The transition from the molten bridge to the metallic phase bridge column arc between electrical contacts opening in vacuum

Abstract

The total arcing time between Ni electrical contacts opening in vacuum was controlled by interrupting a low voltage, low current circuit. The voltage across the opening contacts showed the rupture of the molten metal bridge and then its rapid increase to a value greater than the minimum arcing voltage before dropping back to a value close to the minimum arcing voltage. Using a radioactive tracer technique it was observed that at very short arcing times there was an anomalous net transfer of contact material to the cathode. As the arcing time increased this cathode gain decreased before increasing again at longer arcing times. High-speed streak photographs of Cr and W contacts opening in vacuum at higher currents showed an initial high pressure, metal vapor regime that resulted from the rupture of the molten metal bridge and before the formation of a metallic phase, columnar arc. After the rupture of the molten metal bridge, the voltage across the contacts rose to a value of a few 10psilas of volts before dropping back to a minimum value. These data will be used to discuss the rupture of the molten metal bridge, the formation of the pseudo arc in the high-pressure metal vapor resulting from the bridge rupture and the eventual development of the metallic phase, bridge column arc.