Maximum experimental safe gap
Maximum experimental safe gap 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.Examples
| NEC Class I Group | Gas MESG | Example Gas |
| Group A | 0.25 mm | Acetylene |
| Group B | ≤0.45 mm | Hydrogen |
| Group C | 0.45 mm < x ≤ 0.75 mm | Ethylene |
| Group D | > 0.75 mm | Propane |
