Investigation of elbow and insulated bushing cap flashovers during unloaded switching

Extensive testing has been performed in an effort to identify the phenomena responsible for widespread field reports of unexplained flashovers while switching 25 and 35 kV elbows to drop unloaded and lightly loaded cables. A large number of elbow and cap pulling operations were performed under a wide range of conditions. In addition, mechanical tests were performed to characterize relationships between lubrication, displacement, and partial vacuum developed between elbows and bushings during pulling. While the low flashover rate achieved in the laboratory testing does not support statistically valid correlation of flashovers to circuit, mechanical, and environmental factors, the test results provide significant information. In particular, the results imply that commonly accepted explanations for the flashovers may be incomplete, and that there are complexities to the phenomena which are yet to be fully understood.