A circuit for detecting a DC current in a conductor (L1) includes a current transformer (CT) having a ferromagnetic core (10), a primary winding comprising the conductor (L1) and at least one secondary winding (W1). The circuit further includes an oscillator (12) for supplying an oscillating signal across the secondary winding and means for detecting a dc offset in the current flowing in the oscillator circuit. The circuit includes a capacitor (C1) in series with the secondary winding, and the detecting means is arranged to detect a non-zero voltage across the capacitor above a certain level.