Main protective bonding is the conductor, at least 6 mm² copper and at least 10 mm² copper on a PME supply, that connects the main earthing terminal (MET) to every extraneous-conductive-part entering the building: metal gas, water and oil service pipes, structural steel and a lightning protection system. Its job is to hold all of that metalwork at the same potential as the electrical earth during a fault, so there is no dangerous voltage between a tap and a radiator. It is required by Regulation 411.3.1.2 of BS 7671:2018+A4:2026, sized under Regulation 544.1.1 and Table 54.8, checked on every EIC, Minor Works Certificate and EICR, and coded C2 when it is missing.
- Bonding equalises potential; it is not there to carry fault current back to the supply. That is the earthing conductor's job.
- Bond metal gas, water, oil and heating pipes, structural steel and lightning protection where they enter the building. Plastic incomers, and metal incomers with an insulating section at the point of entry, are not extraneous-conductive-parts.
- Minimum 6 mm² copper on TN-S and TT. Minimum 10 mm² copper on TN-C-S (PME) with a supply neutral up to 35 mm², rising with the neutral under Table 54.8.
- Connect within 600 mm of the meter outlet, on the consumer's side, before any branch, with a BS 951 clamp and the "Safety Electrical Connection – Do Not Remove" label.
- Test with a low-resistance ohmmeter between the MET and the clamp. IET Guidance Note 3 treats 0.05 Ω or less as a sound conductor.
- Missing main bonding is C2 under Electrical Safety First's Best Practice Guide 4. Present, sound but undersized bonding is generally C3.
- Amendment 4:2026 made no change to Regulation 544 or Table 54.8. It added Section 545 (functional bonding, identified pink) and protective neutral bonding (PNB) at 312.2.1.1.
What does main protective bonding actually do?
Automatic disconnection of supply under Chapter 41 assumes that, in the fraction of a second before the protective device opens, a person cannot be touching two things at different voltages. All the exposed-conductive-parts in the installation (the boiler casing, the cooker, the metal light switch) sit at the supply earth potential through their circuit protective conductors. A metal water pipe coming up out of the ground sits at the potential of the ground outside. Under a fault those two potentials are not the same.
Main protective bonding ties the pipe to the MET. If a line-to-earth fault lifts the installation earth to 100 V for the 0.4 s it takes the device to trip, the bonded gas pipe rises to 100 V with it and the voltage between the tap and the radiator stays at zero. On a PME supply the stakes are higher: a broken network neutral can lift the whole installation earth well above true earth for as long as the fault lasts, and the bonding conductor is then carrying real current to the buried pipework, which is why Table 54.8 asks for a heavier conductor.
That is why bonding is sized differently from the earthing conductor. The earthing conductor has to carry fault current back to the source and is sized adiabatically or from Table 54.7. The bonding conductor is sized for the equalising current it might see, which is why Regulation 543.1.1 excludes protective bonding conductors from the general CPC sizing rules.
Which services must be bonded?
Regulation 411.3.1.2 lists the extraneous-conductive-parts that must be connected to the MET: water installation pipes, gas installation pipes, other installation pipework and ducting, central heating and air-conditioning systems, exposed metallic structural parts of the building and, with the consent of the owner, the metallic sheath of a telecommunication cable. A lightning protection system is bonded in line with BS EN 62305. The 18th Edition added the sentence that metallic pipes entering the building with an insulating section at their point of entry need not be connected.
| Service entering the building | Bond required? | Why |
|---|---|---|
| Metal gas pipe | Yes | Extraneous-conductive-part, 411.3.1.2 |
| Metal water pipe | Yes | Extraneous-conductive-part, 411.3.1.2 |
| Metal oil supply pipe | Yes | Extraneous-conductive-part, 411.3.1.2 |
| Structural steel frame | Yes | Exposed metallic structural part |
| Lightning protection system | Yes, where present | BS EN 62305 and 411.3.1.2 |
| Plastic incoming gas or water pipe | No | Cannot introduce a potential |
| Metal incomer with an insulating section at entry | No | 411.3.1.2, 18th Edition wording |
| Internal copper fed from a plastic incomer | Usually no | Test to confirm (GN8) |
| Kitchen sink on plastic plumbing | No | Not extraneous |
Where you cannot tell whether internal metalwork is extraneous, IET Guidance Note 8 gives the test: with the bonding disconnected, measure the resistance between the part and the MET with an insulation resistance tester. A reading of about 22 kΩ or more means the part cannot pass enough current to be dangerous (230 V across 22 kΩ plus body resistance stays under 10 mA) and it does not need bonding. Below that, bond it. Record the reading in your notes; it is the sort of thing a client's next inspector will ask about.
