Active pilot wire apparatus for replacing a passive continuous metallic wire pair coupling between the electromechanical units of a conventional electromechanical pilot wire protective relay is disclosed. For the protection of a power line section, an electromechanical unit is coupled to each end thereof. Each electromechanical unit includes an operating relay coil which when energized operates breaker contacts to interrupt current through the protected power line section. A signal representative of the potential across the operating coil of each electromechanical unit is transmitted through an active communication channel to the other electromechanical unit where it is buffered by a high impedance amplifier. In each case, the output signal of the buffer amplifier is representative substantially of the corresponding operating relay coil potential. An impedance element is coupled between each buffer amplifier and the other operating relay coil. Current is diverted from the operating relay coil of each electromechanical unit through the corresponding coupled impedance element in accordance with the potential of the signals coupled thereacross. The active pilot wire apparatus may be extended to the coupling of a plurality of electromechanical units to form a current differential protective relay operative to protect against faults in a corresponding plurality of sets of power lines with each set constituting a current path of a power system network.