Earthing and Bonding Explained: Main Protective Bonding, Supplementary Bonding and CSA

Quick Answer: Earthing returns fault current to the supply transformer via the circuit protective conductor (CPC) and the main earthing terminal (MET); bonding equalises potential between exposed metalwork to prevent shock. Main protective bonding (MPB) connects extraneous-conductive-parts (incoming gas and water pipes) to the MET. Under BS 7671:2018+A2:2022, MPB conductor CSA must be at least half the earthing conductor CSA, minimum 6 mm² for TN systems and 10 mm² for TT systems. Supplementary bonding in bathrooms is no longer required where all circuits have 30 mA RCD protection AND main bonding meets the standard.

Summary

Earthing and bonding are two distinct functions, frequently confused. Earthing is the deliberate connection of exposed-conductive-parts (metal cases of appliances, light fittings, switchgear) to earth via the CPC, so that a fault to earth creates a low-impedance path that operates the protective device (fuse, MCB, RCD or RCBO) within the disconnection time required by BS 7671 Chapter 41. Bonding is the deliberate connection of extraneous-conductive-parts (metalwork brought into the installation from outside, like incoming water and gas pipes) to the main earthing terminal so that their potential is held the same as the earthing system. Without bonding, an earth fault elsewhere in the supply network can raise the potential of incoming pipework relative to the dwelling's earth, creating a touch-voltage hazard.

Bonding does not protect from a fault inside the dwelling — the circuit protective device does that. Bonding protects against a fault in the supply network or a separate building making contact with a service pipe outside the property. Customers and even some electricians confuse this — bonding is not an electrical protection in normal terms; it is a touch-voltage equalisation measure for abnormal supply conditions.

BS 7671:2018+A2:2022 (the 18th Edition with the second amendment) restated the bonding rules and added several clarifications: minimum CSAs differ between TN and TT systems; supplementary bonding requirements in bathrooms can be relaxed under specific conditions; the earthing conductor CSA at the cut-out depends on whether the supply is PME (TN-C-S) or TN-S.

Key Facts

• BS 7671:2018+A2:2022 — current Wiring Regulations (18th Edition, Amendment 2, 2022); the legal-effective standard for new electrical installations in the UK
• Earthing conductor — connects the MET to the means of earthing (DNO earth terminal, earth rod for TT, etc.); minimum CSA per Regulation 543 and Table 54.7
• Main protective bonding (MPB) — connects extraneous-conductive-parts to MET; minimum CSA per Regulation 544
• MPB minimum CSA (TN systems) — 6 mm² typical; 10 mm² where supply is PME (TN-C-S) with neutral conductor >35 mm² or where neutral fault potential requires
• MPB minimum CSA (TT systems) — at least half the earthing conductor CSA, minimum 6 mm² (rod-earthed TT systems do not have the PME 10 mm² uplift)
• Earthing conductor CSA — TN-S typically 16 mm² minimum; TN-C-S (PME) 16 mm² minimum (commonly upgraded to 25 mm²); TT installations typically 16 mm²
• CPC (circuit protective conductor) — sized per Regulation 543.1; for ring final 2.5 mm² T&E the CPC is 1.5 mm²; for 6 mm² radial the CPC is 2.5 mm²
• Equipotential bonding clamp — BS 951 compliant; brass or copper alloy with "SAFETY ELECTRICAL CONNECTION — DO NOT REMOVE" label
• Bonding clamp position on gas pipe — must be within 600 mm of the gas meter outlet on the consumer side, before any branch (Regulation 544.1.2)
• Bonding clamp position on water pipe — must be within 600 mm of the point of entry to the building, on the consumer side of the stopcock (Regulation 544.1.2)
• Earth electrode for TT systems — typically 1200 mm copper-clad steel rod; resistance ≤200 Ω required, ≤100 Ω strongly preferred; verified by measurement
• Supplementary bonding (601.415) — required in special locations (bathrooms, swimming pools) unless: all circuits have 30 mA RCD AND main bonding meets the standard AND disconnection times are met
• Earth fault loop impedance (Zs) — total impedance from phase to earth fault and back via the supply; must be low enough to operate the protective device within disconnection time per Regulations 411.3 and Table 41.1
• Disconnection time — 0.4 s for final circuits ≤32 A on TN; 0.2 s on TT with RCD; 5 s for distribution circuits and equipment ≥32 A on TN
• MET (main earthing terminal) — a single common terminal block in the consumer unit cabinet or adjacent; all CPCs, the earthing conductor and all main bonding conductors must connect here

Quick Reference Table

Detailed Guidance

TN-S, TN-C-S (PME) and TT — earthing-system implications

The first thing to determine on any installation is the earthing arrangement. This is shown on the DNO cut-out label or the previous Electrical Installation Certificate.