Gas engineers have their own obligation here. Gas Safe Register guidance requires an engineer working on a gas installation to check that main bonding is present and to advise the responsible person in writing where it is not, so a missing gas bond is often first flagged by a plumber, not an electrician.
How big must the bonding conductor be?
Regulation 544.1.1 gives two rules, depending on the supply.
| Supply type | Minimum main bonding conductor (copper) | Source |
|---|---|---|
| TN-S or TT | Half the earthing conductor CSA, not less than 6 mm², need not exceed 25 mm² | 544.1.1 |
| TN-C-S (PME), supply neutral up to 35 mm² | 10 mm² | Table 54.8 |
| PME, neutral over 35 up to 50 mm² | 16 mm² | Table 54.8 |
| PME, neutral over 50 up to 95 mm² | 25 mm² | Table 54.8 |
| PME, neutral over 95 up to 150 mm² | 35 mm² | Table 54.8 |
| PME, neutral over 150 mm² | 50 mm² | Table 54.8 |
The PME sizing is driven by the copper-equivalent cross-sectional area of the supply neutral, not by the tails. A typical domestic service with 25 mm² tails sits in the first band and needs 10 mm². If the intake is a larger commercial supply, ask the distribution network operator for the neutral size before you size the bonding; the DNO's figure is what goes on the certificate. Regulation 544.1.1 permits a metal other than copper if it gives equivalent conductance, but steel wire armour almost never does: GN8's ratio makes 10 mm² of copper equivalent to about 85 mm² of steel.
Amendment 4:2026 left Regulation 544.1.1 and Table 54.8 as they were. What it did add is Section 545 for functional earthing and functional-equipotential bonding of ICT equipment, with conductors identified pink and a main functional earthing terminal (MFET). Do not confuse pink functional bonding with green-and-yellow protective bonding on a report: they serve different purposes and one does not replace the other. A4 also recognised protective neutral bonding (PNB) at Regulation 312.2.1.1; where a PNB supply is connected to a public network, PME conditions can apply and the bonding is sized accordingly.
Where and how is the connection made?
Regulation 544.1.2 requires the connection to the gas, water or other service to be made as near as practicable to the point of entry, on the consumer's side of the meter or any insulating section, and before any branch pipework. Where practicable, IET guidance puts the clamp within 600 mm of the meter outlet. The reason is simple: bond after the first tee and the branch leaving before the clamp is a separate unbonded extraneous-conductive-part.
Use a BS 951 earth clamp sized for the pipe, on bright metal, and fit the "Safety Electrical Connection – Do Not Remove" label that Regulation 514.13.1 requires at the connection. The other end lands on the MET, which is usually the earth bar in the consumer unit or a separate earthing block at the intake. On a single conductor looped from one service to the next, keep the conductor unbroken at the intermediate clamps; a cut-and-reconnected loop depends on each termination.
In flats and houses in multiple occupation, bond where the service enters the building and again at each dwelling where a metal riser or branch enters that dwelling and can be shown to be extraneous. The communal intake bonding is the landlord's; the flat's bonding is recorded on the flat's certificate.
How do you test main bonding continuity?
Use the low-resistance ohmmeter function of your test instrument, with a test current of at least 200 mA and a resolution of 0.01 Ω. Null the leads first: a 10 m wander lead can add 0.3 to 0.5 Ω and will make every reading look like a failure if you forget. Clip one lead to the bonding conductor at the MET and take the other to the far end of the conductor at the clamp, or to the pipe itself just beyond the clamp if you want to prove the clamp as well.
GN3 (10th Edition, April 2026) gives 0.05 Ω or less as the guide value for a sound bonding conductor. It is guidance, not a BS 7671 limit, and a long run can legitimately exceed it: 10 mm² copper is about 1.83 mΩ per metre, so 30 m of it reads around 0.055 Ω before you add the clamp. Read a high value as a prompt to look for a corroded clamp, a loose MET termination or an undersized conductor, not as an automatic fail. Record the result on the certificate's main protective bonding line (conductor material, CSA, and whether continuity was verified) and, on an EICR, against the inspection schedule's main protective bonding item under Regulation 544.1.
