The Transition from to the Metallic Phase Arc After the Rupture of the Molten Metal Bridge for Contacts Opening in Air and Vacuum

The total arcing time between opening electrical contacts interrupting a low voltage, low current circuit in both air and vacuum was controlled. 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 contacts opening in air at higher currents showed an initial shock wave that resulted from the rupture of the molten metal bridge and before the formation of the metallic phase arc. Similar photographs of contacts opening in vacuum showed the initiation of a metal phase arc in the region of the ruptured molten metal bridge. After the rupture of the molten metal bridge, the voltage across the contacts rose very rapidly to a value of a few 10's 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 arc.