A switching circuit, e.g. an RCD, comprises a pair of mechanical contacts (M) in an alternating current electricity supply (N, L) and circuit means (SCR) for opening the contacts to cut off the supply in response to a switching signal (S). The switching circuit includes further circuit means (Rs, Cs, D2, D3, Q1, Q2) for defining successive periods of time (“activation windows”) during which the magnitude of the supply voltage is not at a maximum and allowing the contacts to open in response to a switching signal only during such windows.