Permissible Operating Temperatures of Impregnated Paper Insulation in Which Dielectric Stress is Low

The writer reviews the effect of temperature on insulating materials, abstracting from the 1913 Steinmetz and Lamme report, and the 1905 British Engineering Standards Committee tests. Then he gives surveys of low-tension cables in large distributing systems, and special tests on cables including sheath cracking, high temperature tests, effect of bending on cables heated at high temperatures, distillation of cable compounds, ambient temperatures in subway ducts as affected by thermal conductivity of concrete, amount of moisture in soil, different arrangements of ducts and load factors at which the cables are operated. From this review, conclusions are derived that the permissible operating temperatures are to be a function of the load factors at which the cables operate. The writer recommends 105 deg. cent., 95 deg. cent. and 90 deg. cent. for load factors of 33 per cent, 50 per cent, and over 66 per cent, respectively.