The thermal resistance and current-carrying capacity of three-core screened and S.L.-type cables

A simple experimental method of determining the thermal effect of the screens in 3-core screened-type cables is described and a comparison made between the results obtained and those derived from other sources. A basic diagram embodying the experimental results is given which covers a wide range of conductor sizes and insulation thicknesses and enables the influence of different types of screens to be readily determined. From this diagram a practical chart has been prepared showing the thermal resistance of 0.04- to 0.5-sq. in. 3-core cables with the two types of screens commonly employed, namely (1) 3-mil copper tape; (2) 1.5-mil aluminium foil. In general, it is found that for practical sizes of cables copper-tape screens reduce the thermal resistance by about 30 to 40 per cent, and aluminium-foil screens by about 15 to 30 per cent. A method of calculating the thermal resistance of S.L.-type cables is given from which it appears that, contrary to popular ideas on the subject, this type of cable is not always the best from a thermal point of view.