Behavior of CuCr and WCu contacts during making tests at high voltage and high currents

Contact welding is a major problem in the medium voltage vacuum interrupters. Different solutions can be proposed such as adding a third element or increasing Cr content in the CuCr contacts. Moreover WCu contacts can be used even if the current breaking performance is decreased. To understand the phenomena involved during a making test, an experimental vacuum closing device was designed that recreates the pre-arcing generated under a high voltage of 14kV. Currents from 5 to 50kA are injected so that the contacts can weld. The force to break this welding is then measured and various contact materials such as WCu (10 to 30% of copper) and CuCr (25 and 50% of Cr) can be classified. The arc can also be filmed during the contact closing, thus enabling the understanding of the evolution of the arc and the bounces. Making tests in VI and in the vacuum chamber are in good agreement and the anti-welding ability of WCu was found to be better than that of CuCr. However, during our experiments, no correlation could be shown between the pre-arcing times and the welding forces. The arc modify the contact surface which is strongly influential on the bounces, thus the pre-arcing time has an irregular evolution and is not indicative of a welding.