Abstract: Transmission lines have both their terminals connected with bus lines through breakers, respectively, and connected to earth through earth switches, respectively. Protecting relay devices are fed with not only the outputs of voltage transformers and current transformers but also the contact signals made coactive with the breakers and earth switches at the terminals considered. Those contact signals are transmitted through signal transmission devices to the other terminals, and the contact signals of the breakers and earth switches at the other terminals are transmitted. The protecting relay devices detect the fault of the transmission lines, if any, to trip the breakers and to confirm the open states of the breakers from the breaker contact signals of the terminals considered and the other terminals thereby to close the earth switches.