Earth Bonding and Supplementary Bonding — Main Equipotential Bonding, Cross-Bonding, and BS 7671 Requirements for UK Electricians

Electrical earthing and bonding are among the most critical safety elements in any UK installation. Yet bonding is also one of the most frequently misapplied — either under-specified (leaving dangerous potential differences) or over-specified (adding unnecessary conductors to already-bonded metalwork). This guide covers the two distinct bonding obligations under BS 7671:2018 (18th Edition): main equipotential bonding and supplementary equipotential bonding. It explains when each applies, how to size conductors correctly, and what test evidence you need to leave on site.

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Safety Electrical Connection

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Why Bonding Matters: The Risk of Potential Differences

When metalwork in a building is at different electrical potentials — even small differences of a few volts — a person bridging them can receive a shock or, in wet environments, a lethal current. Bonding brings all exposed and extraneous conductive parts to the same potential as the earthing system. This does not eliminate voltage during a fault, but it prevents dangerous differences from appearing between things a person could touch simultaneously.

The distinction between exposed conductive parts (ECPs) — accessible metalwork that is part of the electrical installation, such as conduit or consumer unit casings — and extraneous conductive parts (XCPs) — metalwork not part of the installation but capable of introducing a potential, such as gas and water pipes — drives the bonding strategy. ECPs are protected by the circuit protective conductor (CPC) within each circuit. XCPs are addressed by main and supplementary bonding.

Main Equipotential Bonding (Regulation 411.3.1.2)

Main equipotential bonding connects the main earthing terminal (MET) of the installation to all extraneous conductive parts entering or leaving the building. Regulation 411.3.1.2 lists the mandatory connections:

• Water installation pipes
• Gas installation pipes
• Structural steelwork
• Central heating and air-conditioning systems
• Lightning protection systems (where present)
• Any metallic sheath of a telecommunications cable (unless service provider specifies otherwise)

The bonding conductor must be connected as near as practicable to the point of entry of the service into the building — before any isolating valve, meter, or tee. On a gas installation, the bond connects to the consumer's pipework, not the meter or service pipe on the network side. This is a frequently failed point on EICRs.

Sizing Main Bonding Conductors

Table 54.8 of BS 7671 gives the minimum cross-sectional area (CSA) for main bonding conductors. The rule is simple: it must be not less than half the CSA of the earthing conductor, subject to a minimum of 6 mm² and a maximum of 25 mm² (copper), unless a risk assessment justifies larger.

In practice, most domestic TN-C-S (PME) supplies have 16 mm² earthing conductors, making 10 mm² the standard main bonding conductor for gas and water. Older properties on TN-S or TT systems with 6 mm² earth conductors may use 6 mm² bonding — always verify the actual earthing arrangement before specifying.

Bonding Clamps and Labels

Bonding conductors must be secured with an approved bonding clamp clamped to clean, uncoated metal. Screwed push-in connectors or saddle-type clamps with a serrated face are acceptable if they maintain a gas-tight, corrosion-resistant connection. Each bonding point must be labelled: "Safety Electrical Connection — Do Not Remove" in accordance with Regulation 514.13.

On plastic or composite pipework (e.g., MDPE service pipes, plastic water meters), the bond connects to the first accessible length of metallic pipework on the consumer's side. If the entire pipework run is plastic, the extraneous conductive part no longer meets the definition — no bonding conductor is required to it, as it cannot import a dangerous potential. Document this assessment.

Supplementary Equipotential Bonding (Regulation 415.2)

Supplementary bonding is a local measure applied within a specific zone where automatic disconnection of supply cannot achieve safe touch voltages within the required time limits. It is most commonly encountered in:

• Bathroom zones (BS 7671 Section 701)
• Swimming pools (Section 702)
• Agricultural locations (Section 705)
• Areas with increased shock risk

The principle: supplementary bonding interconnects all simultaneously accessible ECPs and XCPs within the zone, so even if a fault raises one item to live potential, the person touching it and the nearby metalwork are at the same potential.

When is Supplementary Bonding Still Required in Bathrooms?

Since the 17th Edition, supplementary bonding can be omitted from a bathroom if all of the following conditions are satisfied (Regulation 701.415.2):

1. All circuits serving the bathroom are protected by a 30 mA RCD.
2. All circuits comply with the disconnection time requirements of Table 41.1.
3. Main equipotential bonding has been correctly installed to all extraneous metallic services entering the building.

If these three conditions are met, supplementary bonding in the bathroom is not required. In practice this means most modern installations with a fully RCD-protected consumer unit serving the bathroom circuits will not need local supplementary bonding. However, you must still verify that main bonding is in place and that main bonding conductors are correctly connected and labelled — an omitted or defective main bond negates the RCD exemption.

