DC ground fault overvoltage relays (64P) are commonly used at DC railway substations to detect DC feeding bus ground faults. DC bus ground faults may cause fire or other serious damage. Therefore, 64P relays, which send a command to open all related circuit breakers, are a very important device. However, 64P relays sometimes disrupt railway traffic because it is occasionally activated by irrelevant faults, such as ground fault at external feeding lines or other substations. We are now developing a new method to detect DC bus ground faults that can eliminate the influence of unrelated faults. This paper describes an outline of the new method and results of basic investigation.

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