Abstract: In the location of an earth fault on a sheath of an underground cable, pulses of direct current are applied to the sheath at one location on the cable. A search is made at spaced locations along the cable route for the magnetic field due to a fault current to earth flowing in the sheath which is produced by the applied d.c. pulses using a portable magnetometer, with a visual indicator, and, at each of said spaced locations, the visual indications on the magnetometer are compared with the periods of application of the pulses to the sheath until they correspond. Comparison of the visual indications with the periods of application of the pulses is facilitated by transmitting signals indicative of the periods of current application and using, with the portable magnetometer, a portable receiver arranged to receive the transmitted signals so that the visual indications can be compared with the received signals.