Inrush currents in the dc intermediate circuit of a motor controller necessitate the incorporation of protection circuits. These circuits usually take the form of a resistor inserted in series with the intermediate circuit, and the resistor is short-circuited after completion of inrush by a switch positioned in parallel with the resistor. To reduce cable-transmitted electrical noise an earthed noise-decoupling capacitor is added to the intermediate circuit, but it also requires protection against inrush currents. To minimise the number of components, a protection circuit is described which combines noise filtration and inrush protection. According to the invention, the inrush resistor has the form of a resistor network and is connected to a switch which via a controller alters the resistance of the resistor network, so that when the switch is open, the resistance to a differential current is greater than the resistance to a common mode current, and when the switch is closed, there is greater resistance to a common mode current than to the differential current.