Abstract: A high frequency and two low frequency electromagnetic signals are generated (1), the low ones being coherent with the second being a sub-harmonic of the first. The high frequency is modulated (4) by the second low frequency and applied (7,9) as a composite signal to an underground conductor, while the first low frequency is applied (9,10) additively. A receiver traversed over the line has aerials (12,21) or ground probes (24,25) for picking up these signals in separate channels. The high frequency is demodulated (18) to provide a reference signal for a synchronous detector (20) to which the first low frequency is fed. That suffers a phase reversal if there is a fault, and the output of the synchronous detector is arranged to indicate that.