Earthing & Bonding: Main Equipotential Bonding, Supplementary Bonding & BS 7671

Quick Answer: All domestic electrical installations must have a main earthing terminal connected to the supply earth (PME or TN-S), with main equipotential bonding cables (10mm² for PME, 6mm² for TN-S) connecting all incoming metallic services — gas, water, oil — to that terminal within 600mm of entry. Supplementary bonding in bathrooms is only required where continuity cannot be confirmed through the main bonding.

Summary

Earthing and bonding are the two most misunderstood safety concepts in domestic electrical work. They are related but distinct: earthing provides a fault current path back to the source so protective devices operate; bonding connects metallic parts together to prevent dangerous potential differences appearing between them.

Every installation in the UK must comply with BS 7671:2018+A2:2022 (the 18th Edition Wiring Regulations) on both counts. The specific requirements depend on your supply system type — PME (Protective Multiple Earthing, also called TN-C-S), TN-S (separate neutral and earth), or TT (transformer with separate earth electrode). The vast majority of UK domestic properties are PME.

Failures in earthing and bonding are responsible for a significant proportion of electric shock incidents and electrical fires. Getting this right is non-negotiable — it's also one of the most commonly cited failures on EICR inspections (Electrical Installation Condition Reports), particularly supplementary bonding omissions in older bathrooms.

Key Facts

• PME supply — combined neutral and earth conductor from transformer; earth terminal bonded to neutral at street level. Most common UK domestic supply type.
• TN-S supply — separate earth conductor (usually lead cable sheath). Increasingly rare, found in older properties.
• TT supply — local earth electrode (usually a copper rod). Common in rural areas, caravans, some older properties.
• Main earthing conductor — connects supply earth terminal to main earthing terminal (MET). Minimum 16mm² for PME.
• Main equipotential bonding conductors — 10mm² minimum for PME (Regulation 544.1.1). 6mm² minimum for TN-S.
• Connection point — within 600mm of the point where each service enters the building, or at the meter/isolation point.
• Services requiring main bonding — gas, water, oil, any other metallic incoming service. Not required for plastic incoming pipes.
• Supplementary bonding — no longer mandatory in most bathroom situations where main bonding is confirmed and circuit is RCD protected (Amendment 2, 2015).
• Earth clamps — must be labelled "Safety Electrical Earth — Do Not Remove" per BS 951.
• CPC (Circuit Protective Conductor) — the earth wire within each circuit; sized per Regulation 543.
• Earth fault loop impedance (Ze) — external impedance; test values typically 0.35Ω (PME) or 0.8Ω (TN-S).
• Earthing electrode — required for TT systems; copper rod minimum 1.2m long, resistance as low as possible (typically <200Ω for protection to operate).
• Disconnection times — 0.4 seconds for final circuits ≤32A; 5 seconds for distribution circuits.
• Main switch earth — any metallic enclosure must be earthed regardless of supply type.

Quick Reference Table

Detailed Guidance

Supply System Identification

Before any earthing work, identify your supply system type:

1. Inspect the incoming cable at the meter position. If the earth terminal at the cut-out is connected to the neutral terminal, it's PME. If there's a separate lead cable sheath earth, it's TN-S.
2. The DNO (Distribution Network Operator) can confirm on request.
3. TT systems typically have a local earth rod visible near the property; no connection between neutral and earth at the cut-out.

For PME supplies, exporting PME earth to outbuildings requires careful consideration — BS 7671 Regulation 411.4.1 and Appendix 4 restrict this where the outbuilding contains a bath or shower.

Main Equipotential Bonding — What Needs Connecting

The incoming services that must be bonded:

Gas supply pipe: Bond within 600mm of the gas meter. The bond should be applied to the installation pipework, not the meter itself (the meter may be changed). A 10mm² green/yellow sleeved conductor from the gas pipe to the MET using a BS 951 pipe clamp.

Water supply pipe: Bond the incoming cold water main within 600mm of entry. If the incoming pipe is plastic (MDPE blue pipe), there may be no need to bond — but if there are metal fittings downstream, assess whether bonding is still required. Some installers bond at the first metallic stop valve.

Oil supply pipe: Any incoming oil supply pipe must be bonded as a metallic service.

Structural steel: Large exposed metallic structures (steelwork, reinforced concrete frames) may need bonding per Regulation 411.3.1.2 — assess on a project-by-project basis.

