Abstract: A differential relay includes two apparatus each associated with a different monitoring point on the line being protected by the relay. Each apparatus includes means for deriving digital data representative of the current monitored at the respective point, at time intervals defined by a clock within the apparatus. The two points are linked by a digital data communication channel, and information transmitted between the two points is used to compute digital data for the two points at substantially the same time, from digital data derived at different instants at the two points.

Abstract: In a protection apparatus for a feeder (H) of an electric power transmission system compensation for mutual coupling between the feeder and another feeder (G) is controlled in dependence on the ratio of the zero sequence currents (I.sub.M, I.sub.N) in the two feeders. The mutual coupling compensation signal (I.sub.M Z.sub.M ') is arranged to be proportional to the zero sequence current (I.sub.M) in the other feeder when the ratio of the zero sequence currents is below a predetermined value, and to be limited to a value proportional to the protected feeder zero sequence current (I.sub.N) when said ratio is above the predetermined value. The arrangement reduces the possibility of maloperation of the feeder protection due to faults on the other feeder.