Vacuum life assessment of a sample of long service vacuum interrupters

The electricity distribution industry relies on an ageing population of vacuum interrupters. It is reasonable to ask the question “how long can vacuum interrupters continue to be used?” In order to answer this, the basic principles of a vacuum interrupter and vacuum insulation are outlined, and the difficulty of establishing whether the vacuum will still be good for a further period of use is explained. The shortcomings of the high voltage withstand test which is currently relied on as an indicator of vacuum condition are pointed out. A technique to assess the remaining life of old vacuum interrupters using a vacuum measurement apparatus is briefly described, and results are presented for a typical sample of nine long service interrupters of three types that have been in service for between twenty-five and almost forty years. It was found that some could be certified for another 20 years of use, some for a lesser period, and one should be immediately replaced. It is concluded that revalidation of interrupters in this way is a viable technique and allows their continued use for an extended period.