Positive lightning impulse discharges along cylindrical insulating surfaces bridging a short rod-plane gap

The salient characteristics of the predischarge phenomena and flashover along cylindrical insulating surfaces bridging a short rod-plane gap under positive lightning impulse voltages were measured in a wide range of applied peak voltages. Materials used in the investigation were polytetrafluoroethylene (PTFE), silicone-rubber, nylon and glazed porcelain; the case of air alone was regarded as reference. Corona inception is favoured by the presence of an insulating surface, this being more evident for materials of high permittivity and for initial than secondary corona. At relatively high applied voltages, the initial corona characteristics are directly related to the field enhancement at the tip of the stressed electrode. However, near threshold inception conditions there is a marked reduction of the inception field with material permittivity; an assisting mechanism of initiatory electron production was put forward to explain this behaviour. Initial corona streamers require higher applied voltages to arrive at the earthed plane when propagating along an insulating surface than in air alone. Flashover, closely related to secondary corona characteristics, is always preceded by continuous corona growth, occurring mainly in free air away from the insulating surface because of possible positive surface charge deposition by the initial corona; the latter is characteristic for polymeric insulating surfaces. There is a reduction of the dielectric strength of the gap in the presence of an insulating surface, more pronounced for higher material permittivity, related to the facilitating action of the field enhancement close to the rod tip on continuous corona growth preceding flashover.