Maximum experimental safe gap (MESG) is a standardized measurement of how easily a gas flame will pass through a narrow gap bordered by heat-absorbing metal. MESG is used to classify flammable gases for the design and/or selection of electrical equipment in hazardous areas, and flame arrestor devices. The National Electrical Code classifies Class I hazardous locations into different groups depending on the respective MESG's of gases in the area. Class I: Flammable gases, vapors or liquids
Examples
+ NEC Class I Group and Gas MESG NEC Class I Group Gas MESG Example Gas Group A 0.25 mm Acetylene Group B ≤0.45 mm (Except acetylene) Hydrogen Group C 0.45 mm < x ≤ 0.75 mm Ethylene Group D > 0.75 mm Propane
References
http://site.ul.com/global/documents/offerings/services/hazardouslocations/CI_groups.pdf