A strike on buried services is rarely a freak event. It’s usually the end result of rushed scanning, misplaced confidence in the kit, and poor translation of what the instrument found into a safe digging plan. Cable avoidance tools and signal generators are excellent aids, but they are not magic wands. Used badly, they lull teams into a false sense of security. Used well, they stop strikes, delays and claims.
TL;DR
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– Slow down: scan in multiple modes, in two directions, and cross-hatch the area.
– Prove the kit on a known service before relying on it, and document your sweep on the permit.
– Use the generator wherever possible; don’t trust passive modes alone.
– Rescan after removing spoil or breaking ground; conditions change.
– When in doubt or in congested ground, stop and bring in competent survey support.
What a competent avoidance scan really detects
/> A cable avoidance tool (CAT) will pick up electromagnetic fields from live power or radio sources in passive modes, and from an injected signal in active mode when paired with a generator (Genny). Passive modes can miss dead cables, plastic water pipes, and well-screened services, so treat them as a first look, not a clearance. Active mode lets you force a signal onto a known conductor, giving clearer, traceable lines and allowing you to follow routes through junctions and changes in depth. Depth indications, if available, are estimates and can be distorted by adjacent services, damp ground, or rebar—use them to understand trends, not to decide safe digging depth.
Orientation matters. Scanning with the antenna at 90 degrees to a conductor and then checking with a “null” pass helps confirm location. Fast sweeps and a single line pass are not good enough; slow, overlapping passes in both directions produce a more reliable picture. And if the kit can’t be proven on a known service on site, don’t proceed—either the ground or the instrument is telling you to stop.
From permit to paint: turning scans into safe digs
/> Good practice starts at the desk: gather utility plans, review the scope, and set the area for scanning before any plant arrives. Build scanning into your permit to dig so it’s a defined step with named competence and a time window that isn’t squeezed by deliveries or concrete slots. On site, demarcate the scan zone, brief the crew, and set up basic traffic management so the scanner isn’t dodging tippers or trench boxes.
Complete a controlled sweep: passive power, passive radio (if available), then active with the Genny using direct connection to a known point where possible; if not, use induction but understand its limits. Mark findings with durable, unambiguous paint and pins, photograph the markings against fixed features, and record the method used and the date/time on the permit. Before breaking ground, agree the trial-hole locations, choose insulated tools, and set an exclusion zone. After opening up any surface—planed tarmac, lifted slabs, or scraped topsoil—rescan before advancing. Conditions change as you remove shielding and expose conductors.
Scenario: drainage tie-in near a live carriageway
/> A civils gang was cutting a narrow trench to tie a new gully into an existing line on a housing estate. The ganger had a CAT but no Genny to hand and did a quick sweep in power mode, reporting “nothing live”. The site had poor markings from an earlier phase and the permit was half-complete. As the saw cut advanced, a banksman spotted a slight flicker on the CAT near the kerb but assumed it was street lighting and off-line. The cut intersected a shallow telecom duct, nicking it without full severance; broadband for the street dropped later in the day and the client took the heat. A re-visit with the Genny found the telecom alignment easily by direct connection at a nearby chamber. The lesson landed: passive-only scans are not a clearance, and scans must be tied into a live permit with photos and a fresh sweep after every stage.
Common scanning mistakes that catch teams out
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Treating depth as fact
Depth readouts are approximate and easily fooled by adjacent services, coupling, or rebar. Never use depth to justify deep mechanical excavation without trial holes.
# Scanning only in Power mode
/> Relying on passive power misses dead or well-screened assets. Without the Genny you won’t trace non-energised or plastic services with tracer wire.
# Skipping a rescan after breaking ground
/> Removing surface layers changes how signals couple and reveals previously hidden lines. Always rescan after planing, slab lifting, or spoil removal.
# Ignoring orientation and peak/null checks
/> Single-direction sweeps and no null confirmation produce false confidence. Use cross-hatching and check both peak and null to verify position.
