Progress report on failures of high voltage bushings with draw leads

Over the past several years, several bushings equipped with draw leads have failed in service. These bushings are not isolated to a single manufacturer and all of them have passed standard routine and type tests. A common characteristic of each failure is the arcing marks found between the draw lead cable and the tube in the center of the bushing. Most of the bushings also failed violently, in that the bushing exploded and caused a fire. The purpose of this panel session paper is four fold: First, extensive investigations have already been conducted on aspects of certain failure mechanisms, and the findings will be reported. Second, other possible, but as yet uninvestigated, failure modes are given. Third, it is hoped this report will draw the interest of others in the industry who will add to the knowledge surrounding this subject. Fourth, suggestions are made to prevent or alleviate the problem. It is not known exactly what causes the arcing in the first place, but it is currently thought that it is initiated by a transient of some kind then is sustained by the power frequency service voltage. Possible transient generators and the ensuing failure mechanism are described.