摘要 |
PURPOSE:To prevent a trouble such as a short-circuit between taps by providing a photo detector which detects the light emission of a current limiting resistor. CONSTITUTION:Adjacent to the current limiting resistors Ra, Rb of travelling contact circuits 3a, 3b, photo detectors Da, Db which detect the light emission of the current limiting resistors Ra, Rb are provided respectively. For example, if the breaking performance of bulbs Wa, Wb or both bulbs Ha, Hb and the bulbs Wa, Wb is reduced and the arc is made longer, a circulating current continuously flows in the current limiting resistors Ra, Rb by the continuation of the arc of the bulb Wa or Wb at the side of opening even after the switching is finished and the current limiting resistors Ra, Rb are overheated to be melted but in this case, light generated from the red heat current limiting resistors Ra, Rb is detected by photo detectors Da, Db, e.g., photo diodes and the breaker of a transformer is tripped. |