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.^[1] The National Electric Code classifies Class I hazardous locations into different groups depending on the respective MESG's of gases in the area.^[2]

Examples

NEC Class I Group and Gas MESG^[2]
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

^ Enardo Flame Arrestor Technology. "Flame Arrestor Technology" (PDF).
^ ^a ^b [1]^{[dead link]} Class I: Flammable gases, vapors or liquids

http://site.ul.com/global/documents/offerings/services/hazardouslocations/CI_groups.pdf^{[permanent dead link]}

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[1] Enardo Flame Arrestor Technology. "Flame Arrestor Technology" (PDF).

[UL-2] [1]^{[dead link]} Class I: Flammable gases, vapors or liquids

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