Abstract
THE use of electrical appliances in coal mines needs to be safeguarded against the risk that they may be the means of igniting firedamp (or coal dust) and thus starting an explosion. Two methods are commonly employed. One is to provide a suitable enclosure from which flame cannot be ejected. The other method is to make the apparatus and related electrical circuits 'intrinsically safe', which means that any sparking which may occur when the circuit is broken at any point is incapable of causing ignition. One aspect of the latter method is discussed in a report, Paper No. 104, published recently by the Safety in Mines Research Board. It gives the relationship which has been found between the inductance in a circuit when the circuit is rapidly broken between a pointed platinum electrode and a wire and the minimum current capable of igniting firedamp. The effects of two methods of increasing the minimum igniting current have been measured. These are the introduction into the circuit of a shunting resistance or condenser. They can be of considerable value when the inductance is high and the current correspondingly low, as in apparatus such as bells, telephones, signalling and shot-firing systems, but become almost valueless when the working current exceeds about one ampere, or in the case of condenser shunts when a slow break of the circuit can occur. The results reported in this paper will be used at the Safety in Mines Testing Station in the examination of mining electrical apparatus submitted by the makers for approval.
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Intrinsic Safety of Electrical Apparatus. Nature 159, 159 (1947). https://doi.org/10.1038/159159c0
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DOI: https://doi.org/10.1038/159159c0