Hot Tapping

What is hot tapping?

Hot tapping is the process of drilling a new branch connection into a live water main without isolating or emptying the pipe. The main stays pressurised, the water keeps flowing, and end users on the network see no interruption to supply. Under pressure drilling is the same technique by its more technical UK utility name, used interchangeably across water company specifications and contractor documentation.

Hot tapping exists because shutting a water distribution main down is rarely easy or cheap. A mains shutdown means planning around end users, arranging temporary supply where needed, draining and refilling the pipe, and rechlorinating the network before the main returns to service. Every shutdown also introduces transient pressures during depressurisation and repressurisation, which are a documented cause of secondary leaks and bursts on older networks. Hot tapping avoids all of that by doing the work with the main still live.

How hot tapping works

Hot tapping follows a three-stage sequence: fit a branch saddle and valve onto the live pipe, drill through the pipe wall under pressure while the valve is open, then close the valve to seal the new connection. The technique depends on two safety principles that apply throughout. A sealed chamber around the work area keeps water inside the pipe, and a full-bore isolation valve between the tapping machine and the main keeps the connection controlled after the drill is removed.

Fitting the branch saddle and pressure test

The branch saddle is the fitting that clamps or bolts onto the outside of the live main. Saddle selection depends on the pipe material, the pipe diameter, and the required branch size. On UK distribution mains the default approach is a mechanical saddle to Water UK WIS 4-32-11 specification, which avoids hot works on the live pipe entirely. Welded branches are used where pipe material, operating pressure, or water company specification call for them.

Once the saddle is fitted, a pressure test runs on the saddle and valve assembly before any drilling begins. The test confirms the seal is watertight and the assembly can safely withstand the main’s operating pressure. This step is not optional. HSE guidance under the Pressure Systems Safety Regulations 2000 treats it as a requirement for live-pipework intervention.

Drilling through the pipe wall under pressure

The tapping machine fits to the top of the isolation valve. A pilot drill centres the cutter over the intended bore, then the cutter advances through the open valve and into the pipe wall. Throughout drilling, the main stays pressurised and water continues to flow. The cut section of pipe wall, called the coupon, is captured inside the cutter head so it cannot fall into the main. No water escapes from the pipe because the valve, the saddle, and the tapping machine form a single sealed chamber around the work. Drilling rate, cutter condition, and valve seal integrity all matter at this stage, which is why UK water companies require the operators doing the work to hold EUSR competence registration.

Closing the valve and completing the connection

Once the bore is through the pipe wall, the cutter retracts back through the valve with the coupon still captured inside it. The isolation valve closes, which seals the new branch off from the live main. The tapping machine is removed. The valve stays in place as a permanent part of the new connection, with the branch pipework, fitting, or asset installed above it. The main has stayed live throughout, and no supply has been lost.

When hot tapping is used on UK water networks

Hot tapping on UK water networks covers five common use cases:

• Service connections. New service connections onto distribution mains for developers, new-build estates, and commercial premises, where taking the main down is not practical.
• Air valves, washouts, and hydrants. Network assets installed onto live mains for operational control or for emergency supply access, typically as part of Water UK undertaker maintenance programmes.
• Flow meters, PRVs, and pressure monitoring points. Instrumentation added to active distribution networks, including Tier 2 subcontract work for Tier 1 civils contractors delivering network upgrade schemes.
• Probe insertion and sampling. Temporary taps for water quality sampling, leakage detection probes, or other monitoring work that needs network access without a shutdown.
• Emergency or unplanned modifications. Situations where a shutdown is not an option because of end user impact, critical supply routing, or the time pressure of an emerging network issue.

UK standards and the EUSR competence framework

Hot tapping on UK water mains sits inside a specific regulatory and competence framework. Four documents matter most.

The Pressure Systems Safety Regulations 2000 and their accompanying ACOP L122 set the HSE framework for any work on a pressure system at work. Water distribution mains above 0.5 bar gauge fall within scope. Pressure test records and written schemes of examination apply.

WIS 4-32-11, published through the Water UK Standards Board’s WIS and IGN library, specifies mechanical and compression fittings for polyethylene water supply pipes. The specification governs the branch saddle and fitting selection used on modern PE and MDPE distribution networks. Alongside WIS 4-32-11, the Civil Engineering Specification for the Water Industry (CESWI 8th Edition, 2023) is the contract standard adopted across UK water undertakers for civil engineering works under AMP8.

Operator competence is covered by the EUSR Safe Control of Mains Connections (Water) scheme, through the Mains Connection Drilling and Tapping (Utilities Water) programme. The programme covers the 1/2″, 3/4″, and 1″ connection envelope at typical distribution pressures up to 12 bar. Water UK undertakers expect operatives making connections to live mains to hold a current EUSR registration under this or an equivalent scheme.

Hot tapping, line stopping, and pipe freezing compared

Three techniques allow work to be done on live pipework without emptying it. Each has a specific role, and confusing them is a common cause of specification errors at procurement stage.

Hot tapping and line stopping are often used together on the same job. A hot tap creates the access point for a line stopping head that then isolates a section for repair. Pipe freezing sits in a different scope and is rarely used on utility-scale distribution mains. Full treatment of line stopping lives on the line stopping capability page.