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.

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  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.

  11. hi , now we act as a technical consultant in GIS project ,this project (pilot project ) aim to located all MV cables located on the area of service in very high accurate maps , the input document given from the utility just a freehand sketches , which is not accurate are you have a device or methodology to locate this cable (MV cables only ) which may be burred underground for distance 1 meter

  12. I have a BX underground cable feeding a shed at 100 ft from house panel.
    Question: I put the induction clamp around exposed wires at the end in the shed – now can the induced signa ln the wires inside the metallic shield radiate outside to the locator sensor? Is not that a Faraday shielding cage simply not letting anything to go out-the frequency used for the Leica 550i digicat was 33khz. Please somebody help

    • Steve Davies on

      Hi Mike, thanks for the question.

      So the basics of locating are that we are always working with a simple electrical circuit. We need a conductor, a signal and a good working signal path, the rest is easy as the equipment does all that.

      Some simple tests to make first…does the cable re-radiate the passive radio mode? It may and may give an approximate location as to where the cable may be, this position can be checked when using a transmitted signal. The two positions should match obviously.

      Another simple check would be to use an induced signal method to apply a signal. At the house panel end use the transmitter with no connection leads and lay it directly above with the arrows pointing in line with the cable. Turn on and send the 33 kHz signal ‘along ‘the cable. Look for the cable about 25ft away to start with maybe a little further, you may get a signal this way too.

      The conductor
      As you say the BX cable will seemingly promote a Faraday cage effect when a signal is applied to the cable directly. Meaning that the transmitted signal will get trapped inside the metal tube of the conduit. All perfectly reasonable but maybe not the true case here. The electromagnetic signal travels around the outside of the conductor and because of the metallic nature of the conduit then the actual conduit should re-radiate the frequency and become detectable, invariably the signal always gets out as it seeks a return path back to the transmitted source. To promote good signal flow and complete the signal path circuit, the cable should be terminated (connected electrically) at both ends to ideally get the clamp to work, Is this the case as you say exposed wires, or can you simply just see them. Also the way the building itself is grounded will affect the circuit, if it is grounded.

      The signal
      33kHz will be fine for this example.

      A good working signal path
      The first point of connection for me would be at the opposite end from the shed, at the house panel. A clamp connection around the actual metal conduit of the BX cable and see if that alone will pass the signal along it. You would need to be about 25ft away from the transmitter to start with to look for the signal but will be able to get closer once confident the cable is tracing. Independent grounding of the cable at the opposite end may also help the current flow as all locates work differently.

      If the clamp method does not pass a signal along the cable then switch to the crocodile / alligator clips method and treat the cable like a metal pipe. Attach the black lead to the ground stake and red lead directly to the outer metal part of the cable, ensure there is a good signal (audible tone for transmitter will change) and then see if the cable can be detected, again about 25ft away to start searching for the cable. Again independent grounding of the cable might help pull the signal along.

      If these steps don’t help, then the final solution is switch out the cable at the house panel and disconnect at the shed end and make a direct connection to the conductive core of the cable and send a signal using the connection clips as described previously.

      And finally, if all else fails, complete the circuit using a long (100ft) lead on the switched out conductive core. Connect the red lead to one end of the core and the black lead using the 100ft extension to the other end of the same core, making a circuit using the transmitter set to your frequency. Making sure the extended circuit is far enough away from the actual line to read good signals, this is the most difficult to perform as you need space to separate the cable but is a definite way of proving the line.

  13. Alfazal Engineering gives you the following tips are supposed to be a realistic manual to make certain the secure and right set up of cable ladder and cable tray structures and channel aid and different aid structures. These suggestions aren’t supposed to cowl all info or versions in cable ladder and cable tray set up and do now no longer offer for each setup contingency. It is usually recommended that the paintings defined on this manual are executed with the aid of using in a position individuals who’re acquainted with the goods being set up and the protection requirements related to them. Structural characteristics When thinking about the set up of the cable helps machine its miles vital to keep away from the slicing or drilling of structural constructing participants without the approval of the venture chief on-site.
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  15. Can I do a locate on a live feeder conduit that consist of 200A 120/208V? Do I clamp on the ground wire and the conduit using the connection mode method?

  16. These are some great information that you have shared on how cable locators work on the principles of buried utility detection. I really loved it and thank you very much for sharing this with us. You have a great visualization and you have really presented this content in a really good manner.

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