A cable carrying AC power produces its own signal at 50-60Hz frequencies, together with higher frequency harmonics thus providing a basis for search and location by a passive receiver.

However, the ground is full of power frequency currents flowing between the ground connection points of power systems and cables. These currents automatically take the paths offering least resistance, which of course are all the buried metallic lines and conductors. They will also be coupled into them by capacitance and induction. The result is therefore that 50-60Hz signals, and their harmonics which are strong up to about 3kHz, are present not only on the majority of buried cables, but also on a large number of pipes or other conductors in the vicinity. This implies that it is possible to locate conductors carrying power frequency signals but not to identify them by passive signal location. The signals may be from a live cable, a pipe or simply from reinforcing bars in concrete, but you will know that a conductor is there. Single phase power cables generally radiate clear signals, but with 3 phase cables the signal is largely the result of imbalance between the phase loads, as balanced currents tend to cancel their fields.

The better the balance, the more difficult detection becomes. As high voltage cable loadings are generally better balanced, a simple passive search in ‘P’ mode might easily detect a low voltage cable whilst missing both an 11kV power cable nearby, and live but unloaded cables, such as street-lighting during the day, which radiate no power signal. This is why the availability of the Radio mode (6.2) and active modes (7) is such a valuable complement to the Power mode.