Guide
Safe utility trenching and cable avoidance
How to dig safely near underground services on construction sites. Covers HSG47 safe digging practices, cable avoidance tool (CAT) use, utility record searches, hand digging zones, trench support, and emergency procedures for utility strikes.
Striking an underground utility during excavation can cause fatal electrocution, gas explosions, flooding, loss of supply to the public, and criminal prosecution. HSE guidance HSG47 sets out the safe digging practices you must follow whenever you excavate near buried services.
This guide applies if you are a contractor, site supervisor, or plant operator carrying out any excavation work where underground services may be present. This includes trenching for new utilities, foundation excavation, drainage installation, landscaping, and highway works.
Excavation near underground services is classified as work involving particular risks under CDM 2015 Schedule 3. Your construction phase plan must address these risks before any digging begins.
1. Search utility records before you dig
Before any excavation, you must identify what services are buried in the work area. Utility plans show approximate routes but are not precise -- accuracy is typically plus or minus 500mm. Never rely on plans alone.
Follow these steps to obtain utility records:
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Register with LSBUD (LinesearchbeforeUdig)
Use the free LSBUD service to identify all asset owners with apparatus in your work area. This covers electricity distribution network operators (DNOs), gas distribution networks (GDNs), water companies, telecoms providers, and private utilities.
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Request plans from each asset owner
Contact every identified asset owner and request their service plans for the excavation area. Allow sufficient time for responses before work is due to start.
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Keep plans on site throughout the works
Utility plans must be available on site for the duration of excavation. Brief all operatives and plant operators on the location of known services before work begins each day.
Plans are a starting point, not a guarantee. Services may have been installed, diverted, or abandoned without records being updated. Always use detection equipment to confirm what is in the ground.
2. Scan the area with CAT and Genny
A cable avoidance tool (CAT) scan is mandatory before any excavation. The CAT detects electromagnetic signals from buried metallic services. Used with a signal generator (Genny), it can trace specific utility routes.
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Scan in all three modes
Use the CAT in power mode (detects live electricity cables), radio mode (detects metallic services picking up re-radiated radio signals), and Genny mode (traces a signal applied to a specific known service). All three modes are needed because no single mode detects everything.
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Scan the full excavation area plus margins
Scan the entire area to be excavated plus a suitable margin on all sides. Services do not always follow straight lines or expected routes.
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Mark located services on the ground surface
Use paint, markers, or pegs to mark the indicated line of each detected service on the ground surface before excavation begins.
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Check calibration before use
The CAT must be checked and calibrated in accordance with the manufacturer's instructions. Verify operation against a known service at the start of each shift.
Limitations: A CAT indicates the presence and approximate line of a service. It does not reliably indicate depth. Plastic pipes with no metallic tracing wire will not be detected. Treat every scan result as approximate and confirm by hand digging.
3. Hand dig within the safe zone
Mechanical excavation must not take place within 500mm either side of the indicated line of a located service. Within this zone, you must dig by hand using appropriate techniques.
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Hand dig within 500mm of located services
Use hand tools only within 500mm either side of the indicated line of any located service. No mechanical excavation is permitted in this zone.
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Dig trial holes to expose services
Before main excavation, dig trial holes by hand to confirm the exact position and depth of services. This is essential where services cross the excavation route.
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Use correct hand tool technique
Push spades or forks in vertically alongside (not across) the expected line of the service. Do not use picks, pickaxes, or pointed hand tools directly above buried services.
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Use insulated hand tools near electrical cables
Where electricity cables may be present, use insulated hand tools to reduce the risk of electric shock if a cable is struck.
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Support exposed services
Once exposed, support services to prevent damage from sagging, movement, or loading. Use fine granular material (not sharp stones or rubble) when backfilling around exposed services.
4. Support the trench
Trench collapse is a leading cause of construction fatalities. All excavations where there is a risk of collapse require support, regardless of depth. Even shallow trenches can collapse without warning, and wet or disturbed ground increases the risk.
A competent person must assess what support is needed before any excavation work begins. The competent person must also inspect the excavation at the start of every shift and after any event that could affect stability, such as heavy rain, nearby vibration, or unexpected ground conditions.
5. Ensure your team holds the right qualifications
Everyone involved in utility trenching must be competent for their role. If you are carrying out street works (work in a publicly maintained highway), supervisors and operatives must hold valid qualifications registered on the Street Works Qualifications Register (SWQR) under Section 67 of the New Roads and Street Works Act 1991.
CAT and Genny operators must be formally trained and assessed. Site supervisors must be trained in HSG47 safe digging practices and utility identification. Plant operators working near buried services must hold a valid CPCS or equivalent card and receive a site-specific briefing on service locations before commencing work.
6. Know the emergency procedures for utility strikes
Despite best practice, utility strikes can still occur. Every person on site must know the emergency procedures before excavation begins. The response differs depending on which service is struck.
Gas strike
Evacuate the area immediately, establish an exclusion zone, and eliminate all ignition sources (including mobile phones and vehicle engines). Call the National Gas Emergency Service on 0800 111 999. Do not attempt to repair the damage.
Electricity strike
Do not touch the cable or any equipment in contact with it. Warn everyone to stay clear. If you are in a machine cab, remain in the cab unless there is a fire risk. If you must leave, jump clear with both feet together -- do not step down. Call your DNO emergency number immediately.
Water strike
Evacuate the trench if there is a flooding risk. Call the water company emergency number. Prevent contamination of the water supply.
Telecoms strike
Report to the asset owner. Be aware that fibre optic cables may carry laser light presenting an eye hazard.
All utility strikes
Do not attempt to repair the damage yourself. Make the area safe and await the utility company's response. A strike on underground gas or electricity services is a reportable dangerous occurrence under RIDDOR 2013. Report online at hse.gov.uk/riddor within 10 days, or immediately by telephone on 0345 300 9923 if there is a death or serious injury.
Failure to report a dangerous occurrence is a criminal offence carrying an unlimited fine. Retain records of all utility strikes for a minimum of 3 years.
What to do next
After completing utility trenching safely:
- Record the as-built positions of any services exposed or installed, and provide updated records to the relevant asset owners
- Ensure reinstatement meets the required standard -- if working in a highway, follow the Specification for the Reinstatement of Openings in Highways (SROH)
- If you are working under a street works permit, check your permit conditions and ensure works are completed before the agreed end date to avoid Section 74 overrun charges
- Brief the site team on any lessons learned from the excavation, particularly if services were found in unexpected locations