Abstract: In a power distribution system having a first conductor carrying a current I.sub.1 and a second conductor carrying a current I.sub.2, in the same direction as I.sub.1, a differential current detector sensing when .vertline.I.sub.1 -I.sub.2 .vertline. exceeds a predetermined level, I.sub.3. The detector comprises a ferromagnetic yoke having first and second parallel bores accommodating the first and second conductors respectively. The first and second bores are interconnected by a coplanar slot in the yoke which extends transversely therebetween. The yoke further includes a third bore disposed between the first and second bores and orthogonal thereto. A reed capsule switch having two mutually exclusive conductive states extends through the third bore and is responsive to the flux generated by the currents I.sub.1 and I.sub.2, such that when .vertline.I.sub.1 -I.sub.2 .vertline..ltoreq.I.sub.3 the reed switch exhibits one conductive state and when .vertline.I.sub.1 -I.sub.2 .vertline.>I.sub.