What is the purpose of earthing in an electrical installation?

The primary purpose of earthing is to protect people from electric shock. Under normal conditions, the exposed metalwork of an installation (socket faceplates, appliance casings, radiators connected to metal pipework) should be at zero volts — the same potential as the ground you stand on. If a fault occurs — for example, a live wire touches a metal appliance casing — the earthing system provides a low-resistance path for the fault current to flow back to the source (the transformer). This high fault current causes the protective device (MCB, fuse, or RCD) to disconnect the supply quickly, removing the danger. Without earthing, the metal casing would sit at mains voltage (230V) and anyone touching it would receive a potentially fatal electric shock, because the current would flow through their body to earth instead.

What is the difference between earthing and bonding?

Earthing connects the installation's metalwork to the general mass of earth via the main earthing terminal and the earthing conductor. Its purpose is to provide a fault current path so protective devices operate. Bonding connects different metallic parts together so they are all at the same potential (voltage). Main protective bonding connects the main earthing terminal to incoming metallic services — gas pipes, water pipes, oil pipes, structural steelwork — at the point where they enter the building. This ensures that if a fault raises the voltage on one service, all services rise to the same voltage, so there is no dangerous voltage difference between them that someone could bridge by touching two things simultaneously. Supplementary bonding provides additional connections between exposed and extraneous conductive parts in specific locations (such as bathrooms) where the risk is higher.

How do I know what earthing arrangement my property has?

You can identify the earthing arrangement by looking at how the earth connection is made at the consumer unit or main intake position. For TN-S, look for a separate earth conductor — typically a green/yellow wire connected to the metal sheath or armouring of the supply cable, or to a separate earth terminal provided by the DNO. For TN-C-S (PME), the earth connection is typically made via a terminal on the supply cutout or meter tails — the earth comes from the supply neutral. There is no separate earth conductor in the supply cable. For TT, look for an earth electrode — usually a copper-coated steel rod driven into the ground outside the property, with an earth wire running from it to the main earthing terminal. TT systems also typically have an earth electrode resistance label. If you are unsure, a qualified electrician can identify the arrangement during an EICR by measuring the Ze and inspecting the incoming supply.

What is an open PEN fault and why is it dangerous?

An open PEN fault occurs on a TN-C-S (PME) system when the combined protective earth and neutral conductor breaks or becomes disconnected somewhere in the supply network between the transformer and the consumer's installation. Under normal conditions, the PEN conductor carries the return neutral current for all properties connected to that section of the supply. If it breaks, the neutral current has no path back through the cable — instead, it returns through the earth connections of all connected properties. This can raise the voltage on the earthed metalwork of every property on that section to a dangerous level — potentially close to mains voltage. The risk is that someone touches a bonded metal pipe or appliance casing and receives a shock. This is why PME supplies have strict bonding requirements and why certain locations (bathrooms in particular) have additional restrictions when supplied by PME. TN-S and TT systems are not affected by open PEN faults.

When is a TT earthing system used?

A TT system is used when the electricity supply does not provide a metallic earth connection — typically in rural areas with overhead supply lines, where there is no continuous metallic cable sheath to serve as an earth conductor. The installation provides its own earth by driving an earth electrode (a copper-coated steel rod, typically 1.2m or 2.4m long) into the ground. The earth fault loop impedance on a TT system is much higher than on TN systems (often 20 ohms or more, compared to less than 1 ohm on TN-S or TN-C-S). Because of this high impedance, the fault current under earth fault conditions is low — too low for an MCB to trip within the required time. This is why RCD protection is always required on TT systems. A 30mA RCD can trip at very low fault currents, providing the rapid disconnection needed for safety even with high Zs values.

What size should main bonding conductors be?

The minimum size of main protective bonding conductors is specified in BS 7671 Table 54.8. For a PME (TN-C-S) supply, the main bonding conductor must be at least 10mm² copper if the supply tails are up to 16mm², and 16mm² copper if the tails are up to 35mm². For TN-S and TT supplies, the minimum is 6mm² copper for tails up to 16mm², and 10mm² copper for tails up to 35mm². These are minimum sizes — the bonding conductor must never be less than half the cross-sectional area of the earthing conductor. In practice, 10mm² green/yellow single core cable is the most common size for domestic main bonding. The bonding connection must be made at the point where the service enters the building, on the consumer's side of any meter or stopcock, using a BS 951 clamp or equivalent approved method.