What does NOT require main bonding:

• Plastic incoming pipes with no downstream metallic connection
• Central heating pipework (protected by supplementary or main bonding of water/gas)
• Metal conduit (earthed via circuit CPC)

Supplementary Bonding in Bathrooms

Amendment 2 to the 17th Edition (2015) significantly relaxed supplementary bonding requirements. Under the current 18th Edition BS 7671:2018+A2:2022:

Supplementary bonding is NOT required in a bathroom if:

• All circuits in the location are protected by a 30mA RCD, AND
• Main equipotential bonding is present and confirmed by measurement

Supplementary bonding IS still required if:

• Protective conductors cannot be confirmed to provide satisfactory continuity throughout the installation
• The installation was designed under older regulations and is being like-for-like replaced (EICR may identify it as a C3 improvement)

In practice: for new bathroom installations with all circuits on RCBO or RCD-protected ways, supplementary bonding is not needed. For older properties where you cannot confirm main bonding is present and in good order, supplementary bonding remains good practice.

When supplementary bonding IS installed, all simultaneously accessible metal parts in zone 1 and 2 must be interconnected: bath, shower tray, towel rail, waste pipe, supply pipes, radiator. Minimum 4mm² conductors.

Earthing Electrode Systems (TT)

TT systems require a local earth electrode. Common types:

• Ground rod (earth spike): Most common. 16mm copper rod, minimum 1.2m. Drive in vertically or at 45°. Multiple rods can be connected in parallel for lower resistance.
• Earth mat: Horizontal buried copper tape or mesh. Used where ground conditions prevent deep driving.
• Ring earth electrode: Loop of copper buried 600mm–1m depth around building perimeter.

Electrode resistance must be low enough that the earth fault loop impedance allows the RCD to operate. For a 30mA RCD, Zs ≤ 1667Ω — achievable with most soil conditions. For fuse protection, electrode resistance must be much lower.

Test electrode resistance using a proprietary earth electrode tester (4-terminal method) or clamp meter method on existing electrodes.

CPC Sizing

Circuit protective conductors within wiring systems:

• If the CPC is the same material as the phase conductor: minimum half the phase conductor cross-section area (adiabatic equation per Regulation 543.1)
• Common cables: 1.5mm² twin-and-earth has a 1mm² CPC; 2.5mm² has a 1.5mm² CPC; 6mm² has a 6mm² CPC
• For mineral insulated cables and SWA, the sheath/armour acts as CPC — verify by calculation
• CPC continuity must be tested at every outlet during commissioning

Frequently Asked Questions

Do I need to re-bond if the gas pipe has been replaced with plastic?

If the incoming gas service pipe is now plastic (as many are after replacement), there may be no metallic path to bond. However, if the internal installation pipework is copper or steel with a metallic connection to the meter, you must bond at that metallic point within 600mm. If the entire installation including meter connection has been converted to plastic push-fit, bonding at that service may no longer be applicable — but document your assessment. Other services (water, oil) still require bonding.

Is bonding required on plastic incoming water pipes?

MDPE blue plastic incoming mains need no bonding themselves, but if the first fitting inside is metallic (brass stop valve), the bonding should still be applied to that fitting. The purpose is to prevent potential differences on accessible metalwork — so any exposed metal downstream of plastic still warrants consideration.

What happens if main bonding is missing on an EICR?

Missing main bonding is a C2 (potentially dangerous) code on an EICR — meaning it should be investigated and rectified urgently. It does not mean the building is immediately dangerous, but it significantly reduces protection against step potential and touch voltage in fault conditions. Landlords with C2 codes must remediate within 28 days.

Can I use the gas or water pipe as an earth for socket circuits?

No. This was sometimes done in older installations but is dangerous and non-compliant. Bonding conductors are connected TO the MET FROM the pipes — not using the pipe as an earthing conductor for circuits. Using a service pipe as an earth can cause corrosion, introduce dangerous currents into the pipework, and fails to provide a proper low-impedance fault path.

Regulations & Standards

• BS 7671:2018+A2:2022 — 18th Edition Wiring Regulations; Chapters 41 and 54 cover protective earthing and bonding

• Regulation 411.3.1.1 — Requirement for main protective bonding

• Regulation 411.3.1.2 — Structural steelwork and reinforced concrete bonding

• Regulation 544.1.1 — Main bonding conductor sizing (10mm² for PME)

• Regulation 543 — Sizing of protective conductors

• BS 951:2009 — Clamps for earthing and bonding; labelling requirements

• Approved Document P — Notifiable electrical work in dwellings

• HSE Guidance HSR25 — Memorandum of Guidance on the Electricity at Work Regulations 1989

• BS 7671:2018 18th Edition — IET Wiring Regulations (IET)

• Earthing Requirements for the Protection of Safety — HSE

• The IET On-Site Guide — Practical application guidance (IET)

• NAPIT Bonding Guide — Competent person scheme guidance

• bathroom zones — IP ratings and zones; supplementary bonding requirements by zone

• consumer units — MET location, earthing arrangements at the board

• cable sizing — CPC sizing as part of circuit design

• part p notifications — When bonding work requires notification