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.
|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|