“Gas Groups” refer to the sub-groups of gases that fall under Group II hazards which includes gases, vapors and flammable mists. The purpose of Gas Groups is to provide a simplified framework for use in classifying hazardous areas.

There are three sub-groups of gases (Gas Groups) within Group II which are ranked in order of increasing volatility based on the following:

• Explosive pressure they generate when ignited; and
• Minimum ignition energy or the amount of energy required to ignite the most explosive mixture of the gas with air.

The Gas Groups are defined in the following table:

IIC gases are more volatile group than Group IIB which are more volatile than Group IIA gases. The more easily ignitable the mixture, the more rigorous the conditions are for equipment to be installed in an area containing them.

When designing electrical systems for explosive gas atmospheres, it is critical to explosion prevention that electrical equipment is appropriately rated, for the most volatile gas it is exposed too.  

Equipment rated for use with gases in Group IIC can be installed in areas classified as IIC, IIB or IIA, as the equipment is designed for more volatile and ignitable gases.  Equipment rated Group IIB can be used with Group IIB or Group IIA gases. Equipment marked IIA can only be used in IIA environments.

In short, Gas Groups (IIA, IIB & IIC) are the categories into which a range of gases, vapors and flammable mists are grouped depending on volatility, to allow for simplified area classification and safe equipment selection.

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