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.

5.7.2 FEATURES AND TEST CRITERIA

When completely and properly installed, Type 7 enclosures:

1. Provide to a hazardous gas environment a degree of protection from an internal explosion or from operation of internal equipment

2. Do not develop external surface temperatures that exceed prescribed limits for the specific gas corresponding to the atmospheres for which the enclosure is intended when internal heatsimulating equipment is operated at rated load

3. Withstand a series of internal explosion design tests:

a. That determine the maximum pressure effects of the gas mixture

b. That determine propagation effects of the gas mixtures.

4. Withstand, without rupture or permanent distortion, an internal hydrostatic design test based on the maximum internal pressure obtained during explosion tests and on a specified safety factor

5. Are marked with the appropriate class and groups for which they have been qualified.