Avoiding Underground Utility Strikes: CAT Scanning and HSG47

Summary

Underground utility strikes are one of the most serious and preventable hazards in groundwork. A struck 230V or 400V cable can cause severe arc-flash burns, a struck gas main can lead to explosion or asphyxiation, and a struck water or fibre main causes costly damage and disruption. The HSE's HSG47 is the definitive UK guidance and frames safe digging around three steps: planning the work, locating and marking services, then excavating safely with the assumption that detection is never 100% reliable.

The two core tools are the CAT (Cable Avoidance Tool), a receiver that detects electromagnetic signals radiating from buried conductors, and the signal generator or "Genny", which applies a traceable signal to a known service so the CAT can follow it. Used together — never the CAT alone — they let a competent operator trace and mark the route of most metallic services and live power cables before a spade goes in the ground. PAS 128 sets out a graded standard for professional utility detection surveys where higher confidence is needed.

No tool finds everything. Plastic gas and water pipes, ceramic ducts, and disused or unenergised cables may be invisible to a standard CAT. That is why HSG47 requires services to be physically confirmed by careful hand-digging (or vacuum excavation) and why mechanical excavators must never be used directly over a known service line. The discipline is layered: plans, then electronic location, then trial holes, then safe digging — each layer catching what the previous one missed.

Key Facts

• HSG47 — HSE's "Avoiding danger from underground services" is the primary UK guidance; its model is Plan, Locate, Dig.
• Legal duty — managing excavation risk is a duty under the Health and Safety at Work etc. Act 1974 and CDM 2015 (planning, information and supervision).
• CAT must be used with a Genny — a CAT in passive mode alone is not a reliable locate; the signal generator is needed to trace specific services and find non-radiating ones.
• PAS 128 — the UK specification for underground utility detection, verification and location surveys; defines survey quality levels (e.g. D, C, B, A from desktop to verified).
• Calibration — CAT/Genny units should be checked daily (function check) and formally calibrated, typically every 12 months.
• Gas pipes — typical minimum burial depth ~600mm in a footpath, ~750mm under a carriageway; service pipes to homes often shallower.
• Electricity cables — LV/HV cables typically laid ~450–600mm in footpaths and 600mm+ under roads; depths vary widely and cannot be assumed.
• Water mains — typically 750mm–1350mm cover; communication pipes to properties often ~750mm.
• Telecoms/fibre — frequently shallow, sometimes 250–350mm, and increasingly plastic-ducted (hard to detect electronically).
• Plastic pipes — modern gas and water mains are often MDPE/PE and will NOT show on a standard CAT; trace wires or sondes are needed.
• Permit to Dig — most commercial and utility sites require a documented permit-to-dig system authorising each excavation.
• Safe digging zone — once a service is located, excavate by hand within ~0.5m either side; never use mechanical tools directly above it.
• Insulated tools — use insulated hand tools and spades/insulated shovels near known electrical services; avoid forks and picks over cables.
• Dial before you dig — request utility plans from every asset owner (DNO, gas, water, telecoms, plus any private supplies) before starting.
• Trial holes — physically expose and confirm critical services before bulk excavation begins.

Quick Reference Table

Depths are indicative only — never assume depth; always confirm by safe digging.

Detailed Guidance

Step 1 — Plan: gather information before site

Obtain current utility records from every asset owner: the Distribution Network Operator (electricity), the gas transporter, the water company, telecoms providers (BT/Openreach and others), and the local highways authority for street lighting. Check for private supplies — a buried armoured cable feeding a garage, a private water main, an oil line, or unrecorded drainage. Carry out a desktop study (this is PAS 128 quality level D) and walk the site looking for surface evidence: manhole and inspection covers, stop taps, marker posts, lamp columns, meter boxes, and patched trenches that hint at a previous dig. Plans are a guide, not gospel — services move, get added, and are often recorded inaccurately.

Step 2 — Locate: CAT and Genny technique

A CAT works in three modes. Power mode detects the 50Hz field radiating from live mains cables. Radio mode detects re-radiated radio signals on long metallic conductors (pipes, cables). Generator (signal) mode detects a specific traceable signal applied by the Genny. A safe sweep uses all three, then traces specific services with the Genny.

CAT/GENNY LOCATE SEQUENCE
-------------------------
1. Daily function check pass?          --NO--> Do not use. Replace/recalibrate.
        | YES
2. Sweep area in POWER mode            --> mark all live-cable indications
        |
3. Sweep area in RADIO mode            --> mark all long-conductor indications
        |
4. Connect GENNY to known service,
   sweep in SIGNAL/GENERATOR mode      --> trace & mark each service route
        |
5. Sweep at 90 degrees (cross-pattern) --> confirm direction, avoid missing
        |                                   services running parallel
6. Mark service routes on the ground (spray/pegs)
        |
7. Still assume undetected services exist --> proceed to safe dig

Sweep slowly, keep the CAT vertical, and cross the search area in two directions at right angles so a service running parallel to your sweep line is not missed. Mark everything found directly on the ground. The Genny can be applied by direct connection (clamp/clip to an accessible conductor), by induction (placed on the ground over a suspected route), or via a signal clamp around a cable.

