IEC 60079 Gas Groups — IIA IIB IIC Explained
Gas groups classify flammable gases and vapours according to how easily they can ignite through a gap in an enclosure — measured by the Maximum Experimental Safe Gap (MESG) and the Minimum Igniting Current (MIC) ratio. Equipment must be rated for the gas group present in the hazardous area.
Gas Group Classification
| Gas Group | MESG (mm) | MIC Ratio | Typical Gases |
|---|---|---|---|
| IIA | >0.9 | >0.8 | Propane, methane, butane, hexane, ammonia |
| IIB | 0.5–0.9 | 0.45–0.8 | Ethylene, hydrogen sulphide, cyclopropane |
| IIC | <0.5 | <0.45 | Hydrogen, acetylene, carbon disulphide |
How Gas Groups Affect Equipment
The gas group affects the design requirements for Ex d equipment most significantly. A higher gas group (IIC) requires tighter flamepath gaps, because IIC gases can propagate through smaller openings than IIA gases. This is why IIC equipment has the most stringent flamepath requirements.
Equipment rated for IIC can always be used where IIA or IIB is present. Equipment rated for IIB can be used where IIA is present. Equipment rated for IIA cannot be used where IIB or IIC gases are present. Always match or exceed the gas group rating required for the area.
Group I — Mining
Group I equipment (firedamp — mainly methane in mining environments) is separate from Group II equipment used in surface industries. Group I equipment is specifically designed for underground mining applications and is not interchangeable with Group II.
Gas Group on the Certificate
The gas group is shown in the ATEX/IECEx certificate marking. For example: Ex d IIB T4 Gb — this equipment is rated for IIB gas group. It can be used in IIA areas but NOT in IIC areas.
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