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 Electric Code The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment in the United States. It is part of the National Fire Code series published by the National Fire Prot ...

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

References

http://site.ul.com/global/documents/offerings/services/hazardouslocations/CI_groups.pdf Electrical safety Fire test standards {{Measurement-stub