PURPOSE: To improve information transmitting performance by holding data from a sensor in a first area of a RAM of a transmission unit between a host controller and the sensor and data to be transmitted from the controller in a second area, and transferring and transmitting the data of the first area to the second area according to a command from the controller.

CONSTITUTION: A circuit breaker 1 for wirings containing a sensor 2 and a transmission unit 3 is connected to a host controller C. a first area 4 of the unit 3 is formed of a RAM to store the data from the sensor 2. A second area 5 of the unit 3 is also formed of a RAM to store data to be transmitted to the controller C. When the sensor 2 detects a malfunction, informs it to the controller C through a malfunction data area 4a. The controller C instructs to copy the data of the area 4 to a second area 5 to immediately transmit it to the controller C. Thus, a waiting time is shortened, and rapid interruption is conducted.