Methods for estimation of the vacuum status in vacuum circuit breakers

In this paper several methods are described for the estimation of the pressure inside vacuum interrupters which have been in operation for many years. The methods are applicable without demounting the interrupters and use only electrical measuring techniques. According to these methods the vacuum circuit breakers are subjected to synthetic circuits, switch-off limited currents, either 10 A dc or 300 A ac. With the 10 A dc method the height of arc voltage spikes are measured and represented in a histogram. The shape of the histogram is discriminative for pressures below or above 10^-2 Pa. Switching tests with currents of 200 to 400 A ac in inductive and capacitive circuits also showed different behavior for internal pressures below or above 10^-1 Pa. A three phase 200 A test device with a 10 kV recovery voltage, fed from a standard 380 V supply, showed similar results