Vacuum Interrupters (VI) have been the dominant switching device in medium voltage switchgear for the past 30 years. They have proved very reliable and many have been in service well beyond the 20 year service lives that manufacturers assigned to them. We have examined over 200 life-expired VI withdrawn from service for checking. The only end-of-life failure mechanism found was degradation of vacuum. About seven percent had such poor vacuum that they either did not withstand test voltage or they would be expected to fail quite soon. This is a very high rate of failure compared to that reported for VI within their design life of 20 years, and implies that the sample may be showing signs of end of life faults.
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