For the earthing and external loop side of the intake, see What is Ze? External earth fault loop impedance and Earthing Arrangements Explained: TN-S, TN-C-S and TT. The method for R1+R2 on circuits is at Continuity Testing: R1+R2 Method, Wander Lead and Values.
What EICR code does a bonding defect get?
Electrical Safety First's Best Practice Guide 4 (Issue 7.4, August 2026) is the reference most scheme assessors expect you to follow.
| Finding | Usual code | Reasoning |
|---|---|---|
| No main bonding to a metal gas or water incomer | C2 | A fault could leave pipework at a dangerous voltage |
| Bonding disconnected, and metalwork found live | C1 | Danger present |
| 6 mm² bonding on a PME supply, sound and connected | C3 | Present but does not meet Table 54.8 |
| 4 mm² or smaller, or a visibly poor termination, on PME | C2 | Substantially undersized or unreliable |
| Clamp after the first branch | C2 for the unbonded branch | Branch is an unbonded extraneous-conductive-part |
| Label missing at the clamp | C3 | 514.13.1 |
| Bonding to a plastic incomer with no metal beyond it | No code | Not required; note it as an observation only if it misleads |
| Bonding absent, but the incomer is plastic | No code | Not an extraneous-conductive-part |
The 6 mm² case is the one that generates arguments. BPG4 treats present, sound, connected but undersized main bonding as C3 in a typical domestic installation, because the conductor is still doing most of its job. Move it to C2 if the conductor is far smaller than the table requires, if the clamp is corroded or loose, or if the installation has other earthing defects that make a bonding failure more likely to matter. Whatever you decide, write the observation in a way the landlord can act on: "Main protective bonding conductor to water service is 6 mm² on a PME supply; Regulation 544.1.1 and Table 54.8 require 10 mm². C3." The wording guide at How to Write EICR Observations That Stand Up in 2026 has more examples, and the full code definitions are at EICR Observation Codes Explained: C1, C2, C3 and FI.
What about additions and alterations?
Regulation 132.16 says that an addition or alteration must not impair the safety of the existing installation, and that the earthing and bonding arrangements must be adequate for the new work. The EIC and the Minor Works Certificate both carry a box for the inspector to confirm that main protective bonding is present and adequate, alongside the earthing arrangement and the earthing conductor.
Ticking that box without walking to the meter box is the commonest shortcut on a Minor Works Certificate. If you add a circuit in a house with no gas bond, you have two honest options: fit the bond as part of the job, which is the usual answer and cheap, or decline to sign the certificate as adequate, record the defect and tell the client in writing. What you cannot do is certify the earthing and bonding as adequate when they are not. See Minor Works Certificate: When to Use It and When You Can't for where the line falls between a Minor Works Certificate and an EIC.
Common mistakes with main protective bonding
Bonding after a branch, so the first tee off the incomer is left unbonded. Six millimetre on a PME supply, which is common in 1980s and 1990s work. Missing that the water or gas incomer has been replaced with plastic since the last inspection, or bonding internal copper that no longer needs it. No label at the clamp. Confusing main bonding with supplementary bonding: the Section 701 bathroom requirement at 701.415.2 is separate, is sized under 544.2 and is usually omitted today where the circuits have 30 mA RCD protection and the main bonding is in place (/guides/bathroom-zones-section-701). Bonding the lightning protection system without consulting BS EN 62305. And the one that undoes everything else: a clamp on a painted or corroded pipe that reads 0.8 Ω to the metal underneath.
On a TT installation, bonding matters just as much as on PME, even though the conductor is smaller, because the earth electrode may have a resistance of 100 Ω or more and the installation earth will rise much further during a fault (/guides/earth-electrode-testing-tt-systems).
Related terms
What is Ze? External earth fault loop impedance · What is Zs? Earth fault loop impedance explained · What Is an EIC? Electrical Installation Certificate for 2026 · What Is a Minor Works Certificate? MEIWC Explained for 2026 · What Is a Multifunction Tester (MFT)? Functions, Standards, Cost · Earthing Arrangements Explained: TN-S, TN-C-S and TT
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