Detecting faults in power lines

A system for detecting a fault in a power line (1) includes, in the power line (1), a first circuit (4) capable of attenuating signals within a first frequency band, which is centered on a first frequency, substantially more than it attenuates signals outside the first frequency band, and, a second circuit (5) connected to the power line (1) on one side of the first circuit (4), the second circuit (5) being capable of extracting signals within a second frequency band that includes the first frequency. The relative levels of two signals extracted by the second circuit (5), are detected and signalled, the two signals being at respective frequencies which the first circuit is capable of attenuating by different relative amounts.