Circuit breaker for protecting a power line comprising: a circuit breaking element for disconnecting the power line from a power source, and a control unit arranged to - detect a fault on the power line, - open the circuit breaking element based on said detected fault, which opening starts a pre-determined disconnection time interval, - inject a test signal (t(f)) at a main frequency (f) into said power line, and - measure a response voltage (U(f)) and a response current (I(f)) in the power line, where the response voltage (U(f)) and the response current (I(f)) result from the test signal (t(f)) - determine an impedance to ground of the power line from the response voltage (U(f)) and the response current (I(f)), characterised in that the control unit is further arranged to - analyse the impedance to ground of the power line over time during said disconnection time interval, said analysing including comparing the impedance to ground with a reference threshold (Z 0), - determine a permanent or a temporary fault of the power line based on the change of the impedance to ground during the disconnection time interval, - re-close the circuit breaking element after the expiry of the disconnection time interval if the fault is determined to be a temporary fault, and - keep the circuit breaking element open after the expiry of the disconnection time interval if the fault is determined to be a permanent fault. a circuit breaking element for disconnecting the power line from a power source, and a control unit arranged to - detect a fault on the power line, - open the circuit breaking element based on said detected fault, which opening starts a pre-determined disconnection time interval, - inject a test signal (t(f)) at a main frequency (f) into said power line, and - measure a response voltage (U(f)) and a response current (I(f)) in the power line, where the response voltage (U(f)) and the response current (I(f)) result from the test signal (t(f)) - determine an impedance to ground of the power line from the response voltage (U(f)) and the response current (I(f)), - detect a fault on the power line, - open the circuit breaking element based on said detected fault, which opening starts a pre-determined disconnection time interval, - inject a test signal (t(f)) at a main frequency (f) into said power line, and - measure a response voltage (U(f)) and a response current (I(f)) in the power line, where the response voltage (U(f)) and the response current (I(f)) result from the test signal (t(f)) - determine an impedance to ground of the power line from the response voltage (U(f)) and the response current (I(f)), characterised in that the control unit is further arranged to - analyse the impedance to ground of the power line over time during said disconnection time interval, said analysing including comparing the impedance to ground with a reference threshold (Z 0), - determine a permanent or a temporary fault of the power line based on the change of the impedance to ground during the disconnection time interval, - re-close the circuit breaking element after the expiry of the disconnection time interval if the fault is determined to be a temporary fault, and - keep the circuit breaking element open after the expiry of the disconnection time interval if the fault is determined to be a permanent fault. - analyse the impedance to ground of the power line over time during said disconnection time interval, said analysing including comparing the impedance to ground with a reference threshold (Z 0), - determine a permanent or a temporary fault of the power line based on the change of the impedance to ground during the disconnection time interval, - re-close the circuit breaking element after the expiry of the disconnection time interval if the fault is determined to be a temporary fault, and - keep the circuit breaking element open after the expiry of the disconnection time interval if the fault is determined to be a permanent fault.