Abstract :
In general, the temperature-rise of the surface of a cable, freely exposed to the air, due to the current carried, will only be a few degrees above ambient temperature, whereas if the cable is exposed to direct sunlight a substantial increase of temperature will occur. This increase may be treated as additional to that due to the load, provided it is noted, in calculating the former, that the resistance will be increased at the higher temperature reached by exposure to the sun. It is found that this rise is brought about in part by radiation, but mainly by convection. It therefore varies with the intensity of the solar radiation, the velocity of the ambient air, the diameter of the cable, its location (i.e. the extent to which it is exposed), and the nature of the surface of the cable. The maximum solar radiation in different parts of the world is known approximately, and some tabular information is given in the paper. From this the temperature-rise of a cable of given diameter can be determined for a given air velocity by means of a factorF varying with the diameter of the cable and with the air velocity. It is found, further, that there is in practice an approximate minimum air velocity, and the maximum rise is tabulated for this figure. This maximum will be attained in about half an hour, and will be reached if the sky is clear and the cable is directly exposed normal to the sun´s radiation between, say, 12 noon and 2 p.m. Summer Time. Under these conditions cables of 2 to 2? in diameter,in this country, may show a temperature-rise of 17 deg.C. This figure must be deducted from the permissible rise of, say, 50 deg. C. above shade temperature, thus materially reducing the rating of the cable. The data given are based on results obtained in London, Milan and Buenos Aires, and are in good agreement. An account is given of the methods employed and results obtained, together with supplementary information on certain technical features that presented difficulty during- the investigation.
