Active signal application requires the use of a signal transmitter designed to produce from battery power an AC voltage of known frequency and the means of applying it to the target buried conductors.
The output AC voltage from the signal generator is connected directly to the pipe or cable at an access point such as a valve, meter or end of the conductor, and the circuit is completed by a connection to a ground stake or other ground connection point.
Clamping uses the induction principle to give a similar result to direct connection, but without electrical contact to the line. The output from the signal transmitter is effectively coupled to a particular line by clamping round it with a split toroidal magnetic core, which carries a primary winding, magnetising the core with the AC signal. The line becomes the secondary of the transformer, and will carry a strong signal, provided that it has good coupling to ground on both sides.
Use of the clamp has the advantages over direct connection that, not only is the need for direct electrical contact with line and ground avoided, but any lines sharing a common connection will divide the signal current between them, leaving the line to which the signal has been applied with the strongest signal.