The explosive limits of a gas or vapor are the lower and upper percentages by volume of concentration of gas in a gas-air mixture that will form an ignitable mixture, see Appendix to ISA RP12.13, Part II for U.S. Bureau of Mines Bulletin 627, Flammability Characteristics of Combustible Gases and Vapors.
Source: API Recommended Practice 14FZ, Recommended Practice for Design, Installation, and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class I, Zone 0, Zone 1, and Zone 2 Locations, Second Edition, May 2013. Global Standards
Source: API RP 14F, Design, Installation, and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class 1, Division 1 and Division 2 Locations, Fifth Edition, July 2008. Global Standards
NOTE The terms “explosive limit” and “flammable limit” are equivalent. IEC 60079-20 and IEC 61779-1 use the term “flammable limit” whilst all the other standards use the more widely accepted term “explosive limit”.
Source: IEC 61892-7, Mobile and fixed offshore units – Electrical installations – Part 7: Hazardous areas. Global Standards