Striking buried services remains one of the fastest ways to injure people, stop the job and blow the programme. The CAT and Genny are standard kit on UK sites, but they’re only as effective as the planning, competence and interpretation behind them. Treat them as risk-reduction tools, not magic wands, and build your excavation approach around evidence, hold points and competent people.
TL;DR
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– Scan in multiple modes, mark confidently, and verify with trial holes or vacuum excavation before any machine bucket goes in.
– A permit-to-dig with hold points, photos and a sketch is the control spine; no permit, no dig.
– If the readout is noisy or inconsistent, stop and escalate—assume an unproven absence, not a safe clearance.
– Keep plant and people apart; use barriers and a banksman at the bucket when near any suspected service.
– Refresh competence: operatives must understand Power, Radio and Genny modes, clamp/direct/induction methods, and their limits.
Signs your locating work is about to go wrong
/> A service plan that’s vague, historic or missing is an early warning. If the utility map doesn’t tally with kerb lines, chambers or building ages, you should expect surprises. Equally, a rushed briefing or a lone operator sweeping without a witness and sketching is a red flag.
Watch for environmental noise that makes readings unreliable. Adjacent HV, tramways, temporary power, overhead lines, electric fencing and even rebar-heavy slabs can mask or mimic signals. Wet ground, made ground and deep backfill reduce range; plastic pipes without tracer wires won’t show unless a genny signal is coupled onto them via a connected metallic path.
Competence and battery state matter. A tired CAT with a low battery, out-of-date function check, or a user relying solely on “Power” mode is typical prelude to a strike. If the set-out doesn’t include a gridded sweep (two passes at right angles) plus a Genny-assisted pass, you’re not building a complete picture.
When the readout doesn’t stack up: intervene early
/> Don’t rely on a single pass. Cross-scan at 90 degrees, use Power and Radio to pick up naturally occurring signals, then use the Genny to physically induce a trace. If clamp or direct connection is possible on a known cable or metallic pipe, take it—it’s cleaner and more reliable than induction.
If the display spikes erratically, the depth estimate jumps, or there’s a null directly over a suspected service, stop the excavation. Move noisy kit away, switch off generators or welders nearby if practical, and rescan. If doubt remains, escalate: bring in a more experienced locator, consider vacuum excavation, or commission higher-resolution detection (e.g. radar) as a supplementary survey rather than pushing on.
Hand digging is still a control, but it must be methodical: short controlled bites, never stabbing with a pick, and a banksman watching the scan marks, not just the hole. Keep machine buckets back until the trial hole confirms what the scan suggested.
# Permit-to-dig essentials: quick checklist
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– Confirm competence and function check of CAT and Genny; record serial numbers and battery status.
– Collate available utility plans, highlight confidence gaps, and mark assumed “no information” zones in the permit.
– Scan in Power, Radio and Genny modes; sketch findings to scale with measurements from fixed features.
– Establish hold points: “no machine until trial hole proves route and depth” and “stop if markings don’t match what’s uncovered”.
– Set physical barriers and signage; nominate a dedicated banksman for any plant movements near markings.
– Photograph the mark-up and exposed services; attach to permit and brief the team at the workface.
Scenario: kerb works alongside a busy pavement
/> A small civils team is cutting back a kerb line on a housing development to make space for a lay-by. The service plans show an LV cable and a water main somewhere in the footpath, but the drawings are old and the kerb alignment has changed since. The operative scans in Power mode, gets patchy signals near a streetlight column, and paints a rough line. The supervisor notices conflicting spikes across the footway and calls for a Genny pass with induction, then clamps the streetlight feeder in the column base to get a clean trace. The new trace shows the LV bending closer to the new kerb than expected. A vacuum excavator opens two small trial holes along the proposed cut; the LV is found shallow due to historic resurfacing. The cut line is moved 300 mm, barriers are extended, and the job proceeds without a strike—and with updated photos and sketches filed to the permit.
Progress without gambling on buried services
/> Programme pressure tempts shortcuts, but you can keep momentum by sequencing and isolating risks. Break the work into short sections with defined scan and dig windows, so you’re always working on freshly validated ground. Use vacuum excavation to prove congested areas quickly and cleanly, particularly around street furniture, chambers and known pinch points.
