The testing of compression fittings on steel-cored aluminium conductors for overhead lines

Although midspan joints and anchor clamps for overhead power lines are designed to satisfy stringent specifications, failures occur due to faulty manufacture or erection. Compression fittings are now the only types being fitted by the Central Electricity Generating Board, and statistics are given of their relatively low rate of failure. The usual mechanism of failure is by overheating of the steel core by the current it is forced to carry when high-resistance contacts exist between the aluminium strands of the conductor and the fitting. The testing of fittings during erection or on unenergized lines can be performed by resistance measurements, with suitable precautions, by radiographic examination, or by magnetic location methods. On energized lines, the fitting can be tested with a hook-on voltmeter or thermistor, or with an infra-red bolometer. The results are given of resistance and bolometer tests on 132kV lines in South-East England. The two methods detected a similar percentage of defective fittings. It is concluded that failure of fittings can be practically eliminated if care is taken during manufacture and erection. The testing during erection and at regular intervals during service should ensure that no further faults occur. It is recommended that all jointers be properly trained.