Switching of load currents with vacuum interrupters and the resulting electrical life

Although a considerable amount of effort is focused on the short circuit performance of vacuum interrupters, in practical applications these high current operations rarely or almost never occur. Instead, the bulk of operations in the field are switching load and/or capacitive currents. For load current switching, the low contact erosion and low metal vapor generation of the vacuum arc enables very long electrical life. Although this behavior is very advantageous, it does make the testing and determination of the ultimate electrical life at load currents difficult. Tests on Cu-Cr contacts in vacuum interrupters performed 30,000 operations at 2500 A rms. The vacuum interrupters maintained the low contact erosion, contact shape and composition, dielectric strength, and small change in contact resistance necessary to perform all required duties even after arcing operations with total transferred charge of >300 kA-s per interrupter. Contact erosion was minimal and of limited practical significant. These results confirm that vacuum interrupters can have a very long electrical life when switching currents where the vacuum arc is naturally diffuse.