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In an investigation into a series of falures of 275 kV and 400 kV cable stop-joints, examination of unfailed joints taken from service revealed bands of accumulated debris in the oil channels of the joints. The debris was identified as being mainly copper and its properties were studied. A mechanism of failure of the stop-joints was proposed on the basis of small scale experiments, this was that metallic particles can form lines in an electric field and if the line is sufficiently long gas can be generated at the points of contact. The relationship between the length of line and the field required to generate gas has been determined. The next stage in the failure mechanism is the build up of gas and the formation of bubbles in the oil channel. These bubbles, composed mainly of hydrogen, discharge and eventually cause failure. The proposed mechanism has been tested in various simulants of stop-joints and has been shown to be feasible and consistent with the evidence from failed joints. Some remedial measures for both new and existing circuits are proposed.