Effect of mechanical contact force on withstand voltage of copper-bismuth and CLR vacuum interrupters

The d.c. breakdown voltages of two commercial vacuum interrupters, having different electrode materials, is measured prior to and following axial compression of the contacts in a hydraulic press. In one switch, the contacts are fabricated of copper-bismuth alloy and the other of a propriety material known as CLR. It has been found that the compression of the contacts results in reducing the withstand direct voltage (deconditioning) of the open contacts in both interrupters. After a few compression cycles, the deconditioning effect diminishes and almost disappears for the Cu-Bi contacts. The CLR switch shows some tendency to cold welding of the contacts. However, the welds are easily broken and after a few compression cycles the deconditioning is smaller compared with, for example, the deconditioning due to high current arcing. The d.c. and a.c. breakdown voltages of vacuum gaps can be made to have almost the same values after spark conditioning of both electrodes with alternating voltage.