Electrical Life of Vacuum Interrupters for Load Current Switching

While the short-circuit current of vacuum interrupters (VIs) is a key performance parameter, very high current operations rarely occur in actual applications. In contrast, switching load and/or capacitive currents comprise virtually all the practical operations. Load current switching requires a low contact erosion and low metal vapor generation of the arc to achieve a long electrical life and is achieved by contacts in vacuum. However, the excellent performance of contacts in vacuum makes the testing and determination of the ultimate electrical life at load currents challenging. Tests on Cu-Cr and Cu-W contacts in VIs were performed at 2500-2700 A rms with 20,000-30,000 switching operations. The VIs with both types of contacts retained their dielectric strength, contact shape and composition, and low contact resistance. Even after arcing operations with total transferred charges >200 kA,center dot,s per interrupter, the contacts could continue providing good dielectric and switching performance with low resistance. Contact erosion was minimal and of limited practical significance. These results demonstrate the very long electrical life of VIs when the vacuum arc is naturally diffuse.