Fault Development in Wet, Low Voltage, Oil-Impregnated Paper Insulated Cables

Few studies have been carried out on the failure processes of low voltage distribution cables. This is mainly due to their excellent track record of high reliability, and their use of traditional technologies. Growing quantities of aged cable within the network now make management of these assets an important issue. This paper describes measurements of leakage current, partial discharges and arc formation within 400 V oil-impregnated paper insulated cable structures which have been artificially wetted by immersion in bulk water. A generic picture is produced to show the gradual evolution from a leakage current through discharge growth to final catastrophic failure. Rapid changes to the core temperature of a wet cable are shown to produce transient, high levels of leakage current. It is also shown that by increasing the core temperature more slowly these high levels can be reduced. It is suggested that the characteristics of individual arcs within successive half cycles may provide a basis for predicting whether an incipient fault will lead to failure in the short term.