Permissible operating temperatures of impregnated paper insulation in which the dielectric stress is low

FOR a number of years the standard low-tension direct-current feeder cable in Chicago was 1,000,-000-cir. mil two-conductor concentric paper-insulated cable with three pressure wires laid up with the outer conductor. Apparently there has been the greatest tendency to overload this particular size of cable more than any other as, with both single and concentric cables in service, it was difficult to make a proper distinction between the carrying capacity of the two kinds of cables. About ten years ago we had two cases in which the cables of this kind were called upon to carry loads somewhat larger than had been customary, and steadily throughout the day instead of a short peak in the evening. The two customers in these cases, in different portions of the city, were both manufacturing customers with an eight-hour load, and on account of some increase in business they increased their load on short notice when no cable was available for another feeder to their premises. In each case, the cable has to be ordered from the factory so that this unusual load was carried by these cables for some three or four months. In the first case of this kind we were so disturbed about the temperature that we made a rather extended series of temperature measurements, using one of the pressure wires imbedded in the outer conductor as a resistance, and determining the temperature by the rise in resistance. This, of course, would give us the temperature of the outer conductor which would be probably 8 or 10 deg. lower than the inner conductor. The temperature measured in this manner was slightly above 100 deg. cent., and as there was some doubt regarding the accuracy of the determination made in this manner, it was checked by cutting a small hole in the lead sheath of the cable and in the outer insulation, and measuring the temperature of the outer copper with a thermometer. The temperature taken in this way checked very closely with the temperature as determined by the rise in resistance - f one of the pressure wires. The insulation removed in this manner was carefully preserved and compared with the insulation in a piece of new cable, and it was impossible, by careful examination, to discover any appreciable difference.