Protective Earthing and Bonding: The Complete BS 7671 Guide

Protective earthing and bonding form the foundation of electrical safety in every installation. Without effective earthing, protective devices (MCBs, RCDs, fuses) cannot operate correctly in the event of an earth fault. Without bonding, dangerous voltage differences can develop between the electrical installation and metallic services such as water pipes, gas pipes, and structural steelwork.

When an earth fault occurs — for example, a live conductor touches the metal casing of an appliance — fault current must flow through the protective conductor back to the supply transformer. This fault current must be high enough to trip the protective device within the required disconnection time (0.4 seconds for final circuits up to 32 A supplying fixed equipment, or up to 63 A where socket outlets are present, in TN systems — per Regulation 411.3.2.2 and Table 41.1). If the earthing path has too much impedance (resistance), the fault current will be too low to trip the device quickly, and the metalwork will remain live — creating a risk of electric shock.

Bonding complements earthing by ensuring that metallic services entering the building are at the same electrical potential as the earthing system. Without bonding, a person simultaneously touching an earthed appliance and an unbonded water pipe could experience an electric shock if there is any potential difference between them. Main protective bonding eliminates this risk by connecting all extraneous-conductive-parts to the main earthing terminal.

These are not theoretical concerns. BS 7671 devotes Chapter 41 (Protection against electric shock) and Chapter 54 (Earthing arrangements and protective conductors) to these requirements, and earthing defects are among the most common C1 and C2 observations recorded on EICR reports.

The earthing arrangement of an installation describes how the exposed-conductive-parts are connected to the general mass of earth. The three arrangements used in the UK are:

Identifying the earthing arrangement is one of the first steps in any inspection. It determines the expected earth fault loop impedance values, the bonding requirements, and the type of fault protection that should be in place. Record the earthing arrangement on the EICR and verify it by measuring Ze at the origin.

The main earthing terminal (MET) is the central connection point for the earthing system. Every earthing conductor, bonding conductor, and circuit protective conductor ultimately connects back to the MET. It is the single point where the installation earthing system meets the supply earth or earth electrode.

During a periodic inspection, check the MET for security of connections, signs of corrosion or overheating, presence of the safety label, and accessibility. A corroded or loose MET connection is a common C2 (Potentially Dangerous) observation on EICR reports.

Main protective bonding conductors connect extraneous-conductive-parts to the main earthing terminal. Their purpose is to maintain equipotential bonding — ensuring that all metallic services in the building are at the same potential as the earthing system, eliminating dangerous touch voltages.

Under BS 7671 Regulation 411.3.1.2 and Section 544, the following services must be bonded to the MET:

Each bonding connection must be made with a BS 951 bonding clamp, fitted to a clean section of pipe, and bear a permanent warning notice: "Safety Electrical Connection — Do Not Remove" (Regulation 514.13.1). Run the conductor unbroken from the MET to the bonding clamp wherever practicable; BS 7671 does not prohibit a joint, but Regulation 543.3.2 requires every connection and joint in a protective conductor to remain accessible for inspection, testing and maintenance, so a buried or concealed joint is not acceptable.

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The size of main protective bonding conductors is determined by BS 7671 Table 54.8. The key rule is:

For most domestic work, the answer is simple: use 10 mm copper. It is the default size for TN-C-S (PME) supplies, which account for the majority of UK domestic installations. Using 10 mm eliminates any doubt about compliance and provides a robust connection.

The bonding conductor can be single-core green/yellow cable or twin-core with a green/yellow sheath. Where it is clipped to the surface, it should be run neatly and protected from mechanical damage. Where it passes through walls or floors, use grommets or suitable bushings to protect the cable. The conductor must not be reduced in size at any point along its length.

For larger PME installations (three-phase supplies, commercial premises), refer to Table 54.8 and select the bonding conductor size based on the supply PEN conductor cross-sectional area: 35 mm² or less → 10 mm²; over 35 up to 50 mm² → 16 mm²; over 50 up to 95 mm² → 25 mm²; over 95 up to 150 mm² → 35 mm²; over 150 mm² → 50 mm². These are copper equivalent areas — another metal may be used where it affords equivalent conductance. The note to Table 54.8 warns that the local distributor's network conditions may require a larger conductor, so always check with the DNO for very large supplies.

One of the most debated topics in electrical inspection is: what constitutes an extraneous-conductive-part? Getting this right determines what needs bonding and what does not.

BS 7671 defines an extraneous-conductive-part as: "A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation." The key phrase is "liable to introduce a potential" — the part must be capable of conducting a voltage from outside the equipotential zone into the installation.

The practical test is: can this metallic part introduce a potential from outside the equipotential zone? BS 7671 answers this directly. The note to Regulation 411.3.1.2 states that where non-metallic pipes — plastic, for example — enter the building and are then connected to metallic pipes within the building, the metallic pipes within the building do not normally require protective bonding, as they are unlikely to be extraneous-conductive-parts. Confirm it by assessment or measurement rather than bonding by default, and record the basis of your decision.

During an EICR inspection, you must assess whether all extraneous-conductive-parts are correctly bonded. Missing bonding on a gas or water pipe is a C2 (Potentially Dangerous) observation. Missing bonding on a service that is not actually an extraneous-conductive-part (for example, internal copper connected only to plastic mains) may be recorded as C3 or noted as not required — but you must document your reasoning.

Every circuit in the installation has a circuit protective conductor (CPC) that connects the exposed-conductive-parts on that circuit back to the main earthing terminal. The CPC is the green/yellow conductor in a twin-and-earth cable, or a separate green/yellow conductor in conduit or trunking systems.

Unsleeved CPCs at termination points are one of the most common observations during periodic inspections. While this is typically a C3 (Improvement Recommended) observation, it indicates a lack of attention to detail during the original installation or subsequent alterations. Always sleeve bare CPCs when working on an installation — it takes seconds and demonstrates professional workmanship.

Testing earthing and bonding is a core part of both initial verification and periodic inspection. The key tests are:

Record all test results on the schedule of test results for the EICR or EIC. Any earthing or bonding defects found during testing should be classified according to the observation code system: C1 for absent or disconnected earthing, C2 for deteriorated or inadequate earthing/bonding, and C3 for missing earth sleeving or recommendations for improvement.

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