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.

http://site.ul.com/global/documents/offerings/services/hazardouslocations/CI_groups.pdf

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