Flashover along cylindrical insulating surfaces under positive lightning impulse voltages

Results are presented concerning discharge development and breakdown along cylindrical insulating surfaces in a 12 cm long rod-plane gap under positive standard lightning impulse voltages. Cylindrical insulator specimens, made of silicone rubber and glazed porcelain, could be inserted parallel to the rod electrode, bridging the gap; the case of air alone was regarded as reference. Discharge development patterns were obtained by oscillographic monitoring of the electric field strength at the earthed plane. An insulating surface, depending on material permittivity, may inhibit the development of the initial corona streamers but favour breakdown.