| 摘要 |
PROBLEM TO BE SOLVED: To back up power source supply and transmits a signal to the final end without greatly varying a power source load in a disaster prevention receiving board or a relay amplifying board against a power source reduction in the relay amplifying board. SOLUTION: A detector 5 and a terminal apparatus such as a control apparatus arranged inside a tunnel are connected to a transmission path including a power source line SV led out from the disaster prevention receiving board, a common line SC, and signal lines SA and Sb, while the relay amplifying boards 7-1 and 7-2 are connected at predetermined intervals, and the inside of the tunnel is monitored and controlled by supplying a power source to a descendent side power source line on the terminal side by means of the respective relay boards. The relay amplifying boards 7-1 and 7-2 are provided with a voltage reduction detection circuit 10 monitoring a power source voltage in the ascend side power source line on the receiving board side for detecting a voltage drop, a back-up circuit closing return contacts b1 and b2 arranged to the ascend side power source line SV and the common line SC on the ascend side for supplying the relay board power source as a backup poser source when a detection output from a voltage drop detection circuit 10 is obtained, and a bypass circuit having a bypass contact a2 closing the common line side contact b1 in compliance with a power source drop and bypass contacts a1, a3 and a4.
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