Experiments in high current switching using small contact gaps

A series of tests were performed on two types of commercial vacuum interrupters, herein called Type A and Type B to ascertain the effect of the contact gap on their high current interruption ability. The tests were carried out on a synthetic test plant at 12 kV, 31.5 kA for the Type A and 12 kV, 13.1 kA for the Type B, all values are rms. Three interrupters of each type were tested at contact gaps of between 8 mm and 1 mm. In addition, identical contacts were subjected to short circuit testing in a vacuum demountable chamber which allowed filming of the arc by means of a high speed camera. The results indicate that Type A showed a significant reduction in the probability of interruption of the rated short circuit current at contact gaps below 4 mm, whereas Type B showed no degradation at contact gaps down to 1 mm. The reasons for this are discussed