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Page Title: Description, Features, and Test Criteria of Enclosures for Hazardous (Classified) Locations (PER NEMA 250)
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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.
5-27


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