• TN-S — the DNO provides a separate earth conductor from the substation; older urban supplies (lead-sheathed) and some modern PILC supplies
• TN-C-S (PME, Protective Multiple Earthing) — the DNO provides a combined neutral-and-earth (PEN) conductor, separated at the consumer's installation into N and E; the majority of UK supplies since the 1980s
• TT — no DNO earth; the consumer establishes their own earth via a rod electrode; common for rural supplies, mobile homes, outbuildings with separate supplies

PME implications (TN-C-S):

• Earthing conductor minimum CSA is 16 mm² but very commonly specified at 25 mm² to give margin
• MPB conductor minimum CSA is 10 mm² (not 6 mm²) where the PEN conductor is ≤35 mm² and the supply is PME
• Where the PEN conductor breaks (open-PEN fault on the network), the dwelling's earth potential can rise to phase voltage — bonding ensures gas and water pipes rise with it, so touch voltage stays low

TT implications:

• Earth fault current passes via the earth electrode and back to the substation via the ground — typically 100–500 mA in good soil, much less in dry chalk or rock
• A 30 mA RCD is essential on every final circuit (Zs is too high for an MCB alone to disconnect in time)
• Earth electrode resistance verification at every periodic inspection

Main protective bonding — what to bond and what NOT to bond

Bond:

• Incoming gas pipe (consumer side of meter, within 600 mm of meter outlet, before first branch)
• Incoming water pipe (consumer side of stopcock, within 600 mm of point of entry)
• Oil supply pipe (where steel; some plastic oil supply doesn't require bonding)
• Structural steelwork in commercial/industrial — where it can introduce a potential
• Lightning protection system (separate down-conductor bonded to MET in certain protection schemes)

Do NOT bond:

• Plastic pipework (internal copper supply downstream of a plastic main is not an extraneous-conductive-part — it cannot introduce earth potential from outside)
• Internal metal pipework downstream of plastic stop end — not extraneous
• Stainless steel sinks, baths, taps — not extraneous-conductive-parts under BS 7671; equipotential bonding to these was historic practice but is no longer required if all bathroom circuits have 30 mA RCD

The test for "is this an extraneous-conductive-part?" is whether the metalwork in question could introduce a potential, typically earth potential, from outside the equipotential zone. If the pipe enters the property in plastic and the metalwork starts inside, it is not extraneous — no bonding required.

Bonding clamp installation

BS 951 bonding clamps are colour-coded labels: yellow background, black text reading "SAFETY ELECTRICAL CONNECTION — DO NOT REMOVE". Clamps must:

• Be physically secured to the pipe with the manufacturer's screws (typically two)
• Have the bonding conductor terminated under the dedicated terminal (not the clamp screws)
• Be located where they can be inspected (not buried in a wall or boxed-in cabinet)
• Be located on the consumer side of the meter or stopcock — the meter or stopcock is the property of the supplier and may be removed
• Be the bonding clamp's "main" connection, not via an intermediate fitting

Common installation faults:

• Clamp on the incoming side of the gas meter (supplier side) — wrong, the supplier may remove the meter and you've lost continuity
• Clamp on the first downstream pipe branch rather than the main incoming pipe — branch could be isolated or modified independently
• Clamp on non-extraneous (plastic-supplied) pipe — unnecessary and confusing
• Clamp loose or corroded — periodic inspection required
• No label fitted — the BS 951 label is part of the compliance; without it, the bond may be removed by a future trade

Supplementary bonding in bathrooms

Historically, supplementary bonding was required to connect the bath, basin, radiator, towel rail and any other exposed metalwork in a bathroom to the same equipotential zone. BS 7671:2018+A2:2022 (Regulation 701.415.2) relaxes this:

Supplementary bonding is NOT required in a bathroom where ALL of the following are met:

1. All circuits serving the bathroom (lighting, shower, towel rail, fans, accessories) are protected by a 30 mA RCD
2. The disconnection times of Regulation 411.3.2 are met for all circuits
3. Main protective bonding to extraneous-conductive-parts (gas, water) at the MET meets BS 7671 standards

If ANY of these conditions are not met (e.g. an old circuit without RCD that serves the bathroom), supplementary bonding IS required.

Supplementary bonding conductor CSA:

• Between extraneous-conductive-parts (bath to radiator, etc.): minimum 2.5 mm² with mechanical protection, 4 mm² without
• Between exposed-conductive-part and extraneous-conductive-part: minimum 1.5 mm² (with mechanical protection)

Earthing conductor CSA — Table 54.7 simplified

For TN-S and TN-C-S systems where the conductor CSA cannot be calculated by the adiabatic equation:

For TT systems, the same table applies but the practical minimum is 16 mm² Cu in any case to handle earth electrode connection mechanics.

PME special case: the supplier's "earthing conductor minimum CSA" for PME supplies in TN-C-S is in the DNO's connection conditions; many DNOs require minimum 16 mm² to consumer's MET, but supply the cable in 25 mm² for resilience.

