Gas Groups

In the IEC system, the group allocation for surface and underground (mining) industries are separate. Group I is reserved for the mining industry, and Group II which is subdivided into IIC, IIB and IIA for surface industries.

Gases are divided into subdivisions within these groups, these subdivisions are based upon experimental work conducted with flameproof and intrinsically safe apparatus. These gas subdivisions are based on the possibility of a burning gas mixture passing through a specified gap, the MESG (Maximum Experimental Safe Gap) and the amount of energy needed to ignite that gas, the MIC (Minimum Ignition Current).

Gas Group II refers above ground, surface industries. It is seen in the table that surface industry gases are divided into three subdivisions, IIA, IIB and IIC, where IIA gases are the least easily ignitable and IIC gases are the most easily ignitable in terms of ignition energy.

In practice however, considerable safety margins are applied. For example the maximum gap for flange widths of 25mm is 0.2mm for IIB and 0.1mm for hydrogen IIC