Main Protective Bonding Explained: Gas, Water, Earthing and Why It Matters

Main protective bonding is one of the most misunderstood parts of a UK electrical installation. It is often confused with earthing, hidden near the gas meter or stop tap, and only noticed when an electrician refuses to change a consumer unit until the bonding is corrected.

The purpose is simple: bonding reduces dangerous voltage differences between metal services and the electrical installation during a fault. If incoming metal water, gas or structural metalwork can introduce an outside earth potential into the building, it may need to be connected to the Main Earth Terminal (MET) with a correctly sized bonding conductor.

This guide explains what main protective bonding does, how it differs from earthing, what electricians look for during an EICR or consumer unit upgrade, and why missing bonding is treated seriously.

Earthing and Bonding Are Not the Same

Earthing and bonding work together, but they do different jobs.

Earthing provides a fault-current path back to the supply so protective devices can disconnect the circuit. If a live conductor touches a metal appliance casing, the earth path allows enough fault current to flow for an MCB, fuse or RCBO to operate.

Bonding connects conductive parts together so they stay at a similar voltage. The aim is not to carry normal load current and not to “make a pipe safe” in isolation. The aim is to reduce touch voltage between things a person could contact at the same time.

Related: Types of Earthing Systems Explained: TN-S, TN-C-S (PME) and TT

Think of it this way:

• Earthing helps a fault disconnect quickly.
• Bonding helps prevent dangerous voltage differences inside the building.
• RCDs reduce shock risk, but they do not replace the need for correct earthing and bonding.

What Main Protective Bonding Connects

Main protective bonding connects extraneous conductive parts to the MET.

An extraneous conductive part is a conductive part that is not part of the electrical installation but can introduce a potential, usually earth potential, into the building. Common examples include:

• Metallic incoming water service pipes
• Metallic incoming gas service pipes
• Metallic oil supply pipes
• Structural steelwork, where it can introduce earth potential
• Other metallic services that enter the building from outside

Not every visible metal pipe automatically needs main bonding. The key question is whether it is an extraneous conductive part. Plastic incoming services, isolated short metal sections, or metal pipework that cannot introduce earth potential may not require main protective bonding, but that decision should be made by testing and inspection rather than guesswork.

The Main Earth Terminal (MET)

The Main Earth Terminal is the central point where the installation’s earthing and bonding conductors connect. It may be inside the consumer unit, next to the supply intake, or provided as a separate earth block.

Typical connections at or near the MET include:

• The main earthing conductor from the supply earth or earth electrode
• Circuit protective conductors from the installation
• Main protective bonding conductors for gas, water and other services
• Earthing connections associated with surge protection or other equipment

When electricians inspect an installation, they check that the MET is accessible, secure, correctly labelled where needed, and that bonding conductors are continuous back to it.

Why Bonding Matters During a Fault

Imagine a fault raises the voltage of exposed metalwork connected to the electrical earth. At the same time, a metallic water pipe entering the property may be connected to true earth outside the building.

If the electrical earth and the pipework are at different voltages, a person touching both could become the path between them. In a bathroom, kitchen, utility space or near a boiler, this is a realistic touch-risk scenario.

Main protective bonding reduces that voltage difference by connecting the incoming service pipework to the installation’s earthing system. During a fault, the metalwork rises and falls together more closely, reducing the voltage across a person who might touch two conductive parts at once.

This is why bonding is especially important in properties with:

• Older metallic incoming services
• PME/TN-C-S supplies
• Boiler pipework and metallic heating systems
• Consumer unit upgrades
• EICR remedial work
• Alterations where services have been replaced in mixed plastic and copper

Gas and Water Bonding: What Electricians Look For

For a typical UK home, the main bonding connection to a gas or water service is usually made near the point where the service enters the building, before branch pipework splits away.

An electrician will normally check:

• Whether the incoming service is metallic or plastic
• Whether the service is actually extraneous conductive
• Whether the clamp is suitable for the pipe material
• Whether the bonding conductor is correctly sized
• Whether the connection is mechanically sound and not loose or corroded
• Whether the conductor runs continuously back to the MET
• Whether the connection is accessible for inspection
• Whether warning labels are present where required

A common visual clue is a green-and-yellow conductor connected by a BS 951-style clamp near the gas meter, water stop tap, or incoming pipe. Visual checks are useful, but they do not prove continuity or correct sizing on their own.

Typical Bonding Conductor Sizes

In many domestic UK installations, main protective bonding to incoming gas and water services is commonly 10 mm2 copper, especially on TN-C-S/PME supplies.

