Earth Bonding and Equipotential Bonding: The UK Electrician's Compliance Guide

Earth Bonding and Equipotential Bonding: The UK Electrician's Compliance Guide

Earth bonding is one of the most safety-critical elements of any UK domestic electrical installation, yet it is also one of the most commonly misunderstood and incorrectly installed. Whether you are carrying out a full rewire, adding a new circuit in a bathroom, or working on an existing installation, understanding the difference between main protective bonding, supplementary bonding, and earthing is essential for BS 7671 compliance and Part P notification.

What Is Earth Bonding and Why Is It Required?

When fault current flows — for example, when a live conductor touches an exposed metal part — the goal of the earthing and bonding system is to keep all metal surfaces within reach of a person at the same electrical potential. If two metal objects that a person could touch simultaneously are at different potentials during a fault, the person becomes the conductor between them. Bonding prevents this by connecting all metal together and to the main earth terminal, so that in a fault condition everything rises and falls together.

Without correct bonding, the gas pipe entering a kitchen could be at mains potential during a fault while the tap a person is touching is earthed — with fatal results. This is why the IET Wiring Regulations (BS 7671) require specific bonding conductor sizes and connection methods, verified by the installation's Electrical Installation Certificate (EIC) or Minor Electrical Installation Works Certificate (MEWC).

The Three Types of Protective Conductor

1. Main Protective Earthing (CPC)

The circuit protective conductor (CPC) — the green/yellow wire in each cable — connects all exposed conductive parts within each circuit (metal enclosures, accessories, metal conduit) back to the consumer unit earth bar. This is the primary line of defence: it ensures that a fault to metalwork within a circuit operates the protective device (MCB or fuse) quickly enough to prevent injury.

CPC sizing is determined by BS 7671 Table 54.7 or by calculation. In twin-and-earth cables, the CPC is typically the unsheathed bare conductor. On circuits run in conduit, the conduit itself may serve as the CPC only if continuity is verified throughout.

2. Main Protective Bonding Conductors

Main protective bonding connects incoming metallic services — gas pipes, water pipes, and any other metallic service entering the building — to the main earthing terminal (MET) at the consumer unit. These bonding conductors must be installed as close as practicable to the point of entry of each service into the building, before any branch connection.

Minimum conductor size for main protective bonding (BS 7671 Table 54.8):

• For supplies up to 35mm² (typical domestic TN-S or TN-C-S): minimum 10mm² green/yellow copper conductor
• For TT earthing systems: 10mm² minimum but check protection device coordination
• PME (protective multiple earthing) supplies, where the neutral doubles as earth: main bonding must be a minimum of 10mm²

Clamp requirements: Bonding conductors must be connected to the pipework with WRAS-approved bonding clamps (for water pipes) or appropriately rated bonding clamps on gas pipework. Do not use jubilee clips or improvised clamps — the connection must be permanent and of low resistance. Mark bonding connections with durable green/yellow sleeving labels: "Safety Electrical Connection — Do Not Remove."

3. Supplementary Protective Bonding (Bathroom)

Supplementary bonding is additional bonding within a special location (Zone 0, 1, or 2 of a bathroom or shower room) where the normal protective device disconnection time cannot be guaranteed to be fast enough to prevent danger. In a bathroom, supplementary bonding must connect all simultaneously accessible exposed conductive parts and extraneous conductive parts together.

In practice, this means connecting:

• Metallic baths, shower trays, and basins (if accessible to touch simultaneously with other metal)
• Metal taps and waste fittings connected to metal pipes
• Metallic pipes within zones 1 and 2 (hot and cold water pipes, heating pipes)
• Metal structural members (steel joists, columns) within the bathroom zones
• Metal conduit and accessories within the zones

Conductor size for supplementary bonding: BS 7671 Table 544.1 — minimum 2.5mm² when mechanically protected (within conduit or trunking), or 4mm² when not protected. In practice, 4mm² green/yellow sleeved copper is the standard for bathroom supplementary bonding across a single room.

When can supplementary bonding be omitted? Since the 17th Edition, supplementary bonding in bathrooms can be omitted if the following conditions are ALL met (Regulation 701.415.2):

1. All circuits in the bathroom meet the disconnection times of BS 7671 Chapter 41 (typically RCBO protection or an RCD upstream)
2. All extraneous conductive parts (pipes, baths) are connected to the protective conductor via the wiring system rather than via supplementary bonding
3. The installation complies with Regulation 415.2

In modern installations with RCD-protected bathroom circuits, omitting supplementary bonding has become acceptable practice — but only when an EICR or EIC explicitly confirms the conditions are met. Do not omit supplementary bonding in older installations without assessment.

Earth Bonding Conductor Colours and Identification

All protective conductors must be identified with the bicolour green/yellow colour. Key rules:

• Bare copper CPC in twin-and-earth cable: must be sleeved with green/yellow sleeving wherever visible (in accessories, consumer units, at joints)
• Solid green or solid yellow cable is NOT permitted for new installations — bicolour only
• Bonding conductors at clamps: mark with green/yellow adhesive sleeve or tape at both ends and at any accessible intermediate point
• Bonding clamps: label "Safety Electrical Connection — Do Not Remove" per BS 7671 Regulation 514.13.2

TT Earthing Systems — Particular Attention

Properties on TT earthing systems (common in rural areas, older properties, and where the local DNO supplies a single-phase TN-C-S overhead service but no PME terminal) have no inherent earth fault return path through the supply — earth fault current must return via the local ground rod. This typically means higher fault loop impedance and slower fault clearance.

On TT systems, RCD protection is mandatory on virtually all circuits, and the earth electrode resistance must be tested and verified. Main protective bonding remains required, and bonding conductor sizing should be revisited against the specific protection coordination for the installation.

Part P Notifiable Work: When Must You Notify?

Any work that involves adding or altering the main earthing or main protective bonding of an installation is notifiable under Part P of the Building Regulations, unless carried out by a Competent Person Scheme (CPS) registered electrician. Specifically:

• New consumer unit installations (and associated earthing upgrades) — always notifiable
• New circuits in kitchens, bathrooms, or outdoors — notifiable
• Adding or altering main bonding connections — notifiable if not self-certifying

Supplementary bonding within a bathroom that does not alter the main earthing terminal is generally considered minor work and may be certified with a Minor Electrical Installation Works Certificate without Building Control notification — but check with your Local Authority if uncertain.

Earthing and Bonding Materials at APM Plumbing & Electrical

APM stocks a complete range of earthing and bonding materials for trade use including green/yellow earth cable in 1.5mm², 2.5mm², 4mm², 6mm², 10mm², and 16mm² sizes, pre-sleeved CPC earth sleeving packs, WRAS-approved water pipe bonding clamps, gas pipe bonding clamps, and "Safety Electrical Connection" bonding labels. Available individually or in trade packs.

Browse our earth cable and bonding conductors or visit our Acton trade counter. Orders placed before 2pm ship same day across London and the South East.

Products Available from APM Plumbing & Electrical

Key earthing rods and earth bonding components available for trade collection or delivery:

Get It from APM Electricals

APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or visit www.apmi.uk for same-day trade counter collection and next-day delivery across London and the UK.