Practical fixes you can implement tomorrow
/> A robust scan translates into clear decisions, not just colourful paint. Keep the scanning step calm, deliberate and documented, then carry findings forward into the excavation method—trial holes first, restricted plant, and eyes on for changes in the ground.
# Pre-dig scanning checklist for supervisors
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– Confirm the operator’s competence and that the CAT/Genny passed function checks on a known service that day.
– Review current utility plans and define the scan boundary, access, and time slot within the permit.
– Sweep in passive modes, then in active mode using the Genny; prioritise direct connection points over induction.
– Cross-hatch the area slowly, mark routes and suspected zones, and take photos with scale and reference points.
– Brief the digging team on markings, no-go zones, trial-hole sequence, and insulated tools to be used.
– Rescan after surface removal and after each trial hole before widening or introducing plant.
– Capture findings, changes, and decisions on the permit with initials and time stamps.
# Actions to raise CAT & Genny competence this week
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– Schedule a short, on-site demonstration where crews practise direct connection, induction, and null checks on known services.
– Map out a standard colour code and symbology for markings and stick copies in the site office and permits.
– Swap any ageing or suspect instruments for calibrated units and quarantine anything that can’t prove on a known service.
– Tighten the permit workflow so no excavation starts without scan photos and a named operator recorded.
– Capture one learning per job—what confused the scan, what worked—and share it at the next site coordination meeting.
Interfaces and supervision around scanning
/> Scanning isn’t a solo sport. Supervisors should manage interfaces: plant movements paused, briefings delivered, and nearby trades aware of the temporary markings. Temporary works, such as trench boxes or sheet piles, can mask or reflect signals; plan the scan before these go in, and be ready to adapt after they do. Traffic management and weather matter too—heavy rain and standing water can alter readings, and live carriageways may need extra control to give the scanner safe time on the deck.
Housekeeping helps: clear loose metallic debris that can confuse the instrument, and keep markings legible for the life of the task. If the job spans days, revisit the markings, refresh the photos, and reconfirm with a short sweep before the next shift starts.
When to escalate or change method
/> If you can’t achieve a clear trace, the signals are messy, or the area is congested on the plans, stop and escalate. Engage a competent surveyor or deploy additional methods such as ground-penetrating radar, especially in complex urban corridors. Consider isolations with the asset owner where feasible, or redesign the route to avoid high-risk corridors. No one gets marked down for pausing a dig that feels wrong; they do get marked down for pressing on and hitting something.
Slow scans, two-direction passes, and recorded evidence on the permit keep services intact and programmes moving. Expect closer attention from clients and inspectors on permit-to-dig quality and whether your team can actually demonstrate competent CAT & Genny use.
FAQ
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Do I need a permit to dig for every small trench if I’ve scanned the area?
Even for small, quick cuts, a simple permit or documented task briefing is good practice because it captures who scanned, when, and what was found. The paperwork doesn’t slow you down if prepared sensibly; it preserves the reasoning behind your method and prevents casual starts without controls.
# How often should I rescan during an excavation?
/> Rescan after any change that could affect readings: removing surface layers, opening a trial hole, shifting plant into position, or after a long break. Think of rescans as guardrails at each stage, not a one-off gate at the start.
# What if I can’t find a suitable connection point for the Genny?
/> Use induction as a fallback, but understand it spreads signal and can create ghost traces. Widen your search for accessible chambers or exposed metalwork, and if confidence remains low, pause and seek specialist input rather than proceeding on passive mode alone.
# Are depth readings reliable enough to set dig limits?
/> Treat depth as an indicator, not a control measure. Set dig limits using trial holes, hand-digging with insulated tools near markings, and progressive exposure verified by repeated scans.
# How should I manage markings when rain or traffic wears them away?
/> Use durable paint, pins, and flags, photograph everything with reference points, and refresh markings at the start of each shift. If markings degrade or the workface moves, redo a quick sweep and update the permit so the crew isn’t relying on guesswork.
