GB170728A - Improvements in or relating to a method and apparatus for the measurement of the temperature of electric cables - Google Patents

Abstract

170,728. Fawssett, E. Aug. 23, 1920. Thermometers.- In apparatus for measuring the core temperature of an electric cable, the temperature of the lead or other covering or sheath of the cable is directly measured, and the difference between the temperatures of the core and the sheath is indirectly measured, these measurements being added to give the required temperature of the core. The difference between the temperature of the core and sheath. is reproduced in an artificial cable comprising a heater E, of material having a temperature coefficient similar to that of the core, ,and a bath F consisting of layers of paper impregnated with grease or other semi-solid mixture, the thermal constants of which are such as to give the, same temperature gradient between the heater E and the container G as that existing in the real cable. Alternatively the heater may form the core of a short length of miniature cable, having the same thermal constants as the cable under observation. For alternating currents the heater E may be placed in series with the usual switchboard ammeter C energized from a current transformer D, the primary winding of which is in series with the core A of the cable. For direct currents, the heater may be placed in a shunt circuit. In the arrangement shown thermo-junctions H, J, in the centre of the bath F and on the container G respectively, are used to measure the temperature difference between the heater E and container. In series with the thermo-junctions H, J is a thermo-junction K, measuring the temperature of the sheath B, and a millivolt meter L or other instrument calibrated to read the actual temperature of the core- A junction M kept at a constant temperature by a thermostat or by means of ice may be used to provide a reference temperature for which the instrument L is set. In a modified arrangement the thermo-junctions are replaced by resistance thermometers connected to a wheatstone bridge.