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 mmAcetylene
Group B ≤0.45 mm (Except acetylene) Hydrogen
Group C 0.45 mm < x ≤ 0.75 mmEthylene
Group D > 0.75 mm Propane
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References
  1. Enardo Flame Arrestor Technology. "Flame Arrestor Technology" (PDF).
  2. Class I: Flammable gases, vapors or liquids

http://site.ul.com/global/documents/offerings/services/hazardouslocations/CI_groups.pdf%5B%5D

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