Avoiding Underground Services — Safe Digging and Permit to Dig (2026)

Why Buried Services Are So Dangerous

The Health and Safety Executive's guidance HSG47, Avoiding danger from underground services, exists because people are still seriously injured and killed striking buried apparatus every year. The danger varies by service, but every one of them is capable of causing serious harm:

• Electricity cables: the highest-risk hazard. Striking a live cable with a tool or machine bucket can cause fatal electrocution, severe burns and an arc flash or explosion. Even glancing contact can throw molten metal and ignite clothing. The voltage you cannot see is what kills.
• Gas mains: a damaged main releases gas that can ignite, causing fire and explosion. Plastic (PE) gas pipes are particularly easy to snag with a bucket or to pierce with a tool, and the gas can travel along the trench and into nearby buildings.
• Water mains: a struck main can flood an excavation in seconds, collapse trench walls, undermine foundations and wash out the ground around other services.
• Telecoms and fibre: rarely life-threatening, but damage causes major disruption, loss of emergency line access for others, and substantial repair bills charged back to you.
• Sewers and drainage: striking a foul sewer creates a biological hazard, contamination and confined-space risks as well as expensive damage.

You have a legal duty to plan and risk-assess this work. Under the Construction (Design and Management) Regulations 2015 (CDM 2015) and the Health and Safety at Work etc. Act 1974, anyone carrying out excavation must identify the hazards, plan the work to avoid them, and make sure the people doing it are competent. Treating every buried service as live and dangerous until proven otherwise is the baseline standard HSG47 expects.

The HSG47 Safe Digging Hierarchy: Plan, Locate, Dig

HSG47 reduces safe excavation to three stages that must always be followed in order. Skipping or rushing any stage is how strikes happen. Use this as the backbone of every excavation method statement.

Stage 1: Getting Utility Plans

The plan stage starts in the office, not on site. Before mobilising, request asset records from each relevant network operator: the local Distribution Network Operator (DNO) for electricity, the gas distribution network, the water and sewerage company, and the telecoms providers (Openreach and others). A single service such as LinesearchbeforeUdig (LSBUD) lets you submit one enquiry that is forwarded to many registered asset owners, but it does not cover every operator — you still need to identify and contact any that are not members.

Treat the plans as a guide, not gospel. Records are frequently out of date, drawn to the wrong scale, or simply wrong about depth and exact alignment. Diverted, abandoned and unrecorded services are common, especially on older sites. The plans tell you what is likely to be there and roughly where — they do not remove the need to locate and prove every service on the ground.

Stage 2: Using a CAT and Genny

A cable avoidance tool (CAT) detects signals radiating from buried services; a signal generator (Genny) applies a traceable signal to a specific cable or pipe so the CAT can follow it. Used together and competently, they are the core of locating. But they have real limitations you must understand, or they will give you false confidence:

• Plastic pipes carry no signal. A CAT and Genny will not find plastic (PE) gas or water pipes unless a traceable tracer wire is fitted and connected. Many are not — so the absence of a reading does not mean the ground is clear.
• Depth is not reliable. Depth indication on a CAT is an estimate and should never be trusted to set a safe dig depth. Always prove depth by hand-digging.
• Signals distort. Adjacent services, reinforcing mesh and signal bleed between cables can shift or merge readings.
• It must be calibrated and in date. A CAT must be checked daily before use and calibrated regularly (typically every 12 months). Record the check. A faulty or uncalibrated CAT is worse than none because it is trusted.

Scan in all modes (power, radio and generator/signal), survey the whole excavation footprint not just the centreline, and scan from several directions. Then mark out every detected service on the surface with spray paint, using a consistent colour code, so the gang can see what they are digging around before they start.

What a Permit to Dig Is and When You Need One

A permit to dig is a formal control document, issued by the principal contractor or a competent authorising person, that confirms the locating work has been done and the excavation is authorised to proceed. It records the utility plans reviewed, the CAT and Genny survey results, the marked-out services, any trial holes, and the safe digging conditions that apply. No one breaks ground until the permit is signed off.

On most managed construction sites under CDM 2015 the principal contractor will require a permit to dig for any excavation, and many utility and highways clients mandate one as standard. Even on smaller jobs where no client demands it, a written permit to dig is good practice: it forces the plan and locate stages to actually happen, gives you a record that they did, and makes responsibility clear. If you are the principal contractor, putting a permit to dig system in place is one of the simplest ways to control excavation risk across your gangs.

Stage 3: Safe Digging Near Electricity and Gas

Once services are located and marked, excavate by hand to expose them. HSG47 is explicit that mechanical excavators and power tools must not be used close to a known or suspected service. Keep a safe distance — typically excavate by hand within around 500mm of the marked position, and follow any greater distance specified by the asset owner or your permit. Specific safe digging practices:

• Hand-dig trial holes to physically prove the position and depth of each service before machine digging anywhere near it. Never rely on the CAT depth reading alone.
• Dig alongside a service, not directly down onto it. Excavate parallel and work in towards it horizontally so a tool cannot be driven into the top of a cable or pipe.
• Use insulated tools and avoid spades, forks and picks that can pierce a cable — use a blunt, hand tool such as a shovel worked at a shallow angle near services.
• No power tools or breakers within the safe distance of electricity or gas. Hand dig only.
• Treat every exposed service as live. Never assume a cable is dead because it looks old or disused.
• Support and protect exposed services properly, and shore the excavation in line with your trench support plan so a collapse cannot drag the gang onto a service.

Re-scan with the CAT as the dig progresses. Newly exposed faces can reveal services the original survey missed, and the picture changes as you go deeper.

If You Expose or Strike a Service: Emergency Action

If a service is damaged, struck, or exposed in a way that could be dangerous, the priority is people first, then containment. Do not try to fix it yourself.

• Stop work immediately and keep everyone away from the excavation and any machine in contact with the service.
• Evacuate the area. For electricity, do not touch the plant or the casualty if contact may still be live. For gas, eliminate ignition sources, do not operate switches, and get clear of the gas — it can travel and accumulate.
• Call the emergency line. For a struck or damaged electricity cable, call 105 (the national power cut and emergency number) or the DNO's emergency line. For a gas escape, call the National Gas Emergency Service on 0800 111 999. For water, call the water company's emergency line.
• Call 999 if anyone is injured, there is a fire, or there is an immediate threat to life.
• Report and record the incident. Damage to gas and electricity apparatus, and many service strikes, are reportable and may fall under RIDDOR. Keep your survey records, permit and photos.

Recording, Competence and Training

Safe digging only works if the people doing it are trained and the work is documented. Keep records of your utility plan enquiries, CAT calibration and daily checks, the locate survey, the permit to dig and the trial holes. That paper trail proves you planned and risk-assessed the work as CDM 2015 and the Health and Safety at Work Act require, and protects you if a service that was never recorded is later struck despite a proper survey.

Operatives using locating equipment should be properly trained — a recognised CAT & Genny course, and where relevant the EUSR (Energy & Utility Skills Register) safe digging or SHEA registration that many utility and highways clients require. Competence is not a one-off: refresh training, keep equipment in calibration, and brief every gang on the located services before they break ground. The cost of doing this properly is trivial next to the cost of a strike.

Quick Reference: Safe Digging Essentials