Abstract: A periodic current generator is connected with a long power supply line through a first transformer. The lamp of the traffic light is connected to the long line through a second transformer. First and second safety capacitors with four terminals each are respectively arranged in series on the long line with the first and second transformers. A DC current is generated by a detector when the lamp is operating. The detector is connected to one of the capacitors and the DC current passes through the long line to the second capacitor across which the coil of a relay is connected. The relay is actuated by the DC current and sends information concerning the status of the lamp to a central control station.