Maximum Experimental Safe Gap (MESG) is the largest gap width through which a flame, ignited inside a sealed test chamber, cannot propagate outward to ignite the surrounding flammable gas or vapor mixture. It is measured under standardized test conditions defined in IEC 60079-20-1, using a 25 mm long flame path and the most ignition-sensitive concentration of the specific gas or vapor in air.
MESG is one of two key parameters — alongside the MIC ratio — used to assign flammable gases and vapors to groups under both the NEC and the IEC 60079 series. The explosion group determines the minimum joint-gap construction requirements for flameproof (Ex d) enclosures and the appropriate selection of flame arresters. A smaller MESG indicates that the gas is more capable of propagating a flame through a narrow gap, requiring more restrictive equipment design.
For example, H2 being a small flammable molecule, would be fairly low.
Key Points
- MESG is a property of the gas or vapor, not of the enclosure; the equipment standard (IEC 60079-1) specifies the maximum allowable flange gap for each equipment group based on MESG thresholds.
- Equipment rated for a lower-MESG group (e.g., IIC) is suitable for use with higher-MESG gases (e.g., IIA), but a device rated for Group IIA cannot be used in a Group IIC environment.
- MESG values for common industrial gases and vapors are tabulated in IEC 60079-20-1 and NFPA 497
Example
See Also: MIC ratio, flammable gas, hazardous areas
Cited Sources
- IEC 60079-20-1:2010, Explosive Atmospheres — Material Characteristics for Gas and Vapour Classification
- IEC 60079-1:2014, Explosive Atmospheres — Equipment Protection by Flameproof Enclosures “d”