How to Use Utility Locators on Live Sites

A utility strike can stop a programme, damage critical infrastructure and put people at serious risk. Knowing how to use utility locators properly is therefore not simply a matter of switching on a cable avoidance tool before excavation. It is a planned process of reviewing records, detecting, tracing, marking and proving the position of services before anyone breaks ground.

For site engineers, groundworkers and survey teams, a locator is one part of safe excavation practice. Used with the right transmitter, appropriate settings and a clear survey method, it helps build a reliable picture of buried cables, pipes and ducts. Used in isolation, or with assumptions about depth and ownership, it can create false confidence.

Start with records and a site walkover

Before deploying a locator, obtain up-to-date utility plans from the relevant asset owners and review the proposed excavation area. Plans indicate where services may be, but they should not be treated as precise drawings. Their scale, age and coordinate quality can vary, while private services, abandoned apparatus and later alterations may be absent.

Walk the site with the plans in hand. Look for cabinets, lamp columns, valve boxes, service markers, drainage covers, meter positions and points where services enter buildings. Note changes in ground level, recent reinstatement and congested areas around compounds, highways and property boundaries. These observations help you decide where to start tracing and where a signal may be difficult to interpret.

The aim is to turn desk information into a practical detection plan. Consider the full work area, access routes and storage locations, not just the line of the proposed trench. A service can run well beyond the obvious excavation footprint.

Understand what a utility locator can detect

A cable locator receives electromagnetic signals carried by metallic services. It can detect naturally occurring signals, such as those from live power cables, or signals applied deliberately using a transmitter. It does not directly detect every underground asset. Non-conductive plastic pipes, fibre ducts and unenergised cables may require a trace wire, detectable marker, sonde or another method of investigation.

Most professional systems combine a receiver with a transmitter. The receiver identifies the signal and gives visual and audible feedback. The transmitter applies a chosen frequency to a target service so it can be traced more confidently. This distinction matters: passive detection is useful for an initial sweep, while active tracing is usually needed to follow a known route and separate it from neighbouring services.

Depth readings also need careful interpretation. A locator estimates depth to the centre of the signal it is receiving, not necessarily the top of the service. Accuracy depends on correct signal selection, the line being directly beneath the receiver, ground conditions and whether the signal has coupled onto adjacent conductors. Never use a displayed depth as permission to excavate without further verification.

How to use utility locators: a practical method

Begin by checking the equipment. Confirm the receiver and transmitter batteries are charged, leads and clamps are undamaged, and the locator has passed its daily functional check. Professional equipment should also be within its recommended calibration and service interval. A damaged lead, weak battery or overdue calibration can compromise an otherwise sound survey.

Set the receiver to the relevant passive modes and carry out a systematic sweep. Walk the proposed work area in a grid pattern, holding the receiver upright and keeping it close to the ground. Move steadily rather than rushing. Mark any detected responses, then repeat the sweep at right angles. A line that is difficult to identify in one direction can be clearer from another.

Passive power mode may reveal energised electrical cables, while radio mode can identify long metallic conductors carrying re-radiated signals. Neither mode will find everything. A cable may be de-energised, poorly coupled to a radio signal or screened by its installation. Treat a no-signal result as an indication to investigate further, not confirmation that the ground is clear.

Where there is an accessible connection point, use the transmitter in direct connection mode. Connect only where authorised and safe to do so, following the equipment instructions and site procedures. Apply a low frequency first where possible. Lower frequencies tend to remain on the target line more effectively and are less likely to bleed onto nearby services. Trace the route with the receiver, marking its position at regular intervals and at every change in direction.

If direct connection is not possible, a signal clamp can apply a signal around an accessible cable or pipe without making an electrical connection. Induction mode can also be useful, particularly when there is no access point, but it is less selective. The transmitter can induce a signal onto several nearby conductors, so results require more caution in congested ground.

As you trace, sweep to either side of the apparent route. This checks whether the signal is centred and helps identify signal distortion or coupling. Where a service changes direction, reduce your walking pace and take closer marks. Record the signal frequency and method used if the survey will inform a permit, drawing or handover to another team.

Mark routes clearly and protect the information

Mark detected services on the surface using the site’s approved colour convention and include the direction of travel where practical. In many UK projects, markings identify electricity, gas, telecommunications, water and drainage differently, but teams should follow the agreed project standard rather than relying on memory alone.

A mark on the ground is temporary information. Photograph it, annotate the work plan or capture the route with survey equipment if the task demands a record. This is particularly valuable where markings may be removed by traffic, weather or subsequent site activity. For larger schemes, combining utility detection with GNSS, total station observations or reality-capture data can provide a more useful record for design coordination and future work.

Keep the detection findings available to the excavation supervisor and plant operators. A clear briefing is as important as the survey itself. Everyone involved should understand the proposed exclusion zones, the limits of the detection and the method for exposing services safely.

Prove the service before mechanical excavation

Locator findings should be verified by safe excavation practices. Follow the current requirements of HSG47, Avoiding Danger from Underground Services, alongside your organisation’s permit-to-dig and risk assessment procedures. Within the identified danger area, use appropriate hand-dug trial holes or other approved methods to establish the actual position and depth of each service.

Do not assume that a single detected line represents a single asset. Multiple cables may share a trench, a cable can be offset from a duct, and a metallic pipe may carry a coupled signal from another utility. Equally, an undetected asset may still be present. The purpose of trial holes is to replace assumptions with evidence before using mechanical plant.

Once exposed, protect the service and maintain suitable clearances throughout the works. If the detected route conflicts with drawings, if the signal disappears, or if conditions make the result uncertain, stop and reassess. Escalating an unclear result is faster and less costly than dealing with a strike.

Common mistakes that reduce detection quality

The most common mistake is relying on a single mode. Passive scanning alone can miss non-energised or non-metallic services, while induction alone can make it hard to distinguish the target line from neighbouring conductors. Use more than one detection approach where conditions require it.

Another issue is tracing too quickly or only along the expected route. Utilities rarely follow the neat, straight line suggested by a drawing. Sweep the wider area, investigate unexpected responses and re-check at crossings, joints and changes in surface level.

Operators can also misread depth because the receiver is not directly above the service or because the signal is distorted. Reposition, trace the line again and compare results using another frequency or connection method. If readings do not agree, treat the location as uncertain until it is proved.

Finally, equipment capability cannot compensate for inadequate training. Locator controls may appear straightforward, but frequency choice, signal coupling and interpretation take practice. A competent operator understands both what the equipment is showing and what it cannot confirm.

Choosing the right support for the job

The right utility locating setup depends on the services expected, the size of the survey area and the consequences of getting it wrong. A compact cable avoidance tool may suit routine pre-excavation checks, while congested urban work, long trace runs and survey-grade deliverables can justify a higher-specification locator, transmitter, accessories and formal operator training.

Survey Tech can help teams select, hire and maintain professional utility detection equipment for the work in hand, including practical advice on accessories, demonstrations and aftersales support. For a critical excavation, allow time to plan the detection properly, check the equipment and prove every service that matters before the first bucket enters the ground.