Dependence of contact weld force on arc energy supplied to contact surface in low voltage switches

Abstract

This paper describes a mechanism of contact welding formed by bounce arc upon contact making, based on experiments, theoretic and analytic study. We measured the weld force after every contact making in a low-voltage condition (200 V-AC). The arc current was set to 140 to 500 A by using a reactor load, the arc duration time was within 0.22 to 2.37 ms, and the contact material was pure silver (99.99 %). In this condition, we clarified that the weld force increases depending on arc energy supplied to the contact surface. We also measured the melted area on contact surface and clarified the area dependence on the arc energy. Considering that the tensile strength of silver is 170 N/mm2, the measured melted area is too large as a welded area. Then we found out that the real welded area corresponds to about 25 % of the melted area. As a conclusion, we suggest that a vaporized area on the contact surface may not be involved in contact welding, from the analytical simulation of thermal conduction in the contact model. The real welded area is the region whose temperature is higher than the melting temperature and that remains without evaporation.