NFPA 496 (Standard for Purged and Pressurized Enclosures for Electrical Equipment) is one of the most powerful engineering tools available for hazardous locations, but it is also one of the most abused standards for HAZLOCs. Like the Combustible Gas Detection System (CGDS) provisions in NFPA 70, NFPA 496 allows a facility to install ordinary, non-classified electrical hardware inside a Class I or Class II hazardous area by utilizing a positive-pressure barrier with a clean-air or inert-gas supply.
Properly applying NFPA 496 requires understanding the
- Type of Pressurization required,
- The Enclosure Design, and
- The mandatory Control and Safety Interlocks
NFPA 496 categorizes pressurization into three (3) distinct types (Type X, Y, and Z) based on the classification of the surrounding room and the rating of the equipment inside the enclosure.