The duration of arcing following late breakdown in vacuum circuit breakers

Vacuum interrupters occasionally show unexpected (late) breakdowns after current interruption. As a result, the interrupter conducts briefly, carrying a high-frequency current due to the discharge of various capacitances in the vicinity of the interrupter. Thanks to the capability of vacuum interrupters to interrupt such high-frequency current, the conduction period is very short (normally in the order of 10 microsecond). In the discussion around the assessment of such late break- downs (often termed non-sustained disruptive discharges in the standardisation literature) regarding the possibility of generation of overvoltage as a result of late breakdown in capacitive circuits, the duration of the conductive period is an important parameter. Experiments were carried Out, in which discharges of various single frequencies (72 kHz - 1.1 MHz) were generated following breakdown (spontaneous breakdowns as well as pre-strike) of two commercial vacuum breakers, stressed with ac voltages well above the rated value. From a statistical analysis, it is demonstrated that the dura- of the discharge depends strongly on the discharge current frequency. At higher frequencies, the duration of the discharge tends to be shorter than at lower frequencies due to the domi- nance of thermal reignitions of the arc at high values of dl/dt over dielectric reignitions, observed only at lower frequencies. For practical circuits, allowing simultaneous discharge from various circuit parts, it was observed that the highest frequency component determines the arc duration. Durations of up to 15 us were observed. This duration is too short to allow significant overvoltages to develop in standard capacitive circuits, the main candidate switching duty to be troubled with overvoltage generation.