The breakdown mechanism of impregnated paper cables

A new technique of cable examination has been developed. Incipient faults have been studied before actual breakdown with the aid of a microscope, the papers having been freed from oil and dyed to show up the waxing. The types of waxing observed are classified, and their part in breakdown is discussed. The deterioration is always found to be in connection with the conductor, generally through the gap of the first paper. The occurrence of the maximum deterioration at a distance from the conductor is explained, and a diagram showing the path of failure in an actual case is given. The conclusion reached is that failure originates from the ionization in a void in contact with the conductor, but that subsequent breakdown takes place by carbonization of the compound. Such carbonization requires time for its completion and is the chief factor in the voltage/time-to-breakdown curve. It takes place both longitudinally, over the surface of the paper, and perpendicularly through it. The results obtained in the laboratory are compared with those in service, and the types of failure are described. The effect of d. c. stress on cables is discussed and the advantages of routine d. c. testing of feeders are explained on this basis.