ELECTRICAL BASICS

What Is Earthing? Why Electrical Earthing Matters

Earthing is the single most important safety measure in any electrical installation. It provides a safe path for fault current, ensures protective devices operate, and prevents metal parts from sitting at mains voltage. This guide explains how earthing works, the three UK earthing arrangements, and what gets tested during an EICR.

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11 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1Earthing connects the metalwork of an installation to the general mass of earth, providing a safe path for fault current. Without earthing, a fault could make a metal appliance casing live at mains voltage.
2The three main earthing arrangements in the UK are TN-S (separate earth conductor in the supply cable), TN-C-S or PME (combined neutral and earth in the supply cable, split at the origin), and TT (local earth electrode with no metallic connection to the supply earth).
3Main protective bonding connects the main earthing terminal to incoming services — gas, water, oil — so that a fault on any of them cannot create a dangerous voltage difference between metalwork.
4The earthing arrangement determines the maximum Zs values achievable, the bonding requirements, and the type of RCD protection needed. TT systems always require RCD protection because the earth fault loop impedance is too high for MCBs alone.
5During an EICR, the electrician checks the earthing arrangement, measures Ze (external earth fault loop impedance), inspects bonding conductors, and verifies that the earthing system is intact and effective.

01 · Electrical Basics

What Is Earthing?

Earthing is the connection between the metalwork of an electrical installation and the general mass of earth — the ground beneath your feet. Every electrical installation in the UK must have an effective earthing system. It is one of the most fundamental safety measures in electrical work, and it is a requirement of BS 7671 (the IET Wiring Regulations).

In simple terms, earthing provides a safe path for fault current — if something goes wrong and a live conductor touches a metal part that someone could touch, the earthing system ensures that the fault current flows safely to earth, causing the protective device (MCB, fuse, or RCD) to disconnect the supply before anyone is injured.

Without earthing, a fault would leave the metal casing of an appliance, a socket faceplate, or a light fitting at mains voltage — 230V. Anyone touching it would become the path to earth. That is how fatal electric shocks happen.

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02 · Electrical Basics

Why Earthing Matters: The Danger Without It

To understand why earthing matters, consider what happens when a fault occurs in an installation without earthing:

A live wire touches a metal casing. The casing is now at 230V. It sits there, energised, with no visible indication of danger.
Someone touches the casing. Their body provides a path from the casing (at 230V) to the floor (at earth potential). Current flows through their body. The amount depends on the resistance of the path — skin, clothing, footwear, floor material.
The protective device does not trip. Without an earthing path, the fault current flows only through the person — and at perhaps 50-200mA, it is far too low to trip an MCB (which needs many amps to operate). The MCB sees the fault current as a normal load. The circuit stays live. The person continues to receive a shock.
Injury or death. A current of 30-50mA sustained for more than a second can cause ventricular fibrillation. Without earthing and without RCD protection, the person may not survive.

Now consider the same scenario with earthing: the casing is connected to earth via the CPC (circuit protective conductor). When the live wire touches the casing, a large fault current flows through the low-resistance earth path — hundreds of amps. The MCB trips in milliseconds. The supply is disconnected before anyone is hurt. That is what earthing does.

03 · Electrical Basics

How Earthing Provides Safety

The earthing system works by creating a low-resistance path from every exposed metal part in the installation back to the source of supply (the transformer). This path has three components:

Circuit Protective Conductor (CPC). The earth wire in each circuit cable — the bare copper conductor in twin-and-earth cable, or the green/yellow insulated conductor in other cable types. It connects the earth terminal of each socket, light fitting, and appliance to the consumer unit's earth bar.
Earthing conductor. The main conductor that connects the consumer unit's earth bar to the main earthing terminal (MET). Typically 10mm² or 16mm² green/yellow single core cable.
Earth return path. The path from the main earthing terminal back to the transformer star point. This varies depending on the earthing arrangement — through the supply cable sheath (TN-S), through the supply neutral (TN-C-S), or through the soil via an earth electrode (TT).