Step 3 — Dig: safe excavation

Once services are marked, dig trial holes by hand to physically expose and confirm position and depth before any mechanical excavation. Within the safe-digging zone — roughly 0.5m either side of a located service — use only hand tools, preferably insulated spades and shovels; avoid picks and forks that can pierce a cable or pipe. Where ground is hard or congested, vacuum (suction) excavation is the safest method as it removes spoil without striking services. Re-scan as the excavation deepens — depth readings from a CAT are indicative, not exact. Keep the excavation supported per excavation-safety requirements and never assume the absence of a service just because the plan shows none.

What to do if you strike a service

STRIKE RESPONSE
---------------
GAS:    Stop. Evacuate upwind. No ignition sources (phones, engines, switches).
        Call gas emergency line (0800 111 999). Warn occupants. Do not re-enter.
ELECTRIC: Do not touch the cable or anyone in contact with it. Keep clear.
        Treat as live. Call DNO emergency / 105. Get first aid for burns/shock.
WATER:  Isolate at stop tap if safe. Call water company. Manage flooding/wash-out.
TELECOMS: Report to provider; do not attempt repair.
ALL:    Report as a near miss/incident under RIDDOR if reportable. Record and learn.

A gas strike is treated as a potential explosion hazard: stop work, eliminate ignition sources, evacuate, ventilate, and call the National Gas Emergency Service. An electrical strike is treated as live until proven dead by the network operator. Never attempt a repair on a struck service yourself.

Competence, calibration and permits

CAT operators should be trained and competent (e.g. an industry CAT-use course) — the tool is easy to misread in untrained hands. Units need a daily pre-use function check and periodic formal calibration (commonly annual). On managed sites a permit-to-dig system records who authorised each excavation, what services were located, and the controls in place. CDM 2015 places duties on the client and principal contractor to provide pre-construction information (including service records) and to plan and supervise the work.

Frequently Asked Questions

Can I rely on a CAT scan alone before digging?

No. A CAT is one layer of protection, not the whole answer. It misses plastic pipes, unenergised cables and non-metallic ducts, and depth readings are approximate. HSG47 requires you to combine plans, electronic location, trial holes and careful hand-digging. Treat every excavation as if undetected services may be present.

How deep are gas and electricity services usually buried?

Indicatively, gas mains are around 600mm in footpaths and 750mm under roads, and electricity cables around 450–600mm, but depths vary enormously and services are often shallower than records suggest, especially older or amended installations. Never assume depth — confirm it by safe digging.

Why won't my CAT find a plastic water or gas pipe?

A standard CAT detects electromagnetic fields from metallic conductors. Modern MDPE/PE pipes are non-conductive, so they produce no signal. They can only be traced if they contain a metallic trace wire (which you energise with the Genny) or by inserting a sonde, or located with ground-penetrating radar as part of a PAS 128 survey.

When do I need a PAS 128 survey rather than a CAT scan?

For larger, higher-risk or design-stage works where you need verified, recorded service positions, a professional PAS 128 detection survey gives graded confidence levels (desktop through to verified). A site CAT/Genny scan is appropriate for routine excavation but should still be backed by plans and trial holes.

Who is legally responsible for preventing strikes?

Duties are shared. The client and designers must provide service information and design out risk where possible; the principal contractor and contractor must plan, locate, supervise and provide competent operators — all under the Health and Safety at Work etc. Act 1974 and CDM 2015. The person doing the digging must follow the safe system of work.

Regulations & Standards

• HSG47 "Avoiding danger from underground services" (HSE guidance) — the primary UK guidance; Plan, Locate, Dig method, CAT/Genny use, safe digging.

• Health and Safety at Work etc. Act 1974 — general duty to ensure health and safety of workers and others affected.

• Construction (Design and Management) Regulations 2015 (CDM 2015) — duties to plan, provide pre-construction information and supervise excavation work.

• PAS 128 — specification for underground utility detection, verification and location; defines survey quality levels D/C/B/A.

• RIDDOR 2013 — reporting of certain dangerous occurrences and injuries, which can include serious utility strikes.

• HSG47: Avoiding danger from underground services (HSE) — primary UK guidance on safe excavation near services.

• HSE: Underground services — overview of duties and the Plan, Locate, Dig method.

• CDM 2015 (HSE) — construction design and management duties.

• National Gas Emergency Service — what to do on a gas strike (0800 111 999).

• excavation safety — supporting and working safely in trenches and excavations.

• underground drainage — laying and locating buried drainage runs.

• working at height — companion HSE-regulated site hazard.

• cdm regulations — the legal framework for planning and supervising construction work.