When transmission cables were first operated at potentials exceeding about 7500 volts, it was noted that cable failures occurred in service with loads materially below those which had therefore been found to be permissible with low-voltage cables,and this reduction in carrying capacity increased with in crease of the normal working potential.For
example the author has previously reported that No.0 A.W.G.
four conductor cables operating on a four-wire three-phase system with a maximum normal potential of about 4000 volts between phases carry 200 amperes on each of three conductors without damage due to the overheating,whereas a 250,000-cir.