Purging/Pressurization Systems

DOE-HDBK-1092-98

NEC Article 500

Figure 5-4. Threaded joints can be used as an escape path to cool the hot gases as they pass through the threads

to the outside of the enclosure.

5.2.6 PURGING/PRESSURIZATION SYSTEMS

Purging/pressurization systems permit the safe operation of electrical equipment under conditions

of hazard for which approved equipment may not be commercially available. For instance, most

switchgear units and many large motors do not come in designs listed for Class I Groups A and B.

Whether cast-metal enclosures or sheet-metal enclosures with pressurization should be used for

hazardous locations is mainly a question of economics, if both types are available. As a typical

example, if an installation had many electronic instruments that could be enclosed in a single sheet-

metal enclosure, the installation lends itself to the purging/pressurization system. However, if the

electronic instruments require installation in separate enclosures, use of the cast metal in hazardous-

location housing would almost invariably prove more economical. Pressurized enclosures require:

1. A source of clean air or inert gas

2. A compressor to maintain the required pressure on the system

3. Pressure control valves to prevent the power from being applied before the enclosures have

been purged and to deenergize the system should pressure fall below a safe value.

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