CTG is a family of "core balance transformers". These devices detect the difference in the current flow on the lines running through the coil. They work on either single phase or three-phase mains. When the vectorial sum of the current flowing to the load and the one flowing on the NEUTRAL cable (if present) is different from "0" (zero), a current on the CT secondary circuit is generated. The current difference is usually caused by a leak which may cause an hazard. The output signal can be used by a residual current monitoring device which will disconnect the monitored load or send an alarm signal.