Electromagnetic Location - often referred to as 'Radiodetection'

Electromagnetic location uses the principal that a current flowing along a conductor creates a magnetic field. This field, or signal, can be detected with a receiver.

A signal associated with a conductor can be from two sources being either passive or active in nature.

A passive signal is one naturally occurring around the conductor. Examples of passive signals are from:

• A current flowing in an electric supply cable. NB There may be voltage on a conductor but no current flowing in which case no signal will be generated.
• The earth return current from power systems which use metal pipes or cable sheaths as a convenient conductor
• Radio frequency currents from low wave radio transmissions that have penetrated the ground and flow along buried conductor

Passive signals therefore enable conductors to be located, but not identified, because the same signals may appear on any conductor.

An active signal is one deliberately generated by the user by connection or induction. This mode allows conductors to be identified, traced and their depths measured. Different frequencies (8,33 and 65KHz) can be used to obtain the optimum signal for the conditions.

• Direct connection - a signal from signal generator is connected to a pipe or cable at an access point such as a valve

• Induction - a signal is induced into a buried conductor by placing a signal generator over a know section of the conductor

If it is possible to gain access to the interior of a non-metallic gravity pipe or duct, a sonde can be entered and pushed along on a rod. The signal from the sonde can be located on the surface, so tracing the line of the pipe or duct.