Breakdown Voltage of Conditioned 24 kV-Vacuum Tubes as a Function of Ramp Rate

For the determination of the dielectric strength of different factory-made vacuum interrupter tubes, in the short time range, the breakdown voltage was measured as a function of the voltage ramp rate for various contact distances by applying linear rising impulse voltage with a steepness from 1 kV/ ¿s up to 3 MV/ ¿s. Preconditioning of the contacts was performed with a series of impulse breakdowns. An impulse voltage measuring circuit with a damped capacitive voltage divider and damped lead-in wire was used for the investigations. Adequate steps were taken to avoid flashovers outside the tubes and to give protection against X-rays. After preconditioning, g, some vacuum tubes reached a maximum dielectric strength of 550 kV at the rated contact distance of 14 mm. The minimum values were about 120 kV. The shape of the voltage ramp curves, which is dependent on the contact distance, leads to a conclusion on the effective breakdown mechanisms.