Risk Assessments · Mandatory · Pre-Construction

RAMS — Utility Diversions

This RAMS covers the diversion, disconnection, and reconnection of utility services including gas, electricity, water, and telecommunications. It addresses risks from live services, unexpected energisation, gas leaks, and flooding. Utility company coordination, permit-to-dig procedures, and service location using CAT and Genny equipment are essential requirements.

Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. MHSWR 1999 remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.

Legal basis	HSG47 Avoiding Danger from Underground Services + New Roads and Street Works Act 1991 (NRSWA) S.50 + CDM 2015
Survey standard	PAS 128:2022 Specification for Underground Utility Detection, Verification and Location
NRSWA S.50	Licence required from the street authority before installing apparatus in a street (utility diversions in highway)
SROH	Street Record of Highway — formal record of all apparatus in the highway maintained by the street authority
Safe digging	Hand dig within 500mm of known services; no mechanical excavation within this zone without specific controls
Guide 25 of 25	This is the final guide in the Construction Suite RAMS series
Enforcing authority	Health and Safety Executive (HSE) and street authority (NRSWA)

1. What It Covers

Utility diversion RAMS address the hazards associated with locating, exposing, protecting, and diverting underground and overhead services on construction sites. Striking an underground service during excavation is one of the most common causes of serious injury and death on construction sites. Gas mains can explode, live electrical cables can electrocute, and high-pressure water mains can flood excavations. The consequences of getting it wrong are immediate and often fatal.

HSG47 is the HSE’s primary guidance document on avoiding danger from underground services. PAS 128:2022 provides the specification for underground utility detection, verification and location — replacing earlier ad-hoc survey approaches with a structured four-level system (Types D, C, B and A). Where utility diversions involve apparatus in the public highway, NRSWA 1991 S.50 requires a licence from the street authority, and all works must be coordinated through the Street Record of Highway (SROH).

A CAT scan alone is insufficient for detecting plastic services

Cable Avoidance Tools (CATs) work by detecting electromagnetic signals from metallic cables and pipes. They cannot detect plastic gas pipes, plastic water mains, or fibre-optic telecommunications ducts — all of which are now standard materials for new installations. A CAT scan alone provides a false sense of security. HSG47 requires that CAT scans are supplemented by a Genny (signal generator) to trace metallic services, and by reference to statutory utility records. For plastic services, there is no reliable above-ground detection method — only record searches combined with careful hand excavation can confirm their presence and location. PAS 128 survey Types B and A provide the highest confidence by using GPR and physical verification.

“Underground services should be located...by a combination of plans, cable and pipe locating devices and, where necessary, by hand digging trial holes.”— HSG47 (Fourth edition)

2. PAS 128:2022 Survey Types

PAS 128:2022 defines four survey types, each providing an increasing level of confidence in the location and identification of underground utilities. The survey types are cumulative — each builds on the information gathered at the previous level. The table below sets out the four types, what each involves, and the level of confidence provided.

Survey type	Name	What it involves	Confidence level
Type D	Desktop utility records search	Collation of all available utility record drawings from statutory undertakers, LSBUD searches, and any existing survey data. No site work.	Low — utility records are often inaccurate, incomplete, or out of date. Plans show approximate routes only and do not show depth.
Type C	Site reconnaissance	Visual inspection of the site to identify surface features indicating underground services: manholes, valve covers, marker posts, cable route markers, overhead lines. Combined with Type D records.	Low to moderate — confirms presence of some utilities and identifies surface evidence not shown on plans, but does not locate buried services.
Type B	Detection survey	Geophysical survey using ground-penetrating radar (GPR), electromagnetic location (EML), and signal injection to detect and trace underground utilities. Results mapped to a topographical survey.	Moderate to high — identifies metallic and some non-metallic services, estimates depth and horizontal position. Limitations in congested areas and certain ground conditions.
Type A	Verification survey	Physical exposure of underground services by hand-digging or vacuum excavation at key points to verify exact position, depth, size, material and condition. Confirms or corrects Type B survey data.	High — provides confirmed, measured positions at verification points. The only survey type that gives definitive location data.

