Electrical ignitions and use of flameproof enclosures in coal-dust and methane atmospheres

In recent regulations of the European Union concerned with mine atmospheres the classification of an explosive atmosphere was extended to include dust s. It is demonstrated that, in the context of electrical equipment used in mines, coal-dust atmospheres present no risk in excess of that from methane (firedamp) atmospheres. Current knowledge on the transmission of coal dust and coal dust-methane combustion through safe gaps is reviewed, and some e xperimental work on electrical ignition of coal-dust atmospheres is describ ed. Four main conclusions are drawn. First, it is unlikely that combustion of c oal deposits will propagate through a gap of 1 mm or less in width and 3 mm in length, and gaps designed to methane flameproof standards are unlikely to permit propagation of coal-dust deposit combustion from inside the enclo sure to outside. Second, gaps of 2 mm in width or less are unlikely to perm it external ignition by way of individual heated particles; gaps designed t o methane flameproof standards are unlikely to permit external ignition of dust clouds or hybrid mixtures in this way. Third, coal dust and coal dust- methane mixtures have safe gaps generally greater than that of methane; gap s designed to methane flameproof standards are unlikely to permit external ignition of coal dust or coal dust-methane hybrid atmospheres by direct exp losion transmission. Fourth, coal dust and coal dust-methane mixtures do no t ignite at ignition energy levels below that required to ignite methane.