Abstract: A signal of a predetermined frequency is injected into the served (insulated) sheath of an underground cable and the resultant magnetic H field generated is monitored using two pick-up coils 62 and 63 aligned along the length of the cable. The phase difference between the emfs induced in the two coils is a measure of the H field gradient and a sharp change in phase accompanied by a change in emf. magnitude indicates a fault. The same two coils turned through 90.degree. transversely to the cable can be used to locate the cable by scanning transversely to the cable. As the coils pass over the cable the relative phase of the induced emf signals in the coils reverses. Alternatively, three or four coils may be used arranged in two orthogonal pairs with the coils of one pair being aligned in use along the cable and the coils of the other pair transverse to the cable.