A lightning current responsive alarm device in which an earthed discharge current circuit consisting of a parallel combination of a non- linear resistor and an inductive impedance element is connected between a protective coordination gap in a power transmission system and earth. In the device, a capacitor is charged up with the drop voltage appearing across the nonlinear resistor, and a sensor or counter is actuated by the discharge current of the capacitor, so that the device can operate with a greatly widened operating range which covers a large current region of lightning current.
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