The invention relates to a
circuit breaker and to a method for protecting a power supply formed by
three phase conductors (L1, L2, L3) and a neutral conductor (N). The
circuit breaker is designed to determine current values of the
three phase conductors (L1, L2, L3), determine values of the voltages applied between the phase conductors (L1, L2, L3) and the neutral conductor (N), and evaluate determined current and
voltage values. Virtual circuits are defined between the individual phase conductors (SKL1L2, SKL1L3, SKL2L3) for the evaluation process. The
circuit breaker is designed to determine circuit-specific voltages, currents and impedances for the virtual circuits using the determined current and
voltage values. The
voltage of a
virtual circuit (SKL1L2, SKL1L3, SKL2L3) between two phase conductors corresponds to the difference between the voltages between the respective phase conductors and the neutral conductor, the current of a
virtual circuit (SKL1L2, SKL1L3, SKL2L3) between two phase conductors corresponds to the weighted difference between the currents of the respective phase conductors, and the impedance (ZL1L2, ZL1L3, ZL2L3) of a
virtual circuit between two phase conductors corresponds to the quotient of the voltage and the current of the virtual circuit between the two phase conductors. The variables determined for the virtual circuits are used to diagnose
overcurrent events.