A study of vacuum levels in a sample of long service vacuum interrupters

We report on the inspection and testing of 140 vacuum interrupters that had been in service with utilities. These had significantly exceeded the normal 20 year vacuum lives originally assigned by the manufacturers, but were still considered to be good, and remained in service. Our sample included interrupters of a number of different types from two manufacturers. We found that some interrupters still had an extremely good vacuum. The majority had a slightly degraded vacuum, but we could confidently predict a further 10 or 20 years of use. A few had a poor vacuum and needed to be replaced urgently. A small number had such a poor vacuum that they failed a voltage withstand test, and were not suitable for vacuum measurement. This study concludes that although the reliability of vacuum interrupters is extremely high during the normal service life, interrupters in use for significant periods beyond this life have a significantly higher probability of vacuum failure, following the predictions of the classic "bath tub" curve of failure.