Most utility diversion projects require a minimum of Type B survey

Type D and C surveys alone do not provide sufficient confidence to plan a utility diversion safely. Utility record drawings are notoriously inaccurate — services may be several metres from their drawn position, and unrecorded services are common. A Type B detection survey using GPR and EML should be carried out as a minimum for any project involving excavation near known or suspected services. Type A verification should be used at critical crossings, connection points, and wherever the consequences of a service strike are highest.

3. Service Types

Different service types carry different risks when struck. The following table identifies the principal service types encountered during utility diversion works, the statutory undertaker responsible, and the specific hazards and controls for each.

Service type	Typical operator	Hazards if struck	Key controls
Gas (low pressure)	Gas distribution network (e.g. Cadent, SGN, NGN, WWU)	Fire, explosion, asphyxiation. LP mains are typically PE plastic — invisible to CAT scans. Gas accumulates in excavations and confined spaces.	Record search, hand dig within 500mm, gas monitoring in excavations, no mechanical excavation near PE mains, emergency isolation procedures agreed with operator
Gas (high pressure / intermediate)	National Gas Transmission (formerly National Grid Gas) or regional transporter	Catastrophic explosion, fireball, fatality zone extends well beyond the excavation. HP steel mains operate at pressures up to 70 bar.	Mandatory Type A verification before any excavation near HP mains, standoff distances agreed with operator, on-site supervision by gas transporter representative, emergency shutdown plan
Electricity	Distribution Network Operator (DNO) (e.g. UKPN, NGED, ENWL, SSEN, NPG)	Electrocution, arc flash, burns, cardiac arrest. HV cables (>1kV) can be fatal on contact. LV cables cause severe burns and shock.	Cable plans from DNO, CAT and Genny scan, hand dig within 500mm, insulated tools, HV cable crossings physically exposed and protected, permit to work for HV proximity
Water	Water company (e.g. Thames Water, Severn Trent, United Utilities)	Flooding of excavation, drowning, ground collapse, contamination of supply. Trunk mains under significant pressure.	Record search, Type B survey, hand dig exposure, valve location and isolation plan, dewatering equipment available, emergency isolation agreed with water company
Sewer / foul drainage	Water company (sewerage undertaker)	Contamination, biological hazards, collapse of sewer causing ground subsidence, hydrogen sulphide gas in combined sewers	CCTV survey of sewer condition before works, gas monitoring, biological hazard assessment, structural support for sewer during adjacent excavation, permit to build over/near
Telecommunications	Openreach, Virgin Media, CityFibre, network operators	Loss of critical communications (including emergency services 999 infrastructure), fibre optic cables are invisible to CAT, duct damage	Record search (including LSBUD), physical identification of duct routes, hand dig near known telecoms routes, fibre optic cables marked and protected, no mechanical excavation near ducts
District heating	District heating network operator (e.g. Vattenfall, E.ON)	Scalding from pressurised hot water or steam (up to 120 degrees C), pipe insulation may contain legacy asbestos in older installations	Network operator consulted, isolation and drain-down before exposure, asbestos survey on older pipe insulation, burn risk assessment, emergency shutdown procedure

4. Common Mistakes

Relying on a CAT scan alone without record searches

A CAT scan detects electromagnetic signals from metallic services. It cannot detect plastic gas mains, plastic water pipes, or fibre-optic ducts. HSG47 requires a combination of record drawings, CAT and Genny scanning, and trial holes. Using a CAT alone provides a false sense of security and misses an entire category of underground services. The RAMS must specify the full service location methodology — not just ‘CAT scan before excavation.’

No NRSWA S.50 licence for works in the highway

Any installation of apparatus in a street requires a S.50 licence from the street authority under NRSWA 1991. Starting utility diversion works in the public highway without a S.50 licence is a criminal offence. The licence application process takes time — typically 3 months — and must be programmed into the project timeline. Failure to obtain the licence also means the works cannot be registered on the Street Record of Highway.

