Electrical

Main Protective Bonding Explained

Main protective bonding ties the incoming metallic services — gas, water, oil, structural steel — to the main earthing terminal so that under a fault everything rises to the same potential together, and its conductor size steps up on PME supplies.

Updated 20 Aug 20268 min read

Main protective bonding is one of the quiet fundamentals of a safe installation. It connects the extraneous-conductive-parts entering a building — the metal gas and water pipes, oil lines, structural steelwork — back to the main earthing terminal, so that if a fault raises the potential of the earthing system, every bonded part rises with it rather than leaving a dangerous voltage between, say, a tap and a radiator.

This guide explains what main bonding does and why, what must be bonded, how the conductors are sized (including the larger sizes required on PME supplies), and how it differs from supplementary bonding.

Key takeaways

Main bonding connects incoming metallic services to the main earthing terminal so bonded parts share the same potential under fault.
Bond extraneous-conductive-parts: metallic gas, water and oil service pipes, and structural metalwork that could introduce a potential.
Conductor size is set by BS 7671 Section 544 / Table 54.8 relative to the earthing conductor — commonly 10mm² minimum.
PME (TN-C-S) supplies require larger main bonding — 25mm² or more depending on the supply neutral size.
Main bonding is equipotential bonding across the installation; supplementary bonding is local, in specific locations like bathrooms.

What main bonding does

Main protective bonding creates an equipotential zone. By connecting the incoming metallic services to the main earthing terminal, it ensures that under an earth fault — when the whole earthing system may be lifted above true earth potential — the bonded metalwork rises to the same voltage at the same time. Someone touching two bonded parts at once feels no dangerous difference in potential between them, because there isn't one.

Without it, a fault could put the earthing system at a raised potential while a metal water pipe sat near true earth, leaving a shock voltage between them. Bonding removes that difference. It's a fault-protection measure that works alongside automatic disconnection, not instead of it.

What must be bonded

You bond extraneous-conductive-parts: conductive parts that are liable to introduce a potential (generally earth potential) into the installation from outside it. The classic ones are the incoming metallic gas service pipe, the incoming metallic water service pipe, and any oil supply pipe — bonded on the consumer's side, close to the point of entry, before any branch and within the distance BS 7671 specifies of the meter or point of entry.

Structural steelwork and other extraneous metalwork that could introduce a potential are bonded too. What you don't bond is a part that isn't extraneous — a plastic-fed pipe with only internal metal, for example, doesn't introduce an external potential and isn't an extraneous-conductive-part. Testing whether a part is genuinely extraneous (rather than bonding everything metal in sight) is part of getting this right.

Extraneous, not just metal

Bond parts that introduce an outside potential — the incoming gas and water services. A metal pipe fed entirely in plastic isn't extraneous and doesn't need main bonding.

Sizing the conductors

Main bonding conductor size is set out in BS 7671 Section 544, related to the size of the installation's earthing conductor. The general rule gives a minimum of half the earthing conductor's cross-sectional area, subject to a floor — commonly 10mm² for the great majority of domestic installations, and not less than 6mm² in any case. So for most houses on a standard supply, 10mm² main bonding to gas and water is the familiar answer.

The point to hold onto is that it's a rule tied to conductor sizes, not a single fixed number. Read the earthing conductor size, apply the Section 544 relationship, and take the result subject to the minimum. Don't guess a figure — size it against the standard for the actual installation.

PME supplies need bigger bonding

On a PME (TN-C-S) supply the main bonding requirements step up, and this is where the numbers change. Because a broken PEN conductor in the network could impose the supply neutral current onto the bonded metalwork, BS 7671 Table 54.8 sets minimum main bonding sizes according to the size of the supply neutral conductor — the bonding has to be robust enough to carry that current safely.

The practical upshot is that PME installations commonly require 25mm² main bonding rather than 10mm², and larger still where the supply neutral is big. Use Table 54.8 to read the required size against the copper-equivalent cross-sectional area of the supply neutral — describe it from the table rather than assuming 25mm² covers every case, because larger supplies demand more.

Read Table 54.8 for PME

PME main bonding is sized against the supply neutral — often 25mm², sometimes larger. Don't carry a 10mm² habit onto a PME supply; look it up in Table 54.8.

Main vs supplementary bonding

Main bonding and supplementary bonding are different jobs. Main bonding is the installation-wide equipotential connection of incoming services to the main earthing terminal, done once at the origin. Supplementary bonding is local: it connects exposed- and extraneous-conductive-parts together within a specific location — historically every bathroom — to control potential differences right where a person is most vulnerable.

Under the 18th Edition, supplementary bonding in a bathroom can be omitted where specific conditions are met: all circuits have RCD protection, disconnection times are satisfied, and the main bonding is in place and verified. Main bonding, by contrast, is not optional — it's the foundation the whole equipotential concept rests on, and its absence or undersizing is a common EICR C2.

Frequently asked questions

What is main protective bonding?

It's the connection of incoming metallic services — gas, water, oil, structural steel — to the main earthing terminal, so that under a fault all the bonded metalwork rises to the same potential together and there's no dangerous voltage between parts a person could touch at once.

What size should main bonding be?

It's sized per BS 7671 Section 544 relative to the earthing conductor — commonly 10mm² minimum on a standard domestic supply, and never less than 6mm². On PME supplies it's larger, typically 25mm² or more, read from Table 54.8 against the supply neutral size.

Why is PME bonding bigger?

Because a broken PEN conductor on a PME network could impose neutral current onto the bonded metalwork. The bonding must be robust enough to carry that safely, so Table 54.8 sets larger minimum sizes based on the supply neutral conductor.

Do I still need supplementary bonding in a bathroom?

Under the 18th Edition it can be omitted where all circuits serving the location have RCD protection, disconnection times are met, and the main bonding is present and verified. Main bonding itself is never optional — those conditions rely on it being in place.

From guidance to action

EICR certificate appRecord bonding observations and code missing or undersized bonding.