This standard covers the classification of surface and underground locations in which fires or explosions can occur owing to the presence of flammable gases, vapors or mists, dusts, fibers or filings in the air, in order to permit the proper selection of electrical equipment and mechanical equipment, such as compression-ignition engines (diesel engines), to be used in such locations. The classification is based on the type of substance liable to cause a fire or an explosion, and on the frequency or probability of the occurrence of explosive conditions.