Service strikes remain one of the ugliest ways to lose time, money and credibility on a UK job. A simple dig for bollards, drainage or fencing can turn into a shutdown if a live cable or gas main is clipped. The combination of competent people, a live permit-to-dig, and proper use of CAT and Genny is still the best defence—but only when it’s treated as a control system, not a tick-box.
TL;DR
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– Treat CAT/Genny, drawings, and trial holes as a combined system; none of them is good enough on its own.
– Permits must be live documents with hold points, rescans and change control, not a one-off sheet.
– Mark up services clearly, refresh markings as work moves, and keep non-essential plant out of the zone.
– Use hand-dig and vacuum excavation near suspected services; depth indications are not guarantees.
– Stop work if anything changes: different kit, offset position, new plant arriving, or fading marks.
The basics: what CAT and Genny can and can’t do
/> A cable avoidance tool listens for electromagnetic signals; the generator induces a signal to make buried metallic services easier to find. Used together, they can pick up power cables, telecoms, and metal pipes, and help you trace routes along a run. They are not X‑ray goggles: they struggle with plastic, dead cables with no signal, shallow drops, or areas with interference from nearby kit.
Good operators sweep in multiple modes, use the Genny to place a signal on suspected routes, and keep the head close to the ground. They understand that depth readouts—if the model has them—are only an indication, not permission to dig. The result is a plotted, colour-marked sketch on the ground and on paper, showing both what was detected and the uncertainties. Anything “unknown” is treated as live until exposed safely.
Site scenario: civils team installing EV charge pedestals on a housing development
/> A small civils crew is cutting slots along a footpath to run ducting to new EV chargers outside a row of new-builds. Programme pressure is real: residents are due to move in within three weeks and the street must be reopened for snagging access. The groundworkers have a weekend window with a mini-excavator, a concrete saw, and a labourer with a CAT and Genny. They have utility records that are two months old, showing low-voltage cables on the opposite verge and a water service near the kerb. On first scan, the operator picks up a strong signal right where the slot is planned—contradicting the drawing. The supervisor pauses, raises a permit hold point, and authorises two trial holes by hand. They expose a taped twin LV cable 250 mm deep—the service was diverted in the last surge of fit-out works and never updated on the plan. The slot is re‑routed, vacuum-excavated across the crossing point, and the job finishes late on Sunday—but with the street open and the lights still on.
On the day: practical checks before breaking ground
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– Confirm competence and recent familiarisation with the specific CAT/Genny model; run a function test and check calibration dates.
– Review current utility information and any recent diversions; brief the team and mark exclusion zones for plant and non-essential labour.
– Scan in multiple modes, induce a signal with the Genny where possible, and mark routes and “no‑dig” zones clearly with paint and flags.
– Lift accessible covers to inspect ducts and direction of travel; set permit hold points at crossings and changes in depth or direction.
– Hand-dig and/or vacuum excavate within the last 500 mm to a detected or suspected service; keep the digger bucket out until services are exposed.
– Keep markings fresh as the workface moves; rescan after breaks, shift changes, rain, or if the plan or team changes.
– Record what’s found—photos, sketches, and updates to the permit; escalate if anything doesn’t match expectations.
Permit-to-dig that actually controls the job
/> A workable permit sets the rules of engagement. It names the area, the task, the people, the plant permitted, and the specific steps for scanning, marking and exposing. It has time limits: permits expire daily or with a key change, such as different operators, a route shift, or new plant arriving. It defines hold points—no further until a supervisor inspects, signs and, if needed, updates the CAT/Genny sweep.
Good permits don’t just clip on the fence; they are kept with the team, referenced in the pre-start briefing, and re‑authorised when weather, surface levels, or access changes. The permit also sets the escalation route: if the scan and drawings disagree, or if unidentified signals persist, the job pauses for a managed trial hole or a specialist survey.
