Vacuum-fused circuit breakers for low and medium voltage AC trailing cable protection

Since the enactment of the Federal Coal Mine Safety Act of 1969, the use of fuses has not been allowed for protection for three phase AC trailing cables in underground coal mines. In 1981 a mining equipment manufacturer approached the Mine Safety and Health Administration with a combination fuse-contactor device requesting that it be permitted as a means of protection for three phase AC trailing cables of 1000 volts or less. The concept behind the design of the vacuum contactor-fuse device was to use a vacuum contactor to provide protection to a specific current level during abnormal circuit conditions. If fault currents exceeded this level, a current limiting fuse would be used to interrupt the current before the vacuum contactor could open. The Mine Electrical Systems Division located in the Mine Safety and Health Administration´s Pittsburgh Safety and Health Technology Center was called upon to make an evaluation of this device. This paper details the tests and criteria used to ensure that the device provided no less protection than a molded case circuit breaker. Details of the laboratory tests conducted and the results of extensive field tests are presented