The quality rating of high-tension cable with impregnated paper insulation

During the past five or six years, practically all of the American cable manufacturers have made some changes in their impregnating compound to produce cable having a low dielectric loss. In a number of cases, these compounds were not well chosen. During the same period, some of the operating companies have reduced the thickness of their insulation, resulting in increased dielectric stresses. In a number of cases the compounds, and perhaps also the paper that was used, developed faults that were more serious than high dielectric loss. Some cables have been very deficient in dielectric strength and others have developed a new type of trouble which has been termed “ionization;” after a comparatively short time, the operating voltage of such cable had to be materially reduced or the cable replaced, in order to keep the lines in service. The cable manufactured during this period has generally passed, with a wide margin, the high-voltage tests required by the specifications. Occasionally a lot of cable which passes these tests by a narrow margin will be shipped by a manufacturer and, in such cases, the service record of the cable is unsatisfactory. The investigations which form the basis of this paper consist of, (a) laboratory tests to determine what high-voltage tests cable, known to be of good quality, will withstand; as well as similar tests on other grades of cable, (b) A careful examination and dissection of samples of the cable as received from the factory and samples removed from lines which have failed in service or which have been in service for a number of years without failure, (c) A comparison of the operating records of the cable with the results of the dissection and examination of the samples and with the factory inspection and tests made on the cable before shipment, (d) Accelerated life tests on long samples of cable having a wide range in quality. The results of these investigations have been correlated in a manner which permits of the determination of the relative quality of several lots of cable purchased, as well as changes in the specifications which must be made in order to secure satisfactory high tension cable with impregnated paper insulation.