Abstract: A current-limiting device for an alternating current system has for each phase a pair of saturable reactors each with an iron core, an alternating current winding and a superconducting direct current bias winding which is common to the reactor cores and biases them both to saturation. The two alternating current windings in each phase are connected in series opposition such that under fault conditions they are driven out of saturation on alternate half-cycles and thus limit the overload current. The superconducting winding has a surrounding magnetic flux screen which also forms a loop for shunting alternating current under fault conditions and which may be used as a direct current control circuit for varying the bias level. The screen serves as the outer vessel of an evacuated enclosure for the superconducting winding and a suppot system for the inner vessel is also described.