The duation of arcing following late breakdown in vacuum circuit breakers

Vacuum interrupters occasionally show unexpected (late) breakdown after current interruption. As a result, the interrupter conducts briefly, carrying a high-frequency current due to the discharge of various (stray) capacitances in the vicinity of the interrupter. Thanks to the capability of vacuum interrupters to interrupt such high-frequency current, the conduction period is normally very short (normally in the order of 10 µs). In the discussion around the assessment of such late breakdowns (often termed nonsustained disruptive discharges in the standardization literature) regarding the possibility of generation of overvoltage 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 prestrike) of two commercial vacuum breakers. It is demonstrated that the duration 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. For practical circuits, allowing simultaneous discharge from various circuit parts, it was observed that the highest frequency component determines the arc duration. Duration of up to 15 µs was observed. This duration is too short to allow significant overvoltages to develop in standard capacitive power delivery circuits.