Impulse breakdown voltage and prebreakdown currents of prestressed circuit breaker vacuum tubes

The electric strength of factory-made vacuum tubes was investigated by application of impulse voltages with various rise times between 0.08 and about 700 mu s. The investigations were performed with a contact distance of 3 mm and with the rated contact distance of 14 mm. To get a defined initial state of the contact surface, the vacuum tubes are prestressed first by breakdown conditioning with impulse voltages at the rated contact distance. For 3-mm contact distance, the breakdown voltages are nearly constant over the whole range of rise times, while for 14 mm the breakdown voltages increase for rise times shorter than about 0.5 mu s. No-load operations cause a larger spread and lower impulse breakdown voltages. There is nearly no influence on the values of the maximum breakdown voltages, but these maximum voltages occur more seldom. On the average, emission currents are higher than in the conditioned state. High-current closing operation with subsequent no-load opening of the contacts reduces the breakdown voltage and increases the emission current due to sharp microprotrusions on the contact surface. The shape of the V-t curves is the same as in the conditioned state of the contacts.