Estimation of vacuum condition in vacuum circuit breakers

In this paper a method is described for the estimation of the pressure inside vacuum circuit breakers which have been in operation for several years. The method is applicable for measurements in the field and uses only electrical measurement techniques. The vacuum circuit breakers switches off a 10 A DC current; arc voltage pulses are measured and represented in a histogram. The shape of the histogram is discriminative for pressures below or above 10/sup -4/ mbar.