CAT and Genny — Avoiding Underground Service Strikes Before You Dig (2026)

Why It Matters — The Law and the Risk

Underground service strikes are one of the most common serious-injury events in groundwork. The guidance that governs this work is the HSE's HSG47, "Avoiding danger from underground services". It is not law in itself, but it sets out the standard a court will expect you to have met, and it underpins your duties under the Health and Safety at Work etc. Act 1974 and the Construction (Design and Management) Regulations 2015 (CDM 2015).

Under CDM 2015 the work must be planned so that risks are eliminated or controlled before anyone breaks ground. That means treating service location as a designed-in control, not an afterthought once the digger is on site. A strike that injures someone — or even one that doesn't — can trigger an HSE investigation, prosecution, unlimited fines and, in the worst cases, a corporate manslaughter charge. The buried-services risk assessment and method statement are the documents that show you planned the work properly.

The Safe Digging Sequence: Plan, Locate, Dig

HSG47 builds everything around three steps. Skip any one of them and the others lose most of their value.

1. Plan

Before any equipment turns up, obtain the utility plans and asset records for the site. Request drawings from the relevant asset owners — the DNO (electricity), gas transporter, water company and telecoms/fibre providers — or use a service that aggregates them. Read the plans on site, not just in the office: locate manholes, marker posts, meter positions, street furniture and the direction services are likely to run. Plans are frequently inaccurate, incomplete or out of date, so treat them as an indication of where services might be, never a guarantee of where they are.

2. Locate

With plans in hand, sweep the dig area with a CAT (Cable Avoidance Tool) and a Genny (signal generator). The CAT detects the electromagnetic fields given off by buried services; the Genny applies a traceable signal so you can find services the CAT would otherwise miss. Mark every located service on the ground with spray paint or marker pins, and note depth estimates. The detail of how the CAT and Genny work is below.

3. Dig

Only once services are planned and located do you break ground — and even then, carefully. Hand-dig trial holes to positively expose and confirm the position and depth of any located service before a mechanical excavator goes anywhere near it. Keep the excavator outside a safe margin of any known service and use hand tools (ideally insulated) in the danger zone. Treat every service as live until it has been proven dead by the asset owner.

How a CAT Works — The Three Modes

A Cable Avoidance Tool is a passive-and-active receiver. It does not "see" pipes and cables directly — it picks up electromagnetic signals associated with them. There are three modes, and competent operators use all three, because each finds different things.

Power mode

Power mode detects the 50Hz field radiated by live, current-carrying electricity cables. It is passive — no Genny needed — and it is your first line of defence against energised mains cables. The limitation is obvious: it only finds cables that are actually carrying load. An unenergised or lightly loaded cable may give little or no signal.

Radio mode

Radio mode is also passive. Long metallic services — water pipes, steel ducts, some telecoms — act as aerials and re-radiate low-frequency radio signals present in the environment. The CAT picks up that re-radiated signal. Radio mode helps find metallic services that aren't carrying power, but it relies on a usable ambient radio field and won't detect non-metallic services at all.

Genny / Signal mode

This is the mode that turns a CAT from a rough warning device into a reliable locator. The Genny (signal generator) applies a known signal to a service — by direct connection to an accessible conductor, by an induction clamp around a cable, or by induction over the ground. The CAT, switched to the matching signal frequency, then traces that specific service with far greater confidence than passive modes allow. Crucially, the Genny lets you find non-live and unenergised services that power and radio modes would miss, and it gives the most reliable located signal of the three. Use the Genny wherever it can be applied — it is the difference between guessing and locating.

Limitations — What a CAT Can and Cannot Find

A CAT is a tool, not a guarantee, and the strikes that injure people usually happen because someone trusted the device beyond what it can actually do. The key limitations:

• Unenergised cables can be missed in power mode — no current, no 50Hz field. Always back up with radio and Genny modes.
• Plastic gas and water pipes cannot be detected by a CAT in any standard mode. Plastic doesn't conduct and doesn't re-radiate. This is one of the most dangerous gaps: a CAT will happily report "clear" over a live plastic gas main. You need the plans, and where possible a sonde or trace wire in the duct, plus a Genny signal applied to any accessible metallic fitting.
• Depth is an estimate, not a certainty. CATs that display depth give an approximation that is easily distorted by congested services, signal distortion and operator technique. Never dig to a CAT depth reading — prove depth with a trial hole.
• Signal distortion from multiple parallel services, deep services or interference can pull a located position sideways. Always confirm by hand-digging.

The practical takeaway: the CAT supports the plans and the trial hole — it never replaces them.

Daily Checks and Calibration

A CAT that isn't working properly is worse than no CAT, because it gives false confidence. Two routines keep yours trustworthy.

Pre-use function checks should be carried out every day before work starts. Confirm the unit powers up, the battery is healthy, and it responds correctly in each mode — for example, by checking it detects a known signal source or a live service you can verify. A quick functional check takes a couple of minutes and catches a flat battery or a faulty receiver before it costs someone a hand.

Calibration is the periodic, traceable verification that the CAT is reading accurately. Industry practice — and most manufacturers' recommendation — is calibration at least annually. Keep the calibration certificate and a log of daily function checks. On an HSE investigation or a managed-site audit, those records are how you demonstrate the locating equipment was fit for use. An out-of-calibration CAT undermines your whole safe-digging case.

Operator Competence and Training

Locating is a skill, not a button-press. The most accurate CAT in the world will mislead an untrained operator who sweeps too fast, holds it wrong or ignores the limitations above. HSG47 expects services to be located by trained, competent operators.

Recognised utility-detection and CAT/Genny awareness training — including EUSR-registered utility avoidance schemes commonly required on utility and managed sites — gives operators the underpinning knowledge and a verifiable record of competence. For any business doing regular excavation, putting your diggers through accredited training is both a safety control and a commercial advantage when tendering for work that demands proof of competence.

Best Practice on the Ground

How you actually use the equipment matters as much as owning it. The habits that separate safe operators from lucky ones:

• Scan in multiple directions. Services run in straight lines and the CAT responds best across a service, not along it. Sweep the area in two directions at right angles to catch services running in any orientation.
• Sweep the whole working area, not just the trench line. Spoil heaps, access routes and machine standing all need to be clear.
• Mark up your findings on the ground as you go — spray lines, depth notes, service type — so the whole crew can see what was located.
• Re-scan as you progress. A single scan at the start isn't enough; conditions change, you uncover new ground, and a service missed on the first pass can show up on the next.
• Treat all located services as live until the asset owner has positively proven them dead. "It's probably disused" has killed people.
• Use the Genny wherever you can. Passive scanning alone is the weakest form of detection.

Permit to Dig and the Role of the Trial Hole

On managed sites — utilities work, principal-contractor sites, highways — you will usually need a permit to dig (permit to excavate) before breaking ground. The permit is a formal control that confirms plans have been obtained, the area has been scanned, services are marked, and the safe system of work is agreed and signed off. No permit, no digging.

The trial hole is the final proof in the sequence. Hand-dug carefully — often with an air or vacuum excavator on sensitive sites — it physically exposes the located service so you can confirm its exact position, depth and type with your own eyes before any mechanical excavation starts nearby. The CAT and Genny tell you roughly where to dig the trial hole; the trial hole tells you the truth. Together they are what "safe digging" actually means.

Quick Reference: The Three CAT Modes

Whatever the modes tell you, the rule never changes: plan, locate, then dig carefully — and prove every service with a trial hole before the excavator moves in.