Arrangement for detecting short-circuits in switched-off branches of electrical AC power networks, especially in branches for feeding railways

The method involves charging a circuit branch with an operating current. The current is limited under application of a test resistance than that of a circuit resistance of the circuit branch. Superimposition of the operating current at a time period along a fundamental component of a sine wave in the voltage drop at the switch is carried out at a correction angle of between 0 degree and 90 degrees before maximum voltage is reached.