Electrical Earthing and Bonding Guide UK — 18th Edition Requirements for Tradespeople (2026)

Why earthing and bonding matters

Earthing and bonding are the primary protection against electric shock and fire in any electrical installation. Their purpose is to ensure that dangerous fault currents — caused by insulation failure, damaged cables, or faulty equipment — are directed to earth quickly, enabling protective devices (fuses, MCBs, RCDs) to operate and disconnect the supply before a lethal shock can occur.

Without an effective earth path, fault current has nowhere to go. A person touching a live piece of metalwork becomes the fault path to earth — with potentially fatal consequences. Bonding ensures that all metalwork in an installation is at the same potential, so there is no voltage difference between parts a person could simultaneously touch.

Poor earthing is one of the most common defects found on EICRs, frequently coded C2 (potentially dangerous) or even C1 (danger present). Missing main bonding to the gas installation pipe, undersized earth conductors, corroded bonding clamps, and absent earth electrode test records on TT systems all appear regularly in inspection reports. Understanding the requirements thoroughly means fewer defects on new work and faster, more accurate reporting on inspection work.

Distribution network operator (DNO) earthing systems

The earthing arrangement for any installation depends on how the distribution network operator (DNO) supplies the building. BS 7671 uses the TN-C-S, TN-S, and TT designations to describe these arrangements, and each has different implications for protective bonding and fault protection.

• TN-C-S (PME — Protective Multiple Earthing): the most common system in the UK. The DNO combines the neutral and earth into a single PEN (Protective Earth and Neutral) conductor from the substation, which is earthed at multiple points along the low-voltage distribution network. At the service head, the PEN is split into separate neutral and earth terminals. The installation earth is derived from the neutral conductor.
• TN-S: a separate neutral and earth are maintained throughout — from the substation to the installation. Older and increasingly rare, TN-S provides a clean, independent earth that is not shared with the neutral. Often found in older urban properties and those supplied via steel-armoured cables where the armouring serves as the earth conductor.
• TT: the DNO provides no earth terminal. The installation must provide its own earth electrode — typically one or more earth rods driven into the ground. Common in rural areas and properties supplied by overhead lines, where the distribution network does not include an earth conductor.

Identifying the earthing system before starting work on any installation is essential. The system determines the earthing conductor size, the need for an earth electrode, and the RCD requirements for the installation.

PME advantages and risks

PME (TN-C-S) provides a low-impedance earth return path via the combined PEN conductor, which is excellent for fault protection — fault currents are high enough to operate fuses and MCBs quickly. Under normal conditions, PME is safe and highly effective.

However, PME carries a specific risk. If the PEN conductor breaks between the DNO substation and the installation — due to corrosion, mechanical damage, or a failed joint — the installation's earthing terminal is no longer connected to true earth. Instead, it rises to the neutral voltage, which can be significant under load conditions. Any metalwork bonded to the MET (main earthing terminal) — gas pipes, water pipes, structural steel, exposed equipment — rises to this voltage. Anyone touching bonded metalwork and a true earth point (such as damp ground) simultaneously would be at serious risk of electric shock.

This is why BS 7671 18th Edition places specific restrictions on PME earthing in certain locations where the consequences of a PEN conductor break would be particularly severe:

• Electric vehicle charging equipment (Section 722 — additional protection required)
• Swimming pools and paddling pools (Section 702)
• Caravan parks and marina berths (Sections 708 and 709)
• Agricultural and horticultural premises (Section 705)
• Locations with equipotential bonding to buried metalwork

In these locations, either the PME earth must not be used, or additional protective measures must be implemented to mitigate the PEN conductor break risk.

Main protective bonding

Main protective bonding is required under BS 7671 Regulation 411.3.1.2. Its purpose is to bring all extraneous conductive parts entering or passing through the building to the same potential as the main earthing terminal (MET), so that no dangerous voltage difference can exist between them during a fault.

The following services must be bonded to the MET as close as reasonably practicable to their point of entry into the building:

• Gas installation pipe
• Water installation pipe (incoming mains)
• Oil installation pipe
• Structural steel (where accessible and electrically continuous)
• Lightning protection system down conductors
• Central heating and air conditioning systems (metallic pipework entering from outside)

Main bonding must be made as close as reasonably practicable to the point of entry of the service into the building — before any branch or isolation point. A bonding connection made downstream of a stopcock or gas meter valve may not provide bonding to all the metalwork within the building. Connections must be made to clean, bare metal using approved bonding clamps, not to painted, coated, or corroded surfaces.

