Factors governing specifications for electric power cables

Abstract

For electric cables, as used for indoor wiring, the only standard of reference now existing in this country is B.S.7, which covers only rubber insulants; for lack of any other specification, the dimensions therein specified for rubber compounds are being misapplied to other insulating materials, without taking into account the different properties of the latter.A new “basic specification” is proposed, composed of a series of electrical, physical, thermal, mechanical and chemical tests; and the author suggests the use of such an agreed basic specification for the determination of minimum dimensions of any approved materials of which the cable may be composed.Following out this suggestion, the author deals with such test requirements under the two headings: (a) electrical specification, which covers the primary purpose of the cable to transmit electrical power, and (b) mechanical specification, which covers the adequate protection of the cable in service.Under the electrical specification some calculations and graphs of interest are given as illustrations only of the more comprehensive survey that is necessary.The author further deals with practical and economic considerations, which may have some bearing on purely echnical conclusions, e.g.:(a) The lack of importance of eccentric insulation, provided the required minimum thickness is used.(b) The incorrectness of stipulating an average thickness with a plus and minus tolerance as against the correct method of specifying either a minimum thickness or a maximum, or both.(c) The increase of cable costs, basically as the square of the radial thicknesses imposed.In particular, the properties of pre-war rubber, reclaimed rubber, p.v.c. compounds and polythene are compared.