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 .