A circuit breaker (10) for use in a power/load circuit formed by a three-phase AC power supply (11) and a load (12) connected to the power supply (11) by means of three live power lines (R-R', S-S' & T-T'). The circuit breaker (10) comprises a tripping circuit (60) including switches (SW1) connected in series with the respective power lines facing the power supply (11), a transformer (T1) having at least three balanced primary windings (P1 to P3) connected subsequently in series along the respective power lines and a secondary winding (S1), and a voltage comparator (50) connected to the secondary winding (S1) for comparing a voltage induced in the transformer (T1) with a reference voltage and then in the event of a leakage fault providing a tripping signal for the tripping circuit (60) to open the switches (SW1). A circuit path (X) is provided, which has a combined voltage sensor/comparator (30) and extends across opposite sides and externally of the transformer (T1) for sensing and comparing two phase-to-phase voltages with each other and then in the event of a voltage imbalance fault simulating a leakage fault condition for the transformer (T1) to detect and the voltage comparator (50) to provide a tripping signal.