An arc fault detector comprises a current transformer (CT) having a primary winding and a secondary winding (W1), the primary winding being formed by at least one conductor (L) of an AC or DC supply circuit. The inductance of the CT is selected so that the CT has a lower frequency operating point (LFOP) of at least 10 KHz. The detector also comprises circuitry (10, 12, SW) for disconnecting the supply if the voltage induced in the secondary winding meets predetermined criteria as to magnitude and duration.