This standard was developed in two parts by the Technical Committee on Electrical
Equipment in Chemical Atmospheres. The first part, addressing purged enclosures for
electrical equipment in Class I hazardous (classified) locations, was adopted as a tentative
standard at the 1966 NFPA Annual Meeting and as an official standard at the 1967 NFPA
Annual Meeting. The second part, addressing pressurized enclosures for electrical equipment
in Class II hazardous (classified) locations, was tentatively adopted at the 1970 NFPA
Annual Meeting and officially adopted at the 1971 NFPA Annual Meeting.