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