Explosive gases, vapors and dusts have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Based on the nature of the explosive gas/dust, gases are grouped into IIA, IIB and IIC and dusts into IIIA, IIIB and IIIC.
Temperature classes are defined for equipment based on its maximum surface temperature. When selecting a piece of equipment for a potentially explosive atmosphere, the equipment’s maximum surface temperature must be lower than the ignition temperature of the possible potential gas or dust mixture.