The residual current operated circuit breaker have a measuring system consisiting of a summation current transformer with a permanent magnetic tripping device. There are RCD detecting only AC fault currents and RCD ranges detecting both, AC fault currents and pulsating DC fault currents up to 63A. Transient fault current pulses can occur due to voltages pulses superimposed on the network, e.g. due to switching of fluorescent lamps, X-ray apparatus an the use of thyristor controls.