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Earthing & Bonding Requirements

Complete earthing and bonding guide — TN-S, TN-C-S, TT systems, CPC sizing, and bonding conductors

TN-STN-C-STTAdiabaticBonding

Earthing Systems Explained

BS 7671 recognises three main earthing arrangements for UK supplies. The system type determines your Ze value, CPC sizing, and bonding requirements.

UK earthing system characteristics

BS 7671 Part 3, Reg 411.4
SystemEarth PathTypical ZeNotes
TN-SSupply cable sheath0.8 ohms maxOlder supplies, separate earth conductor
TN-C-S (PME)Combined neutral/earth0.35 ohms maxMost common modern supply
TTLocal earth electrodeUp to 200 ohmsRCD protection mandatory

Ze is measured at the origin with all installation earthing disconnected.

TT Systems Require RCDs

On TT systems the earth fault loop impedance is too high for MCBs to disconnect within the required time. An RCD rated at 30mA or less is essential for all circuits. The product of RCD rated residual current and electrode resistance must not exceed 50V.

CPC Sizing — Table 54.7 and Adiabatic Equation

The circuit protective conductor must withstand fault energy without exceeding its thermal limit.

Minimum CPC size by line conductor cross-section

BS 7671 Table 54.7
Line Conductor S (mm²)Minimum CPC (mm²)
S <= 16Same as line conductor
16 < S <= 3516
S > 35S / 2

These are minimum values. Where the CPC is not the same material as the line conductor, apply correction per Regulation 543.1.3.

S = sqrt(I²t) / k
S
= Minimum CPC cross-sectional area (mm²)
I²t
= Energy let-through of the protective device (A²s)
k
= Factor from Table 54.4 (e.g. 115 for copper/PVC)

BS 7671 Regulation 543.1.3

Main Protective Bonding

Main bonding connects extraneous conductive parts to the MET to prevent dangerous potential differences during a fault.

Minimum main bonding conductor sizes

BS 7671 Table 54.8
Supply TypeMin. Bonding ConductorServices Bonded
TN-S6mm² CuGas, water, oil, structural steel
TN-C-S (PME)10mm² CuGas, water, oil, structural steel
TT6mm² CuGas, water, oil, structural steel

Plastic Service Entries

Where gas or water enters a building in plastic pipe, establish whether the internal metallic pipework constitutes an extraneous conductive part before omitting the bond. A long-lead earth resistance test reading above 22k ohms indicates the pipe is not extraneous.

Supplementary Bonding

Supplementary bonding connects exposed conductive parts and extraneous conductive parts within a location to limit touch voltage. For bathrooms, conductors of 4mm² (mechanically protected) or 2.5mm² (where the connection is made using a BS EN 60998 connector and is protected) are typical.

When You Can Omit Bathroom Bonding

Since the introduction of Regulation 701.415.2, supplementary bonding in a bathroom can be omitted provided all circuits serving the location meet the disconnection time requirements of Chapter 41 and are protected by a 30mA RCD. Always verify Zs values before relying on this exemption.

Frequently Asked Questions

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