PURPOSE: To prevent system down by protecting a semiconductor element for driving a transformer upon occurrence of short circuit on a transmission line although the voltage level of a control signal, fed from the head vehicle of a composed train, is boosted through a booster in order to enhance noise resistance characteristics.

CONSTITUTION: A signal P is fed from a notch commander 2 to a transformer driving circuit 3 which produces a signal QH having high voltage level for enhancing noise resistant characteristics and the signal QH is delivered on a transmission line 5 to each vehicle. Upon occurrence of short circuit on the transmission line 5, output voltage +V1 drop of a constant voltage power supply 1 is detected and stored in a gate block circuit 8 which forcibly turns OFF a semiconductor element for driving the transformer in the transformer driving circuit 3 thus preventing breakdown.