Abstract: For use in D-C power transmission systems there is provided a circuit breaker system which is responsive to leakage currents to ground to break the circuit from the power source to the D-C transmission lines. A toroidal saturable core surrounds the D-C transmission line feed conductors and carries on such core a primary and a secondary transformer winding. An A-C source is connected to the primary winding in series with one of two field coils in the electromagnetic circuit of an electromagnetically actuated circuit breaker. The other field coil in the electromagnetic circuit of the breaker is connected to the secondary winding on the saturable core.