Interfaces that make or break service safety
/> Most strikes happen at edges—where the plan hands off to the machine, or where one gang replaces another. Keep plant out of marked service corridors unless the permit explicitly allows it. Place barriers and clear signage to stop passing trades from walking kit through the no‑dig zones. Street works need traffic and pedestrian protection that doesn’t hide your markings; refresh paint after cones and barriers are moved.
Temporary works awareness is part of it: trenches and trial holes need support, safe access, and protection from collapse and flooding. If you’re exposing services in poor weather or fading light, your controls must reflect that—task lighting, covers, and secure walkways. Housekeeping matters too: loose spoil or slurry can coat markings and swallow flags, so keep edges clean and re‑mark before recommencing.
Common mistakes to avoid
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Treating the depth readout as a promise
Depth indications are estimates and can be wildly off in congested ground. Always expose with care and assume shallow until proven otherwise.
# Scanning once, then digging all day
/> Signals drift, paints fade, and the dig line moves. Rescan whenever the workface shifts, after breaks, and when different hands take over.
# Believing the drawing is the ground
/> Records can lag behind diversions and private feeds are often missing. Let the ground tell you the truth via CAT/Genny and controlled exposure.
# Handing the permit to the quietest person
/> Permits need authority and presence. Give them to the supervisor who will actually hold the line and stop the job when conditions change.
When scans and drawings don’t agree: practical fixes
/> Start by trusting uncertainty. Where the CAT/Genny shows an unexpected run, mark it, brief it, and raise a permit hold point. Use hand-dug or vacuum-excavated trial holes at strategic points—junctions, bends, or proposed crossings—to expose what’s really there. If you repeatedly pick up untraceable signals or interference, consider a specialist survey with higher-spec detection or arrange temporary power isolation in controlled circumstances.
Document what you find with photos and quick sketches, then immediately reflect those changes on the permit and the site plan. If a reroute or sequencing change is needed, manage it via your change process so everyone—not just the dig team—knows the new plan.
Before the next ground break: five moves
/> Walk your planned dig lines with the permit author and the CAT/Genny operator together, so the intent and the detection line up. Confirm who owns hold points and who has the authority to stop the dig when the world doesn’t match the paper. Line up vacuum excavation early for crossings or congested areas, rather than phoning around at 3 pm. Sort traffic and pedestrian control so it doesn’t wipe out your markings or force the team into the live carriageway. Capture lessons from near-misses and fold them into the next briefing, especially on how often you rescanned and where you changed method.
The bottom line is simple: you can’t out-argue a cable you didn’t see. The crews who blend live permits, competent scanning and disciplined exposure keep power on, gas in the pipe, and the programme intact.
FAQ
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Do I always need both a CAT and a Genny for small digs?
For small, simple tasks you might detect some services in passive modes, but relying on that alone is risky. The Genny gives you a stronger, more targeted signal on metallic services and helps trace routes more confidently. Treat them as a pair whenever you can, especially in congested or previously developed ground.
# How often should we rescan during a shift?
/> Good practice is to rescan when the workface moves, after breaks, when markings fade, or if the team or plant changes. Wet weather, heavy foot traffic, and moving barriers can erase or shift your reference points, so build rescans into the permit hold points. If anything feels uncertain, stop and rescan before the next bite.
# What if the drawing shows no services, but the CAT/Genny is noisy?
/> Trust the instrument and your competence. Mark the suspected run, brief the team, and create a hold point for trial exposure. If you can’t verify safely, escalate for a supervisor decision or a specialist survey rather than pushing on.
# When is vacuum excavation worth bringing in?
/> It’s particularly useful around known crossings, congested corridors, or shallow services where hand-digging is slow and risky. It reduces strike potential while maintaining pace, provided operators are competent and exclusion zones are managed. Plan it early so it’s available at the right time rather than as an emergency fix.
# What should a solid permit-to-dig include for service avoidance?
/> It should define the area and task, reference the latest records, name competent operators, and set clear steps for scanning, marking and exposing. Include expiry, hold points, escalation routes, and conditions for change—such as different machinery or a shift to night work. Keep the permit live with the team, update it as findings change, and close it out with as‑found records.
