PURPOSE: To simplify wirings by encoding the output of various protective relays contained simultaneously by a CPU, and simultaneously serially sending data sent to a data transmitter by a communication controller in the same case.

CONSTITUTION: A ground-fault relay unit GR connected to various protective relays such as, for example, 3-phase power lines 4 through zero-phase current transformer ZCT, an overcurrent relay unit OCR connected to phases through CT and an insufficient voltage relay unit UVR connected through PT are contained in the same case 2. Data detected by various relays are processed to be encoded by a CPU (not shown), and sent to a data transmission line 6. The line 6 simultaneously inputs the data to a serial interface I/F in the case 2. The interface I/F interprets the oscillating relay of the data encoded by the contained CPU, and serially sends it to a communication channel L.