Switching behavior of different contact materials for vacuum interrupters under load switching conditions

In the past years the vacuum switching principle has gained wide acceptance in medium voltage systems. While vacuum circuit breakers, where high short-circuit currents have to be mastered, nearly exclusively utilize CuCr25wt.-% to 50wt.-% as contact material, the choice in vacuum interrupters for switching lower load currents (e.g. contactors) is less definite. The material must meet the requirements of low erosion losses on a high number of switching operations, low chopping currents, low tendency to generate HF transients and a satisfying quenching capability as well. Often materials based on refractory components are used, such as WCu, MoCu, WCAg, but also CuCr seems applicable. The aim of this work has been to investigate such contact materials with respect to their interrupting behavior on load and overload conditions. The experiments were carried out with a vacuum test chamber in a synthetic test circuit. The contacts were stressed by arc currents up to 7 kA_RMS and a transient recovery voltage of 23 kV_peak. Different contact materials have been compared with respect to their breaking capability and the state of their contact surfaces after arcing by means of scanning electron microscopy