Supplementary bonding

Supplementary bonding provides additional local protection in specific locations where simultaneous contact with two conductive parts could be dangerous, even with main bonding in place. It supplements — but does not replace — main protective bonding.

The primary locations requiring supplementary bonding under BS 7671 are:

• Bathrooms and shower rooms (Zone 1 and Zone 2 metalwork)
• Swimming pools and paddling pools
• Wet areas in agricultural premises
• Locations where livestock are kept

In a bathroom, supplementary bonding connects metallic bath/shower trays, exposed pipework, accessible structural metalwork, and the metallic parts of electrical equipment (such as the casing of a SELV shaver supply unit) to each other and to the protective conductor. The minimum conductor size for supplementary bonding is 4mm² copper where not mechanically protected, or 2.5mm² where mechanically protected.

Supplementary bonding can be omitted where both of the following conditions are met under Regulation 415.2:

• All circuits in the bathroom are protected by a 30mA RCD
• All extraneous conductive parts in the bathroom are connected to the protective conductor of the circuits supplying equipment in the location (i.e. at the consumer unit via main bonding)

In modern installations with RCD protection and correctly installed main bonding, supplementary bonding in bathrooms is often not required. In older installations where main bonding is absent or circuits lack RCD protection, supplementary bonding will be required. On EICRs, missing supplementary bonding in older bathrooms is commonly coded C3 (improvement recommended) in current assessments, though this depends on the overall earthing and bonding arrangement.

Earth continuity testing

Testing earth continuity is a mandatory part of verifying any new installation and a key element of periodic inspection. The tests required depend on the work carried out.

For circuit wiring, the R1+R2 test (earth fault loop impedance of each circuit) is carried out using a loop impedance tester. This measures the impedance of the fault current path from the circuit protective conductor back through the supply to confirm that the impedance is low enough for the protective device to operate within the required disconnection time. Results are recorded on the Schedule of Circuit Details as Zs (measured loop impedance at the furthest point of each circuit).

For protective conductors and bonding conductors, continuity is verified using a low-resistance ohmmeter or continuity tester. Main bonding conductors must be tested end-to-end to confirm an unbroken, low-resistance connection from the bonded service to the MET. Check that bonding clamps are tight, correctly positioned on bare metal, and labelled. Corroded, loose, or unlabelled clamps are a common EICR finding coded C2.

All test results — including Ze (earth fault loop impedance at the origin), Zs for each circuit, and continuity readings for protective and bonding conductors — must be recorded on the Electrical Installation Certificate or EICR. Incomplete test records invalidate the certificate.

Earth electrodes (TT systems)

Where the DNO does not provide an earth terminal — a TT system — the installation must create its own earth connection via an earth electrode buried in the ground. The earth electrode provides the path for fault current to flow into the earth and return via the general mass of earth to the source.

BS 7671 recognises several types of earth electrode. The earth rod (a copper-bonded steel rod driven vertically into the ground) is by far the most common on domestic and light commercial TT installations. Other types include earth plates, earth tapes, earth mats, and the metal reinforcement of concrete foundations — though the latter is only used in specific circumstances.

For earth rod installation:

• Standard rods are 1.2m long; multiple rods can be coupled together to reach greater depths
• The top of the rod should be buried below frost level (typically 500mm minimum) and be accessible for testing via an inspection pit or chamber
• Where ground resistance is high (dry or rocky ground), multiple rods driven in parallel and spaced at least twice their driven depth apart will reduce the overall resistance
• The connection between the earth rod and the earthing conductor must be made with a listed clamp and protected from corrosion
• The earthing conductor from the rod to the MET must be protected against mechanical damage and clearly identified

The resistance of the earth electrode to true earth must be measured using a dedicated earth electrode resistance tester (not a loop impedance tester). A resistance of less than 200 ohms is generally considered acceptable for RCD-protected circuits on a TT system, but lower is always better. The measured value must be recorded on the EIC or EICR. Missing earth electrode test records are a frequent EICR finding, commonly coded C2 or C3 depending on the age and condition of the installation.

