EMI

Electromagnetic induction (EMI) using Pipe and Cable Locators

A current flowing along a conductor creates a cylindrical magnetic field that is known as the “signal”.  While it is possible to insulate against the flow of electricity it is not possible to insulate against a magnetic field.  Therefore, the shape of the magnetic field is not changed by the pipe/cable characteristics (composition, insulation) or by the presence of different types of soil.

 Two different types of electric current are available for the transfer of electric charge, direct current (dc) and alternating current (ac).  Direct current produces a field in which the magnitude and direction are constant.  It is very difficult to measure this static field in the presence of earth’s magnetic field and therefore dc fields are generally very difficult to detect.  Alternating current creates a detectable magnetic field or signal because it not only provides a field but also an oscillating frequency of reversals. The reversal frequency is measured in cycles per second or Hertz (Hz).  The two conditions of ac fields make effective utility location possible through the principle of electromagnetic induction. The principle of electromagnetic induction, therefore, states that any change of magnetic flux linking a conductor will induce a voltage in that conductor. This electromagnetic induction principle is applied in pipe and cable location in two ways: Signal application and signal detection.

Why does this matter? Every locate you’ll ever do will involve these principles. Many of the difficulties you encounter during a stakeout will be explained by this simple concept: Pipe and Cable Locators do not locate pipes or cables, but locate a frequency applied to a conductor through current.