Classification of Gases and Vapour

Gases are classified according to the group or subdivision of the equipment required for use in that particular gas, based on the gas ignition properties.

Group I equipment is for use in underground mines susceptible to firedamp (the mining term for methane), and the surface installations of such mines.

Group II equipment is for places with explosive gas atmospheres, other than mines, i.e. the non-mining surface industries.

Group II equipment is subdivided, according to the ignition risk properties of the atmosphere, into subdivisions IIA, IIB, and IIC.

The Group II subdivisions (still referred to by many people as gas groups) IIA, IIB and IIC are based on experimental work conducted with flameproof and intrinsically safe apparatus, and the gases may be categorised into subdivisions by one (or both) of two test methods:

• Subdivision based on Minimum Ignition Current (MIC). MIC is the greatest value in determining standards for intrinsically safe apparatus (in which the energy available in the circuit is insufficient to ignite a gas).
• Subdivision based on Maximum Experimental Safe Gap (MESG). MESG is based on work undertaken to develop flameproof equipment.

A test rig is specified in the standards, and this is used to measure the Minimum Igniting Current (MIC) of the gas. Gases and vapours may be classified according to the ratio of their Minimum Igniting Current (MIC) with the ignition current of the laboratory methane.
Group II equipment is subdivided and, for the purpose of classification of gases and vapours, the MIC ratios are:

The other method determines the ‘maximum experimental safe gap’ (MESG) using an 8 flameproof litre sphere situated inside a gas-tight enclosure.

The halves of the sphere have 25mm flanges, and the gap dimension between the flanges can be varied. The area inside and outside the sphere is filled with a gas in its most explosive concentration in air, and the gas inside the sphere is ignited by a spark-plug. The maximum experimental which prevents ignition of the gas/air mixture outside the sphere is the maximum experimental safe gap. The more dangerous a gas, the tighter the gap.

The two experimental methods have been used to subdivide gases according to the degree of risk, and both proved that hydrogen and acetylene present the most risk (most easily ignited), IIC; and propane the least risk of ignition, IIA.

The following table shows that flammable materials fall into the same order for both MIE and MESG, and shows the gas subdivision and required equipment group for the gases listed. (Ref: IEC 60079-20-1:2010)