Direct current relay that has two coils, wound on the same magnetic core, powered separately; they have a number of turns that can be different and refer to two different circuits with currents, which can be different, which circulate in the opposite direction. If the difference between amperspire, the first reel and the second reel is zero, no magnetic flux is generated, so the relay does not energize; if the difference is different from zero, a magnetic flux is generated which causes the relay to excite. A typical example of a differential relay is the circuit for controlling the call signals. See Relay FS
Link... for your business