The influence of CuCr25 characteristics on the interruption capability of vacuum interrupters

This paper presents an extended survey on the influence of material characteristics of CuCr2S on the overall performance of Axial Magnetic Field Vacuum Interrupters (VI'S). Three distinct production processes are used to manufacture this material which can be discriminated by the maximum operating temperature; these processes are : solid state sintering (Tmax = 1050°C) , liquid stale sintering (Tmax = 1200°C) and melting (Tmin = 1600°C). In this way materials with varying density (95% - 99.9%) and conductivity (23 MS/m - 34 MS/m) are obtained. These materials are characterised for their welding strength, chopping current, cold dielectric behaviour, interruption performance and gas content. The materials are tested as 20 mm diameter flat contacts in an asymmetrical V1 with external axial magnetic field coil. Short circuit interruption performance was highly dependent on the production process. We performed our tests at a short circuit current of about 10 kA in a strong AMF (15 mT/kA) and at a recovery voltage of 12 kV (TRV peak 21 kV). Under these conditions most materials interrupt the current but produce late to very late breakdowns. We correlate this behaviour to the above mentioned material characteristics, the gas and impurity content of the contacts and arc erosion products. We conclude that the solid stale sintering process yields a reliable and economical CuCr25 contact material with good overall performance in axial magnetic field VI's.