The breakdown of cables by thermal instability

An approximate method is described by which the problem of thermal stability of the cylindrical dielectric of a cable can be investigated. This involves the problem of deducing the dielectric loss angle against electric stress characteristic, for the given dielectric, from measurements of dielectric loss angle against voltage made upon a cable; and methods are described for carrying out this conversion. Predictions made by these methods are supported by experimental results. The post-mortem characteristics of thermal-instability failures are described, so that these can he diagnosed and distinguished from failures of the tracking and other types. An account is given of the cable failures met with in excess voltage testing in the laboratory, in which there is a transition from breakdown of the tracking type to a final breakdown by thermal instability.