Abstract: Apparatus for detecting current flow in any one or more of the three lines of a three phase electrical system which have been connected together by means having negligible impedance which comprises three current transformers which may be magnetically coupled to the lines and a further transformer for combining the outputs of the current transformers. The further transformer is connected to an amplifier and a rectifier and the rectified voltage is used to control a visual and audible alarm system to indicate the presence of current flow in one or more of the lines. The rectified voltage may also be used to prevent re-opening of a switch which is used to interconnect the lines.