Where supplementary bonding is required, it must connect all simultaneously accessible ECPs and XCPs: bath taps, shower fittings, towel rails, metallic pipework, heating radiators, and the protective conductors of all electrical equipment within the zone.

Sizing Supplementary Bonding Conductors

Regulation 415.2.2 sets the minimum CSA:

• Between two ECPs: not less than the smaller CPC of the circuits to those ECPs (minimum 2.5 mm² if mechanically protected, 4 mm² if not)
• Between an ECP and an XCP: at least half the CPC of the circuit to the ECP (minimum 2.5 mm² mechanically protected, 4 mm² unprotected)
• Between two XCPs: minimum 2.5 mm² mechanically protected, 4 mm² unprotected

In domestic bathrooms, 4 mm² green/yellow single-core cable is the typical choice where the bonding run is exposed (within plastic conduit or visible). Where run within the fabric of the building and mechanically protected, 2.5 mm² is acceptable, but many electricians use 4 mm² throughout for consistency and to allow future verification.

TT Systems and Bonding

On TT systems (common in rural properties without a PME earth), the earth electrode provides the return path for fault current. The electrode resistance can be high enough that automatic disconnection within the required time is not achievable without RCD protection. For TT systems:

• Main bonding conductors still connect XCPs to the MET — same rules apply
• The earthing conductor to the earth electrode is typically 16 mm² copper (or as calculated for the electrode resistance)
• All circuits should be RCD-protected at 30 mA to ensure prompt disconnection
• The earth electrode resistance must be tested and recorded: R × Ia must not exceed 50 V (typical limit)

On TT systems, the absence of a PME earth means the risk of potential rise on metalwork connected to the PME network (exporting a PEN conductor fault back into the building) does not apply — one of the few advantages of TT.

PME (TN-C-S) and the Bonding Obligation

The vast majority of UK urban domestic supplies are PME (Protective Multiple Earthing), also called TN-C-S, where the combined PEN conductor in the distribution network is used as both neutral and earth. The main concern with PME is that a PEN conductor fault on the network can raise the earthing system to a dangerous potential relative to true earth. This is why BS 7671 Regulation 543.3.3 warns against using the PME terminal for certain outdoor and mobile installations.

For main bonding under PME:

• The earthing conductor connects the MET to the PME earth terminal at the service head (supplied/maintained by the DNO)
• All XCPs must be bonded to the MET
• Any metalwork forming part of an outdoor installation (EV charge points, garden buildings) connected to a PME earth should be assessed under Engineering Recommendation G12 — the DNO may require a local earth electrode

Testing Bonding Connections

Bonding continuity is verified with a low-resistance ohmmeter (also called a continuity tester). The test procedure:

1. Use a wander lead from the MET (or a known-good earth point at the consumer unit).
2. Measure resistance to each bonding conductor connection point: gas meter tail, water meter bypass, structural steelwork, etc.
3. Maximum acceptable resistance for main bonding conductors: typically less than 0.05 Ω for a short run in a domestic premises — this is judgement-based; no absolute limit is prescribed in BS 7671, but the test confirms physical continuity.
4. Where long runs (>10 m) are involved, calculate expected resistance from cable CSA and length to confirm the measured value is consistent.

For supplementary bonding in bathrooms (where fitted), test between each bonded item and record the results. The EICR schedule of test results should include continuity of main and supplementary bonding under the protective conductor section.

Common EICR Observation Codes for Bonding

Bonding defects are among the most common items on EICRs:

Practical Notes for Installation

Routing: Main bonding conductors should be run as directly as possible from the service entry point to the MET. Avoid running long distances — if the MET is remote from the service entry, a local bonding bar (sub-MET) can be used, with a single conductor returning to the MET, but this is unusual in domestic work.

Plastic pipework at service entry: Where the gas or water service enters in plastic (increasingly common with MDPE or polyethylene gas mains), the first metallic fitting on the consumer side is the bonding point. If the entire consumer installation is plastic, no bond is required — document this. However, if there is metallic pipework downstream (e.g., copper heating pipework connected to a plastic cold water supply), assess whether that metalwork is truly an extraneous conductive part introducing potential from outside — typically it is not, if isolated from external earth paths.

New build coordination: On new builds, the bonding is often installed by the plumber before the electrician arrives. Confirm that bonding clamps are correctly positioned and labelled, and include bonding inspection as part of your initial site walk before the EICR or minor works cert is issued.

Related Guides

• Earthing Systems — PME, TN-C-S, TN-S, and TT: A UK Electrician's Guide
• Consumer Units, MCBs, and RCBOs: Selecting and Installing a Split-Load Board
• RCD, RCCB, and RCBO Protection — Types, Testing, and Selection for 18th Edition Compliance
• EICR Landlord Electrical Safety Inspections — Requirements, Process, and Remedial Works
• Bathroom Electrical Zones and IP Ratings — BS 7671 Section 701

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