An RCD detects the residual current between the phase and neutral conductors using a summation current transformer. The phase and neutral conductor currents generate magnetic fields which no longer equalise in the event of current differences. In this case, a magnetic flux occurs in the toroid of the summation current transformer. A secondary coil is wound around the toroid in which a voltage is induced by the magnetic flux. If the current flow in the secondary coil is too high and the rated residual current is exceeded, a relay in the RCD is activated and the current flow is interrupted. This can only be switched on again once the fault current has been rectified.