Purging/Pressurization Systems

DOE-HDBK-1092-2004

Figure 5-5. Flat (ground) joints can be used as an escape path to cool the hot gases as

they pass through the flat (ground) joint.

In Class II locations, the enclosure shall keep dust out of the interior and operate at a safe

surface temperature. Because there will be no internal explosions, the enclosure may have

thinner wall sections. The construction of these enclosures is known as dust-ignition-proof.

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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