The ignition temperature of an explosive gas atmosphere or dust cloud is the lowest temperature of a heated surface at which, under specified conditions, either a combustible in the form of gas or vapour, or as a dust/air mixture, ignites.

The ignition temperature of gases, vapours and dusts is therefore a crucial criterion for the selection of equipment in potentially explosive atmospheres.

Combustible gases and vapours are classified in temperature classes according to their inflammability. The maximum surface temperature of electrical equipment must always be lower than the ignition temperature of the gas/air or vapour/air mixture in which it is used. Equipment which correspond to a higher temperature class (e.g. T5) are, also permitted for use in applications in which a lower temperature class is required (e.g. T2 or T3). Note: In North America, there is a system incorporating further classification into temperature subclasses.
Combustible dusts are not divided into temperature classes and so the ignition temperature of the dust cloud must be compared with the maximum permissible surface temperature of the equipment. However, since dusts can build up on the equipment, the ignition temperature of the dust layer (glow temperature) must also be taken into account. In both cases, the specified safety factors must be considered. Higher layer thicknesses also increase the thermal insulation, which is why maximum permissible surface temperature on the equipment must be further reduced.