An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. Usually, an area designated as susceptible to explosion danger is one where two of the three requirements for a combustive explosion are or can be present: a combustible material, an oxydizer and a source of ignition. Areas classified into zones (0, 1 and 2 for gases, vapours and mist or zone 20, 21 and 22 for flammable dust) must be protected from effective sources of ignition. Zone 0 and 20 represent the highest risk. The intrinsic safety certificate of a piece of equipment classifies it for use in certain zones, depending on how likely it is to act as a source of ignition. Areas classified as zone 0 or 20 require category 1 equipment; zones 1 and 21 category 2; while zone 2 and 22 require category 3 equipment.