Maximum experimental safe gap

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 classifies Class I hazardous locations into different groups depending on the respective MESG's of gases in the area.
References.
http://site.ul.com/global/documents/offerings/services/hazardouslocations/CI_groups.pdf