The U-curves of five different test objects, three of which contain a rod floating object at different positions within the gap, are characterised. During the testing, a high speed camera was used to photograph the discharges. The results are compared to Rizk’s theoretical model for determining the flashover voltage of gaps with floating objects are presented. It is concluded that the position of the floating object within the gap affects which discharge mechanism exists in each of the gaps. The effect that each discharge mechanism has on the flashover voltage and time to crest of the gap is shown. Time interval photographs are presented showing the formation of a discharge channel due to the streamer mechanism. In evaluating the high speed photographs it is seen that the extent of the branching of the discharge channel is a function of the time to crest of the applied impulse, more branching is evident for shorter times to crest.

A growing trend in the electrical industry is to move away from overhead lines and towards underground distribution. The cables necessary for underground distribution are stressed in various ways, and one of the most ignored stresses is that of overvoltage caused by switching. The focus of this research is to determine the effects that high voltage switching impulses have on the electrical strength of medium voltage cable insulation. Accelerated aging was performed on multiple samples of XLPE and EPR cables by applying multitudes of switching impulses. At various stages of the aging process, partial discharge measurements were taken. At the end of the aging process, the ac breakdown voltage of each cable sample was determined. While more testing is necessary to gain a greater understanding of this subject, the results obtained in this study show that switching impulses weaken cable insulation, which may lead to premature failure of distribution cables.