Mechanical excavation within 500mm of known services

HSG47 is clear: within 500mm of a known service, excavation must be by hand using hand tools or vacuum excavation. Mechanical excavation within this zone — even careful use of a toothed bucket — is the most common direct cause of service strikes. The RAMS must define the hand-dig zone, the method of marking out service positions on the ground, and the supervision arrangements for excavation near services.

Not consulting the statutory undertaker before diversion works

Utility diversions involve connecting to, diverting, or protecting apparatus belonging to the statutory undertaker. The undertaker must be consulted and will normally require that their own approved contractor carries out the connection and disconnection. Starting diversion works without the undertaker’s involvement risks damaging live services, invalidating warranties, and creating liabilities that the contractor cannot insure against.

Assuming utility record drawings are accurate

Statutory utility records show approximate routes and are frequently inaccurate. Services may be several metres from their drawn position, at a different depth, or on a different alignment. Unrecorded services — including private connections, abandoned mains, and legacy infrastructure — are common. The RAMS must treat record drawings as indicative only and require physical detection (PAS 128 Type B minimum) and verification (Type A at critical points) before excavation.

5. Frequently Asked Questions

Is a PAS 128 survey legally required before excavation?▾

PAS 128 is a BSI specification, not legislation. However, HSG47 requires that underground services are located by a combination of plans, locating devices, and where necessary trial holes. PAS 128 provides the structured framework for delivering this. While not strictly mandatory, a PAS 128 survey is increasingly accepted as the industry standard, and failure to carry out a detection survey to at least PAS 128 Type B standard would be difficult to defend in any investigation or prosecution following a service strike.

Can I use GPR to detect all underground services?▾

No. Ground-penetrating radar is effective at detecting metallic and some non-metallic services, but its performance depends on ground conditions, service depth, and service material. In clay soils, GPR penetration is reduced. Very deep services or services in congested utility corridors may not be resolved clearly. Plastic services without a metallic tracer wire may not be detected. GPR should be used as part of a multi-technique approach (GPR + EML + signal injection) rather than as a standalone method.

What is the 500mm hand-dig zone?▾

HSG47 states that when excavating near underground services, hand digging should be used when within 500mm of the indicated position of a buried service. This means no mechanical excavation within this zone — only hand tools, spades (not picks or forks), or vacuum excavation equipment. The 500mm zone is measured from the indicated position of the service, not its confirmed position. Given that record drawings can be several metres out, this reinforces the need for detection surveys to narrow down the actual position before excavation.

Who is responsible for the cost of utility diversions?▾

This depends on the circumstances. Where a developer requires services to be diverted to enable a development, the developer typically bears the cost under the terms of the C3 or C4 estimate provided by the statutory undertaker. Where utility diversions are required for highway works promoted by a highway authority, the cost is shared according to the provisions of NRSWA and any relevant cost-sharing agreements. The RAMS does not address cost allocation directly, but the programme must allow for the statutory undertaker’s lead times, which can be 3 to 12 months depending on the service type and complexity.

Guide 25 of 25 — Construction Suite RAMS series complete

This is the final guide in the Construction Suite RAMS series. The 25 guides cover the full scope of RAMS topics commonly required on UK construction projects, from the Construction Phase Plan and F10 notification through to specialist activities including demolition, confined spaces, hot works, working at height, groundworks, plant and machinery, temporary works, night works, refurbishment and strip-out, and utility diversions. Each guide is designed to be used alongside a project-specific risk assessment and method statement — providing the legal framework, practical controls, and common mistakes that should inform the content of every RAMS document.

Generate your RAMS — Utility Diversions on Construction Suite

Construction Suite walks you through every required section with a guided Q&A — built to MHSWR 1999 — and generates a professionally formatted document in minutes.

Get started free