Residual current transformers with characteristic A detect sinusoidal AC currents as well as pulsating DC residual currents. The transformer covers all of the active conductors leading to the consumers and uses its output signal to illustrate the time curve of the sum of all conductor currents flowing through it. Its output signal is proportional to the residual current, which flows back to the earthing point of the supply mains via the protective earth conductor in the case of an insulation fault or via the earth.