Abstract: Disclosed herein is an apparatus for connection to the secondary terminals of a CT to prevent CT damage and electric shock when the secondary terminals are opened, by providing an input signal and operating a relay circuit to short the CT secondary terminals. The apparatus monitors opening of the CT secondary terminals, and to prevent accidental opening, determines opening and provides an over voltage breaking upon opening. To prevent false operation of the protecting relay connected to the secondary terminals caused by the over voltage generated by accident other than opening of the CT secondary terminals, a secondary delay time of operation of the protecting relay is provided that can be variously set by DIP switch or otherwise, allowing the user to choose a desired setting. Further, expected over voltage values can be set according to load characteristics, allowing the apparatus to operate during voltage above the set value.