Chopping Current and Quenching Capability of Low-Voltage Vacuum Arcs

The chopping current and the quenching capability during switching-off in a vacuum are investigated using Cu/Cr, Mo/Cu, and W/ Cu with added materials of high vapor pressure. Cylindrical contacts of small dimensions (15 mm diameter) with antimony, bismuth, cadmium, and zinc alloyed or added in solid form in holes or slots are used. The chopping current is determined duiing interrupting a 50-Hz current of 45 A. The investigation shows that alloying 2 percent of antimony or 3.7 percent of zinc to W/Cu 70/30 reduces the chopping current to about half of the value of W/Cu 70/30. Adding antimony, bismuth, cadmium, or zinc in solid form leads to a reduction from 6 A to about I A. The quenching capability is investigated in a synthetic test circuit using current injection with switchlng-off currents from 800-2400 A and values of the recovery voltage of 2.7 kV. The interruption properties of W/Cu 70/30 are deteriorated when components of high vapor pressure are added. If the contacts are separated shortly before current zero, multiple reignitions are possible due to the interruption of transient high-frequency currents. For the given circuit conditions, switching with short arcing times and small contact distances shows that multiple reignitions occur with all investigated materials. A dependence between reignition voltage and contact distance is given. Materials with a high vapor pressure generally reduce the reignition voltage.