Passive signals
Electrical power frequencies – A cable carrying a.c. 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 50-6OHz 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 unbalance 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 street lighting cable whilst missing an 11 KV main power cable nearby, and live but unloaded cables, which radiate no power signal. This is why the availability of the Radio mode is such a valuable complement to the Power mode.
Radio frequencies
Very low frequency (long wave) radio energy from distant transmitters is present in the atmosphere worldwide. The ground provides return paths for this radiation, and buried metallic lines form-preferred paths. They then act as aerials re-radiating these signals. The signal strength will vary with coupling to ground, size of line and soil conductivity, the strongest signals emanating from lines with good grounding at each end, or of substantial length so that capacitance coupling is maximized.
These radio frequency signals enable the presence of the conductor to be detected by Radio detection locators in the Radio mode, which can then find dead power cables or well-balanced high voltage cables, which could well be missed by power-frequency-only detectors.
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