However, this is not a universal rule for every property. The required size depends on the earthing arrangement, supply characteristics, conductor material and BS 7671 sizing rules. In some cases a different size may be required, and the electricity distributor may have additional requirements for PME supplies.

As a practical guide:

• Do not assume old 6 mm2 bonding is automatically acceptable on a modern PME installation.
• Do not upgrade bonding by guesswork without confirming the earthing arrangement.
• Do not rely on pipework itself as an earthing conductor.
• Do not disconnect bonding clamps during plumbing work and leave them off.

If a consumer unit is being replaced, bonding size and continuity are normally checked before the new board is energised.

Main Bonding vs Supplementary Bonding

Main protective bonding and supplementary bonding are related, but they are not the same.

Main protective bonding is installed near the origin of the installation and connects incoming extraneous conductive parts, such as gas and water services, to the MET.

Supplementary bonding is local bonding within a particular location, historically common in bathrooms, where simultaneously touchable conductive parts are connected together.

Modern bathroom installations with correct RCD protection, automatic disconnection, and verified bonding conditions may not need supplementary bonding in the same way older installations did. But removing or ignoring supplementary bonding is not a DIY decision. The electrician must verify that the conditions for omission are met.

Related: How to Wire a Bathroom: UK Zone-by-Zone Guide

Common Bonding Problems Found on EICRs

Bonding problems are common during EICRs because the conductors are often hidden behind cupboards, under floors or near service entries that have been altered over time.

Typical findings include:

• No bonding conductor visible at the gas or water service
• Bonding connected to the wrong side of a plastic section
• Clamp fitted to painted, corroded or unsuitable pipework
• Clamp loose or damaged
• Bonding conductor too small for the installation
• Bonding conductor cut during plumbing work
• Multiple conductors joined poorly instead of run continuously or correctly jointed
• Bonding connected to pipework but not continuous to the MET

Missing or defective bonding can make an EICR unsatisfactory if it creates a safety defect. The exact code depends on the installation, earthing system and risk, but it should never be dismissed as a cosmetic issue.

Related: EICR Codes Explained: C1, C2, C3 and FI

Bonding and Consumer Unit Upgrades

Before replacing a fuse board or consumer unit, an electrician needs to confirm that the earthing and bonding are suitable. A modern RCBO board cannot compensate for missing main bonding.

This is why consumer unit quotes often include a line such as:

Upgrade main protective bonding to gas and water services if required.

That is not upselling. It is part of making sure the installation has a complete protective system before new devices are fitted.

If the bonding is missing or undersized, the electrician may need to install or upgrade bonding conductors before completing the board change. In some older properties, access can be more difficult than the board replacement itself.

Related: Consumer Unit Upgrade: What to Expect When Replacing Your Fuse Board

When Bonding May Not Be Required

Main protective bonding is required for extraneous conductive parts, not for every piece of metal in a house.

Bonding may not be required where:

• The incoming water service is plastic and does not introduce earth potential
• The incoming gas service is plastic and the internal metal pipework is not extraneous
• A short metal pipe section is isolated by plastic pipework
• The conductive part is not simultaneously accessible or does not introduce a potential

The important point is that this should be confirmed by inspection and testing. A pipe that looks plastic in one cupboard may become metallic underground, and a short visible plastic section does not automatically prove that bonding is unnecessary.

DIY Checks You Can Do Safely

You should not alter bonding conductors unless you are competent to work on electrical installations, but you can make basic observations:

• Look near the gas meter and water stop tap for green-and-yellow bonding conductors.
• Check whether clamps look loose, damaged or heavily corroded.
• Check whether a plumber has removed a clamp during pipework changes.
• Check whether the conductor disappears back toward the consumer unit or MET.
• Tell your electrician if services have been changed from metal to plastic.

Do not disconnect clamps to “clean them”, move conductors, or assume a missing clamp means bonding is not required. If in doubt, book an electrician to test it.

Quick Reference

The Bottom Line

Main protective bonding is not decorative wiring and it is not optional when the installation needs it. It is part of the shock-protection system that keeps metal services and electrical earth at a safer relative voltage during faults.

If you are planning a consumer unit upgrade, buying a property, arranging an EICR, or altering gas and water pipework, bonding should be checked before the work is treated as complete.

Use ElectraSim to explore how fault paths, earthing and protective devices interact in a circuit. For real installations, main protective bonding must be inspected, tested and installed by a competent electrician.