Abstract: A current differential fault detection circuit. This circuit is used with a plurality of main current transformers respectively coupled to a plurality of separate conductors through which current flows into and out of a sectionalized portion of an a-c electric power system. The secondary windings of the main current transformers are connected in parallel with one another. A normally non-conducting solid state a-c switch is connected in series with the primary winding of an auxiliary transformer and the series combination is connected in parallel with the secondary windings of the main current transformers. Circuit means is provided for sensing the voltage across the secondary windings of the main current transformers. When the secondary winding voltage exceeds a predetermined level the a-c switch is triggered into conduction thereby allowing the fault current to flow through the primary winding of the auxiliary transformer.