Patent ID: 8279567

Claim:
A 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 at a main frequency into said power line while the circuit breaking element remains open, measure a response voltage and a response current in the power line, where the response voltage and the response current result from the test signal (t(f)), determine an impedance to ground of the power line from the response voltage and the response current, 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, 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.