As demand for electric power has increased, EHV cable systems have been introduced into city areas; in this situation, problems of system protection that may arise when pilot-wire protection is used include electromagnetic induction, limitation of line length, etc. Last year, we solved those problems by developing a PCM current differential relaying system using fiber-optic data transmission. This system has been installed and operation has started on the 275 KV cable line (MINAMI OSAKA s/s -- SENBOKU s/s).