Glossary — Cable locating
Active/Passive Cable Locating: How Each Mode Finds Lines
A cable locator can trace a buried line in two ways: by picking up signals the line already carries, or by sending its own signal along it and tracking that.
Quick AnswerIn active/passive cable locating, passive mode sweeps for signals already on buried conductors: 50 Hz current in live power cables, or radio energy re-radiated by long metal lines. Active mode applies the locator's own frequency to one line by direct connection, clamp or induction, then follows it. Passive is a fast first sweep; active identifies a specific line. Other terms are in our GPR, SUE & Land Survey Glossary guide.
What is the difference between active and passive locating?
Passive locating uses no transmitter: the receiver detects electromagnetic fields that buried conductors already carry. Active locating uses a transmitter to put a known signal onto a chosen pipe or cable. Passive tells you something metallic is there; active tells you which line it is, and gives more reliable depth readings.
How does passive mode work in active and passive locating?
Power mode tunes the receiver to the 50 Hz field, plus its harmonics, that surrounds any cable carrying load current. A cable with no load current, such as a switched-off street-light circuit, may stay silent. In radio mode it picks up VLF radio energy from distant transmitters that long conductors re-radiate. Passive sweeps of underground utilities are quick and should be done in a grid before any dig. They cannot, however, show which of several parallel lines is which.
How is an active signal applied?
| Method | Set-up | Suits | Limitation |
|---|---|---|---|
| Direct connection | Transmitter lead clipped onto the pipe, valve or tracer wire and earthed with a stake | The strongest and most selective signal | An exposed access point is needed |
| Clamp (signal coupler) | Coupler closed around the cable at a feeder pillar or pit | Live cables where no bare metal can be reached | Only the target cable may pass through the clamp |
| Induction | Transmitter laid on the surface over the line | Lines with no access point | Neighbouring lines pick up the signal as well |
Locating is done alongside GPR on the same visit; details on the GPR survey cost calculator show the per-metre price.
Lower frequencies, a few hundred hertz to around 1 kHz, travel far along utility lines and bleed less onto neighbours. High frequency signals of tens of kilohertz couple more easily, especially by induction, but spread to other conductors.
Worked readings on a crowded footway
Signal current and depth readings together tell you whether you are on the right line. Suppose the transmitter is connected directly to a steel water service and the receiver reads about 58 mA over one line outside the chamber. A metre away a second line reads 12 mA: that is bleed-over onto a neighbour, not the target. Following the strong line, the receiver shows a peak and a depth of 1.1 m. Tilting the antenna through 45° and walking sideways to the null gives a horizontal distance of about 1.1 m too, which confirms the depth reading by an independent method.
Common locating mistakes
Most utility detection errors with a locator come from a weak signal, or from trusting the strongest one.
- A poor earth. A dry stake gives a weak return path, so little current flows down the target.
- Following bleed-over. The strongest response is not always the connected line; check the current value.
- Reading depth at a bend or tee. Fields distort there; take depths on straight runs a few metres away.
- Ignoring nearby steel. Rebar, fences and parked vehicles bend the field and shift the peak.
- Reading silence as absence. An unloaded cable or a plastic pipe gives nothing in passive mode.
How traced lines are shown on Mezux drawings
Electromagnetically traced lines carry a method tag, so readers can see how each was found. Each line in the CAD and GIS layers is labelled as active direct, active clamp, active induction or passive power, with depth readings at intervals and a confidence note. Ground penetrating radar (GPR) images the subsurface with radar pulses and so also finds plastic lines; those confirmed by both GPR and the locator are marked as such. In ASCE 38-22 terms all of these are quality level B, obtained by surface geophysical survey methods. PAS 128:2022 in the UK treats electromagnetic location and GPR as complementary detection techniques.
A footpath example
A footpath holds a live LT cable, a street-light cable and an HDPE fibre duct with a tracer wire. A passive sweep finds the LT cable clearly and the street-light cable only when lamps are on. Clipping the transmitter to the duct's tracer wire at a chamber puts a signal on that line alone, and the locator follows it past the others. Without a tracer wire the duct is plastic and invisible to the locator; a transmitter pushed inside it works instead, as described under Sonde (Locating Transmitter).
Once traced, the data collected go into GIS or KMZ layers, and the two formats are compared in Shapefile vs KMZ for Utility Data. A full utility investigation is covered in a closer look at SUE survey; when you are ready, send your drawings for a quote for your project.
Frequently asked questions
Will a cable locator pick up a plastic pipe?
Not on its own. It needs a tracer wire, a metallic element, or a sonde pushed inside the pipe; GPR, which responds to metallic and non-metallic objects, is the usual alternative.
Is active locating always better?
It is more certain for a known line, but passive sweeps are faster for finding lines you did not know about. Good practice uses both.
Do you also show located lines on a boundary drawing?
Yes, where it helps; the method is in the land survey services process step by step.
Why does the depth reading change near a bend?
At a bend or tee the magnetic field from the two arms overlaps and distorts, so the receiver's depth calculation no longer applies. Readings are taken on straight sections.
Can a locator find a disused cable?
In active mode, often yes, if a signal can be applied and the cable is continuous. In passive power mode a dead cable gives no 50 Hz field.
What is a signal current reading?
An estimate of the transmitter current flowing in the line beneath the receiver. Comparing it along the route helps tell the target from neighbours.
Should passive and active sweeps both be done?
Yes, where time allows. A passive sweep helps detect and map live cables you did not know about; active tracing then identifies and follows the specific lines that matter for the works.
Get lines traced and identified
Related utility terms
Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026