The main object of the tests described in the paper was to investigate why a certain carrier-current equipment, protecting a feeder incorporating a considerable length of 132-kV cable, operated incorrectly on a number of occasions during the war, other attempts to locate the trouble having failed. It was considered worth while to pursue this investigation, so far was was practicable, in order to establish whether carrier-current protection was unsuitable for feeders incorporating lengths of cable.During the tests, the following observations and measurements were made:(a) Power-frequency currents and voltages.(b) The behaviour of switchgear when making and breaking large fault-currents at full voltage.(c) General characteristics and appearances of power-system fault arcs.(d) Induction in neighbouring Post Office communication circuits.The test procedure and equipment used embodied certain novel features, and special attention had to be paid to co-ordinating the activities of the large number of observers employed. Whilst the tests yielded the information sought in items (a)–(d), they did not immediately reveal the cause of the incorrect protective-gear operations during the war. Subsequent examination of the test records drew attention to certain unexpected phenomena which were later investigated by further tests. These phenomena are thought to be the cause of the trouble.