Vacuum Interrupter Impulse Voltage Testing Procedures Should Recognize Initial Breakdowns as Reconditioning Events

It is well known that the ability of a gap in vacuum to withstand high voltages is a property enhanced by conditioning with high voltage[1]. Moreover, the conditioning effect is actually produced by small spark discharges that occur while this high voltage is applied. These spark discharges, or disruptive discharges in the language of test standards, are therefore required to produce the desired conditioning effect in the vacuum interrupter. In addition, the few disruptive discharges that sometimes occur during the process of performing impulse voltage testing in a completed circuit breaker are normal. Although observing such discharges is not new to those active in vacuum technology, they have only recently become an issue in performing impulse voltage certification testing as design voltages have increased to 36 and 38 kV and as the demand for ever smaller interrupters has led to higher stressed designs. The procedure for performing impulse voltage tests must therefore allow for a few disruptive discharges in a way that does not erroneously consider them as evidence of failure in the test.