How cable locators work – Principles of buried utility detection

Undertaking any excavation will inevitably bring site workers into close proximity to underground utilities (electricity, gas, telecommunications, water). Consideration should always be given to knowing the exact location of all buried utilities before and during the excavation process.

To safeguard against utility strikes, a cable locator is used for detecting the presence and proximity of buried utilities.

When an alternating current (AC) travels along a cable, an electromagnetic field is generated. The alternating current not only creates a magnetic field but also the oscillation of the current between positive and negative creates a frequency known as Hertz (Hz). The electromagnetic field generated by an AC current can be detected by a cable locator, such as the Leica Digicat 550i.

There are two main detection principles:

  • Passive location – Used to locate an electromagnetic field already present on a utility
  • Active location – Adds a specific signal using a signal transmitter onto a located utility

Passive Location

Some signals may already be present on a buried utility, either through signals radiating off a power cable or the re-radiation of a radio signals induced onto the utility from a radio transmitter.

Principles behind Power Mode

When an AC current travels along the utility it generates an electromagnetic signal. Using a cable locator (Leica Digicat 550i), a surveyor can detect the buried cables’ position by searching for the electromagnetic field.

Locating electrical cables using a cable locator on its own will only allow the operator to detect utilities with a live current, like a street light cable during the night. However, when lights are turned off, no current flows and the buried electrical cable is not detectable using a cable locator.

Principles of Radio Mode

Low frequency long wave radio signals transmitted from a radio mast can pass into the ground, inducing a signal onto metallic utilities. The utilities re-emit these signals and they can be located and traced using a cable locator in RADIO mode.

Principles behind Auto Mode

Leica Digicat cable locators feature Auto mode, combining the benefit of simultaneous detection in Power and Radio modes. Auto mode helps confirm the presence of utilities on first site visits.

Active Location

Up to 60% of buried utilities could be missed when detecting in passive modes alone. Just because they are not detected in a simple sweep, does not mean they are not there and it is safe to excavate.

A signal cable locator and signal transmitter (Leica Digitex t100) will significantly improve the detection process. This small portable unit induces a signal to a cable or pipe, which can be traced by the cable locator. This is called active locating.

Applying an active signal

A majority of buried utilities may not be detected by searching for passive signals using the locator on its own. These hidden utilities may not carry a live current or radiate radio signals, requiring a signal to be induced directly onto the utility to locate them. To detect these additional utilities, an electrical current or signal will need to be applied onto the buried metallic utility, which enables the utility to be traced and identified by the locator.

Induction mode

Induction is a quick and simple way to apply a signal to a utility without the need to make any physical connection. An internal aerial generates a magnetic field into the ground. Any buried metallic utilities routed within close proximity to the signal transmitter will be induced with the signal, allowing the utility to be located and traced with a cable locator.

Connection mode

This is the most efficient way of applying a signal to a utility and should be used whenever possible (especially when taking a depth reading). The output from the signal transmitter can be directly connected to a cable or pipe. A circuit is completed by a connection to an earth stake or ground connection point.

Applying a signal directly to the utility allows the operator to positively identify and trace its path.

Locating the signal

To detect the magnetic fields emitted from a buried utility, the locator uses aerials built up of wire wrapped around ferrite rods. The aerials are used to amplify the small electromagnetic signals emitted by the utility and to provide an input to the locators’ circuitry.

Leica Digicat locators feature an enhanced Signal Strength Indicator (SSI), a graphical display detailing the signal strength as a bar graph and a numerical SSI reading. The highest signal reading (peak response) is obtained when the utility is directly below the locator.

To learn more about how cable locators work download the White Paper Principles of Buried Utility Detection

Written by Anabela Ferreira Fernandes


  1. It makes sense that simply because something doesn’t show up doesn’t necessarily mean that it isn’t there when cable locating. Hiring some professionals who have multiple ways to check for what they are looking for would be really important. That way you can be sure that they have found what needs to be found and they won’t run the risk of hitting anything.

  2. “Excellent information!
    Since 1972, Line Ward has hand-crafted the highest quality line layers and boring equipment that are specifically designed for seamless underground cable and pipe line installations. All Line Ward line laying equipment is based on the original design patents held by its founders.
    For more details, visit

  3. Hire underground water pipe locating service experts who have the needed technology to locate hidden faulty pipes and drainage system. Get the one who does a total package to save time and money.

  4. Fantastic blog post about “How cable locators work.” Thank you for sharing the Principles of buried utility detection and please keep sharing this type of informational posts regularly.

  5. Induction is a quick and simple way to apply a signal to a utility without the need to make any physical connection. An internal aerial generates a magnetic field into the ground.

  6. Pingback: 5 Best Underground Wire, Cable, Pipe & Utility locator in 2019

  7. My wife and I decided to build a pool in our back yard. Before we start digging, we want to make sure that there are no pipes or cables that could be damaged. Hiring professionals who have multiple techniques and tools to detect any wiring is a great idea, so we will consider the advice.

  8. We’ve a specific requirement for water supply pipe leak detection below several rooms, tiled floors.
    We’re keen to avoid destroying all floors if unnecessary. We’re losing over 20 litres in 24 hours.

    Anyone experience of relevant equipment recommendation ?

  9. Well described post.I just want to let you know that I have checked out your site and I found it very interesting and informative.Keep sharing this type of informational posts regularly.

  10. Sathya Varaprasad on

    Sir Hackathon question:
    Solution for initiating some kind of alarm to protect underground fibre while digging the road for construction any other purpose. Frequent fibre cut is a serious problem being faced by Telecom industry, leading to poor up time of the Telecom Network and poor service to Citizen. Fibre cuts are primarily due to road construction activities. If at the starting time of digging of road, an alarm can be generated, it may avoid cutting of fibre.

Leave A Reply

  • Recent Posts

    More >
  • Never Miss an Update