Abstract: A protective relay for providing an infinitely variable family of curves for use in ground-fault protection. In one embodiment, the protective relay includes a saturable reactor across the output terminals thereof. The saturable reactor impedance varies in accord with the load or fault current such that a trip characteristic based on the zero sequence current and the load or fault current is obtained. The trip characteristic is selected by a potentiometer that limits the amount of dc current to the saturable reactor. Once established by setting the potentiometer, the trip characteristic defines the amount of 3I.sub.O neutral current, as a function of three-phase current, necessary to operate the protective relay. Varying the potentiometer setting provides the infinite family of curves.

Abstract: Protective relay apparatus which performs a segregated phase current comparison of a three-phase protected line section, with a single relay at each end thereof for making the phase current comparison. The single relay includes a rectifier arrangement which provides a signal responsive to the largest phase current of a predetermined polarity, at any instant. In response to a fault detection, it successively phase compares the signals from its rectifier with the signals of the corresponding phases from the rectifier in the relay at the other line end.