Heat or ignition energy must be supplied in order to ignite a flammable substance (gases, vapours or dust-air mixtures) which may cause fire or explosions. Ignition energy may be supplied in the form of sparks (no radio sets in paint shops), flames, hot surfaces, static electricity or through chemical reactions. The minimum ignition energy depends on the substance and therefore is a yardstick for the fire or explosion hazard of a substance. The Regulation relating to Explosive Atmospheres (Verordnung über explosionsfähige Atmosphären, VEXAT) stipulates that areas in which explosion hazards exist, must be free from effective ignition sources. Potential ignition sources must be avoided or reduced to the absolutely necessary minimum. Electrical plant must be positioned outside areas with explosion hazards wherever this is possible.