise suitable for the environmental conditions

expected. See ANSI/UL 913, Intrinsically Safe Apparatus and Associated Apparatus for Use in

Class I, 11, III, Division I, Hazardous (Classified) Locations for detailed requirements.

Purged and pressurized equipment should be installed in enclosures suitable for nonhazardous

locations. Hazards may be reduced or eliminated by adequate positive pressure ventilation from

a source of clean air in conjunction with effective safeguards against ventilation failure. See

NFPA 496, Purged and Pressurized Enclosures for Electrical Equipment in Hazardous

Locations for detailed requirements.

5.7

TYPE 7 ENCLOSURES

Type 7 enclosures are designed for indoor use in locations classified as Class I Groups A, B, C,

or D as defined in the NEC.

5.7.1

DESCRIPTION AND APPLICATION

Type 7 enclosures shall be capable of withstanding the pressures resulting from an internal

explosion of specified gases and containing such an explosion sufficiently that an explosive gas-

air mixture in the atmosphere surrounding the enclosure will not be ignited. Enclosed heat-

generating devices shall not cause external surfaces to reach temperatures capable of igniting

explosive gas-air mixtures in the surrounding atmosphere. Enclosures shall meet explosion,

hydrostatic, and temperature design tests.

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