Ignition of a flammable mixture may occur following an arc, spark or hot surface during the use of electrical apparatus. Arcs can result from the discharge of stored energy or from switching contacts.

Although electrical equipment can be one of the sources of ignition, it should also be recognized that non-electrical equipment may also be the source of ignition. Hot surfaces sufficient to cause ignition can arise from electrical enclosures, or components, or even some types of mechanical equipment. In addition ignition could also be initiated by frictional sparking and electrostatic action.

Other sources of ignition energy are open flame, stray electric currents, lightning, compression, engine exhausts, heat from chemical reactions, spontaneous combustion, & heat from the sun.

Taking into consideration that equipment that is intended for use in explosive atmospheres must be specially protected so as to avoid becoming the source of ignition, the following categorisation of apparatus has been adopted.

Apparatus Group Application
1 Underground Mining
2 Surface Industries