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 sheetmetal 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 hazardouslocation 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.

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 addition, door-interlock switches are required to prevent access to the equipment while the circuits are energized. All of these accessories can add up to a considerable expenditure. For a detailed description of purging/pressurizing systems see NFPA 496, "Purged and Pressurized Enclosures for Electrical Equipment in Hazardous Classified Locations."