Verifying earthing and bonding on inspection

The Electrical Installation Condition Report (EICR) tests include:

1. Continuity of MPB — measure resistance from MET to bonding clamp on gas and water; should be <0.05 Ω
2. Continuity of CPC — measure resistance from MET to earth terminal at extreme of circuit
3. Earth fault loop impedance (Zs) — measured at each accessory, compared to BS 7671 Table 41.3 maximum
4. RCD operation — trip time and trip current
5. Earth electrode resistance (TT only) — measured by ART tester or 3-pin / proprietary methods

A common failure on older properties: the gas pipe MPB is fitted to a copper pipe that has since been replaced (downstream of meter) with plastic. The bond is now on a "stub" of copper and does not extend to the rest of the gas system — but functionally it's still a valid bond to the incoming pipe. EICR coding: this is acceptable as long as the bond is on the incoming metal pipe section, even if downstream is plastic.

Common faults and EICR coding

• No main bonding to gas or water — Code C1 (danger present); requires immediate remedial
• Bonding clamp loose or corroded — Code C2 (potentially dangerous); requires remedial
• Bonding clamp inaccessible (boxed in) — Code C3 (improvement recommended); does not fail EICR
• No supplementary bonding in bathroom, no 30 mA RCD on all circuits — Code C2; requires remedial
• No supplementary bonding in bathroom, all circuits 30 mA RCD — compliant per BS 7671:2018+A2:2022; no code
• Earthing conductor CSA insufficient for PME supply — Code C2; needs upgrade

Frequently Asked Questions

Why is the bond on the gas pipe before the meter, not after?

It should be after the meter (consumer side), not before. The supplier owns the meter and may remove it for maintenance — if the bond is on the supplier side, removing the meter loses continuity. Always fit the bonding clamp on the consumer side of the gas meter, within 600 mm of the meter outlet, before any branch.

Do plastic water pipes need bonding?

No. Plastic pipes cannot conduct earth potential into the property, so they are not extraneous-conductive-parts. If the internal copper pipework starts inside the dwelling (downstream of a plastic incoming main), the internal copper is also not extraneous — no bonding required. This is a common confusion; many older installations were over-bonded based on historic rules.

Why are some installations TT instead of TN?

TT is used where the DNO cannot provide a reliable earth — typically rural overhead-line supplies, mobile homes, marinas, building sites or properties with long supply runs from the substation. TT requires an installed earth electrode (rod, mat or plate) at the consumer's installation, and 30 mA RCD protection on all final circuits because earth fault loop impedance is too high for an MCB alone to operate in time.

Can I bond to an existing copper pipe that's painted?

No — the clamp must make a bare-metal contact with the pipe. Scrape the paint, clean the surface, apply a thin film of jointing compound (electrically conductive) under the clamp, then fix the clamp. Re-paint around the clamp afterwards if desired, but the contact surface must be bare metal at install.

What's the difference between MPB CSA of 6 mm² and 10 mm²?

For TN-S supplies the MPB conductor minimum is 6 mm² (half the typical 16 mm² earthing conductor, minimum 6). For PME (TN-C-S) supplies, BS 7671 requires 10 mm² minimum where the PEN conductor is ≤35 mm² — this allows for higher fault current in an open-PEN scenario. Some installations historically used 6 mm² on PME and EICR will code this as C2 if the supply is PME — upgrade to 10 mm².

Regulations & Standards

• BS 7671:2018+A2:2022 — IET Wiring Regulations, 18th Edition Amendment 2 (the controlling standard for UK electrical installations)

• Regulation 411 — Protection against electric shock; ADS (Automatic Disconnection of Supply)

• Regulation 543 — Protective conductor CSA

• Regulation 544 — Protective bonding conductor CSA

• Regulation 701.415.2 — Bathrooms and shower rooms; supplementary bonding requirements

• BS 951:2009 — Specification for electrical earthing — clamps for earthing and bonding

• Approved Document P (2013) — Electrical safety, dwellings; references BS 7671 for compliance

• ENA Engineering Recommendation G12/4 — Requirements for the application of protective multiple earthing

• IET On-Site Guide — practical guide accompanying BS 7671

• IET BS 7671 information — current Wiring Regulations standard

• Approved Document P electrical safety — competent person scheme requirements

• Energy Networks Association guidance — PME and DNO earthing requirements

• NICEIC technical bulletins — common compliance issues and EICR coding guidance

• Electrical Safety First — guide to earthing — consumer-facing safety guidance

• consumer units — RCD/RCBO selection and SPD requirements under Amendment 2

• cable sizing — CPC sizing within composite cables

• bathroom zones — Zone 0/1/2 equipment and supplementary bonding interaction

• rcd tripping — diagnosing earth fault triggers on 30 mA RCDs

• safe isolation procedure — proving dead before working on bonded systems

• part p notifications — notification of electrical work under Part P