Committee SP12 of the Instrument Society of America (ISA) established the first Design Standard in 1966, entitled "ISA s12.4 - Instrument Purging For Reduction Of Hazardous Area Classification." In 1967, the National Fire Protection Association (NFPA) Technical Committee on Electrical Equipment in Chemical Atmospheres established recommended practices, entitled "NFPA 496 - Purged and Pressurized Enclosures for Electrical Equipment." Since then, the NFPA has expanded their document by adding recommendations for enclosure ventilation and dilution.

The NFPA and ISA define "Types" of pressurization based on the Division rating of a hazardous location and [ electrical ratings ] of the protected equipment. [ General purpose ] and Division 2 rated electrical equipment requires different means of protection, depending on their location.

Protects general purpose equipment in Division 1 Areas

The Type X system reduces the classification within protected enclosures from Division 1 to non-hazardous. It is required to automatically control electrical power to all protected equipment.

Protects Division 2 rated equipment in Division 1 Areas

The Type Y system reduces the classification within protected enclosures from [ Division 1 ] to [ Division 2 ]. All protected equipment must be rated for Division 2. Automatic power control disconnects are not required, but visual and/or audible alarms must be initiated when there is loss of pressure.

Protects general purpose equipment in Division 2 Areas

The Type Z system reduces the classification within protected enclosures from Division 2 to non-classified. Automatic power control disconnects are not required, but visual and/or audible alarms must be initiated when there is loss of pressure.