The influence of composition and Cr particle size of Cu/Cr contacts on chopping current, contact resistance, and breakdown voltage in vacuum interrupters

A range of commercially produced Cu/Cr materials with different compositions (25% to 75% Cu) and Cr particle sizes (up to 160 μm) has been investigated for their differences in chopping currents, contact resistance, breakdown voltage, contact erosion, and contact appearance. Test parameters have been chosen to reflect actual switching conditions. The properties of the Cu/Cr contact materials have been investigated after prestressing the electrodes with defined numbers of break arcs of 1 and 5 kA with alternating and unchanged polarity. It was found that the differences in properties of Cu75/Cr25 and Cu50/Cr50 contacts are minor as long as the same Cr particle size is used. Fine Cr particles increased the breakdown voltage and the rise in breakdown voltage after prestressing and the total erosion of contacts. While the mean chopping current values remained almost unaffected by fine Cr particles, the maximum values of chopping currents were deceased by using fine Cr particle sizes. Contact resistance and cathode surface structure after prestressing remained unaffected by a change in Cr particle size