Electrical apparatus for use in flammable gases and vapours are divided into groups:
Group I is for electrical or mechanical apparatus for mines susceptible to fire damp. Group I equipment will not be
discussed in this guide.
Group II is for electrical and mechanical apparatus used in places other than mines.
Gases of Group II are further divided into sub-divisions i.e. IIA, IIB and IIC. The sub-divisions are based on experimental
work conducted with flameproof and intrinsically safe apparatus. This sub-division is also called the explosion group.
To categorise gases the Maximum Experimental Safe Gap (MESG) and/or the Minimum Ignition Current (MIC) can be used.