Mark clearly and consistently. Use different colours for different utilities and add arrows for direction. Write depths only if you’ve proven them and state where the measure was taken; unproven depth estimates should not be treated as clearance. Refresh the mark-up after rain, foot traffic or when a shift changes—faded paint is not a control.
Keep interfaces tidy. Manage traffic, pedestrians and plant routes so that scanning and trial holes aren’t compromised by parked tippers or wandering operatives. Treat exposed services as temporary works: protect, support and cover as necessary, and record any changes so the next shift doesn’t re-learn the same lesson.
Common mistakes with CAT and Genny work
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Relying on Power mode alone
Only using Power mode misses dead cables, plastic pipes and poorly energised circuits, producing a false sense of security.
# Assuming no signal means no service
/> A quiet scan can be due to shielding, depth, materials or interference; absence of evidence is not evidence of absence.
# Induction used when direct connection was possible
/> Skipping clamp or direct connection because it takes longer leads to bleed-over and vague paths, especially in congested ground.
# Marking without a witness and record
/> Unwitnessed paint marks, with no sketch or photos, disappear with the next shower and leave the next shift to guess.
Signals, modes and practical technique
/> Power mode detects 50 Hz fields from live cables; it’s noisy near substations and can miss poorly loaded circuits. Radio mode picks up re-radiated signals on metallic services; useful for long runs but unreliable near large metallic masses or mesh. The Genny adds a controlled signal: clamp mode is best on accessible live metallic services, direct connection is second-best if you can reach exposed metal, and induction is last resort for inaccessible lines.
Technique matters. Use a slow, overlapping sweep in a grid, then “bracket” the peak and find nulls to centre the path. Depth readouts are approximate; never use them as clearance. Mark width as well as line if the signal is broad, and widen your no-dig buffer accordingly. Keep batteries fresh and do a daily function check; replace damaged leads and clamps.
# Seven-day actions to de-risk scanning on your job
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– Map your high-risk zones (near street furniture, historic reinstatements, tight corridors) and schedule vacuum excavation for proofs.
– Standardise your mark-up: colours, arrows, initials and date; brief all subcontractors to the same convention.
– Upgrade the permit-to-dig template to include photos, a simple sketch and explicit hold points that supervisors can sign off at the trench.
– Arrange a short refresher for CAT and Genny users focusing on clamp/direct/induction choices and dealing with interference.
– Set a rule that any conflicting or noisy readings trigger escalation before plant is allowed within two metres of the suspected line.
The big gains come from slowing down at the right moments, not from heroic last-minute rescues after a strike. Expect closer scrutiny on permits, competence and proof digs as clients and insurers react to costly service incidents; don’t wait for an enforcement visit to tighten your approach.
FAQ
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Do I always need a permit to dig for small hand-dug holes?
For any ground penetration where services may be present, a documented control like a permit-to-dig is good practice. Even for small hand excavations, a short form with a scan record, photos and hold points helps keep discipline and provides a reference when shifts change.
# How close can I machine dig to a marked service?
/> Treat marked lines as indicative, not precise. A conservative approach is to hand or vacuum excavate to prove the exact location and depth, and then maintain a safe buffer with a banksman guiding the bucket. The exact stand-off should be set in the permit based on ground conditions and service criticality.
# What if I can’t get a clean signal with the CAT and Genny?
/> Step back and reduce interference: move generators, switch off nearby plant if possible, and try different modes and orientations. If clamp or direct connection isn’t possible and induction remains noisy, escalate to a more experienced locator or consider supplementary surveys and vacuum excavation to prove the route.
# Are depth readings on the CAT reliable?
/> Depth functions can help, but they rely on strong, centred signals and correct technique. Treat them as approximate and never as a clearance guarantee. Only a proven trial hole or vacuum excavation should be used to confirm depth for safe digging.
# How should I manage other trades working near my marked area?
/> Brief them on the mark-up and exclusion zones at the start of the shift, and refresh the markings if they fade or routes change. Use barriers and signage to keep plant and pedestrians out of no-dig areas, and set a simple escalation rule: if any mark is unclear or missing, stop and call the supervisor before continuing.
