The Effect of Contact Closure in Vacuum with Fault Current on Prestrike Arcing Time, Contact Welding and the Field Enhancement Factor

Experiments were performed with vacuum interrupters containing Cu-Cr (25 wt%) and W-Cu (10 wt%) contacts. The vacuum interrupters were placed in a spring mechanism, which was placed in a tuned, capacitor bank electrical test circuit. The capacitor bank was charged to 25 kV, which allowed a symmetrical fault current of 50 kA (peak) at 30 Hz. As the vacuum interrupter´s contacts closed a prestrike arc occurred when the contact spacing was small enough. This contact gap was recorded. The prestrike arc initiated the ac current, which was interrupted by the test circuit after one half cycle. The contacts were then opened with no current. This process was repeated 5 times. As the experiment progressed the prestrike arcing time increased; i.e. the contact gap broke down at larger and larger gaps during the closing operation resulting in longer and longer prestrike arcing times. We explained this phenomenon by considering the effect of the prestrike arc and the subsequent contact welding on the surface structure of the contacts. The change in the contact´s surface structure resulted in an increase of the field enhancement factor, which, in turn, led to the vacuum breakdown of the contacts at increasing contact gaps. For the Cu-Cr contacts the prestrike arcing time was eventually long enough that the contacts formed a weld that the mechanism could not break. Although the prestrike arcing time with the W-Cu contacts did increase, the mechanism always broke any welds that formed.