RCD protection on TT systems

On a TN system, the low-impedance earth path means fault currents are high enough for fuses and MCBs to disconnect the supply within the required time. On a TT system, the earth path runs through the soil — which has significantly higher resistance than a metallic conductor. Fault currents are correspondingly lower, and fuses and MCBs alone may not disconnect the supply fast enough to prevent a dangerous shock.

RCDs are therefore essential on TT systems. Regulation 411.5.3 requires that all circuits in a TT installation be protected by an RCD where the product of the operating current (IΔn) and the earth electrode resistance (Ra) does not exceed 50V. In practice, for domestic TT installations:

• All socket outlet circuits, lighting circuits, and circuits supplying portable equipment must be protected by 30mA RCDs
• Some equipment circuits (permanently connected appliances with no risk of contact) may be protected by 100mA or 300mA RCDs, but 30mA provides the best protection for domestic use
• A 100mA or 300mA whole-installation RCD at the consumer unit (used as a main switch) must be backed up by 30mA RCDs for individual circuits where required
• RCD operating times must be tested and recorded — at IΔn the RCD must trip in no more than 300ms; at 5×IΔn in no more than 40ms

On a TT system, an installation without adequate RCD protection is a serious defect — a C1 code on an EICR if a dangerous situation exists, or C2 if the risk is potential rather than immediate.

18th Edition Amendment 2 changes to earthing

Amendment 2 to BS 7671:2018, which came into effect in March 2022, introduced significant changes relevant to earthing — particularly for EV charging equipment and solar PV installations.

The revised Section 722 (Electric vehicle charging installations) sets out new earthing requirements for EV supply equipment. PME earthing cannot be used directly for EV charging points without additional protection. The options are:

• Installing a PEN conductor fault detection device (which disconnects the EVSE if the PEN conductor breaks) — most modern EV charger manufacturers incorporate this or provide it as an option
• Providing a separate TT earth for the EV charging point via an earth rod, isolated from the PME earthing arrangement

This requirement applies to all new EV charging installations. Existing installations that predate Amendment 2 do not need to be retrospectively updated, but any new installation or significant modification must comply.

Section 712 (Solar photovoltaic power supply systems) also received clarifications on earthing of PV array frames, inverter enclosures, and DC wiring. The functional earthing of PV systems and the protective earthing of accessible metalwork must both be correctly addressed, particularly for transformer-less inverters where the DC side is not isolated from the AC side.

Amendment 2 also includes updated guidance on earthing in locations with a risk of loss of protective earthing (Section 560) and reinforces the requirements for earthing of generator supplies used in parallel with the public supply.

Common earthing defects found on EICRs

Earthing and bonding defects are among the most frequently encountered findings during periodic inspection. The following are the most common, with their typical EICR codes:

• No main protective bonding to gas installation pipe (C2): extremely common in older properties and post-work installations where the gas pipe was replaced but bonding was not reinstated. A C2 in almost all cases.
• Undersized bonding conductors (C2): bonding installed with 6mm² or 4mm² where 10mm² is required, often from older installations or DIY work.
• Bonding clamps corroded, loose, or painted over (C2): a bonding clamp that is not making good contact with bare metal is not providing effective bonding regardless of conductor size. Corroded or painted-over clamps must be replaced.
• No bonding clamp label (C3 or C2): clamps must be labelled “Safety Electrical Connection — Do Not Remove” under BS 7671. Missing labels are typically C3, but combined with other defects can be coded higher.
• No earth electrode test record on TT system (C2 or C3): if the electrode resistance has never been measured and recorded, there is no evidence the earthing arrangement is adequate. C2 if the installation is old or electrode condition is unknown, C3 if recently installed with a likely good result.
• Missing supplementary bonding in older bathrooms (C3): where circuits lack RCD protection and main bonding is absent, missing supplementary bonding may be coded C2. In modern-standard bathrooms with RCD-protected circuits and correct main bonding, omission of supplementary bonding is not a defect.
• Earthing conductor not sleeved green-and-yellow (C3): unsleeved earth conductors at terminations are a minor but recordable defect.
• No protective earthing to exposed metalwork of fixed equipment (C1 or C2): equipment with accessible metalwork that is not connected to the protective conductor creates an immediate shock risk if the equipment develops a fault.