The total impedance of this path — from the fault point, through the CPC, through the earthing conductor, through the earth return path, through the transformer, and back on the line conductor to the fault — is the earth fault loop impedance (Zs). The lower the Zs, the higher the fault current, and the faster the protective device trips. That is why low-resistance earthing connections matter.

04 · Electrical Basics

Earthing Arrangements: TN-S, TN-C-S (PME), and TT

The UK has three main earthing arrangements, classified by how the earth connection is made between the installation and the supply transformer. The arrangement is determined by the Distribution Network Operator (DNO) — the electrician must work with whatever arrangement the supply provides.

TN-S — Separate Earth

The supply cable has a separate earth conductor — typically the lead sheath or steel wire armouring of the supply cable. The earth is continuous and metallic all the way back to the transformer. Typical Ze: 0.35-0.8 ohms. Found in older properties with underground lead-sheathed cables. Generally provides the most reliable earth, but the sheath can corrode over time, especially on cables installed before the 1960s. If the sheath is damaged, the earth can be lost entirely.

TN-C-S — Combined Earth and Neutral (PME)

The supply cable has a combined neutral and earth conductor (PEN — Protective Earth and Neutral). At the consumer's main earthing terminal, the PEN is split into separate neutral and earth conductors. This is Protective Multiple Earthing (PME). Typical Ze: 0.2-0.35 ohms. The most common arrangement for newer UK domestic properties. Provides a very low Ze, but carries the risk of an open PEN fault — if the PEN conductor breaks, the earthed metalwork of all connected properties can rise to a dangerous voltage. This is why PME supplies have strict main bonding requirements.

TT — Earth Electrode

The installation provides its own earth via an earth electrode driven into the ground. There is no metallic earth connection through the supply cable. Common in rural areas with overhead supply lines. Ze depends on the soil resistivity and electrode type — can be anywhere from 2 ohms to over 200 ohms. Because Ze is typically high, MCBs cannot provide fast enough disconnection under earth fault conditions. RCD protection is essential on all circuits in a TT installation.

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05 · Electrical Basics

Bonding: Main Protective and Supplementary

Bonding is closely related to earthing but serves a different purpose. While earthing provides a fault current path, bonding ensures that all metalwork in and around the installation is at the same voltage — so a person cannot bridge a dangerous potential difference by touching two things simultaneously.

Main Protective Bonding

Connects the main earthing terminal (MET) to incoming metallic services at the point where they enter the building: gas supply pipe, water supply pipe, oil supply pipe, central heating pipework, structural steelwork. The bonding conductor is typically 10mm² or 16mm² green/yellow single core cable, connected using a BS 951 earth clamp. This is required by Regulation 544.1 of BS 7671 and must be present in every installation. Missing or disconnected main bonding is a C1 (Danger Present) observation on an EICR.

Supplementary Bonding

Additional bonding connections between exposed conductive parts and extraneous conductive parts in specific locations. The most common location is bathrooms, where supplementary bonding connects metal pipes, radiators, bath/shower trays (if metal), and any other metalwork within arm's reach. Under BS 7671 Regulation 701.415.2, supplementary bonding in bathrooms may be omitted if all circuits in the bathroom have RCD protection and the main bonding is confirmed present.

06 · Electrical Basics

Visual Checks for Earthing: What to Look For

Whether you are a homeowner checking your own property or an apprentice learning the basics, here is what to look for when visually inspecting an earthing system. Note: only a qualified electrician should test or modify earthing and bonding connections.

Main earthing terminal (MET). Usually located near the consumer unit or electricity meter. All earthing and bonding conductors connect here. Check that all connections are tight and there is no corrosion or damage.
Earthing conductor. The green/yellow cable from the MET to the consumer unit earth bar. Check it is the correct size (at least 16mm² for PME, 10mm² for TN-S) and is securely connected at both ends.
Main bonding conductors. Green/yellow cables from the MET to the gas pipe, water pipe, and any other incoming metallic services. Check they are present, connected with proper BS 951 clamps (not jubilee clips), and labelled with a "Safety Electrical Connection — Do Not Remove" label.
Earth electrode (TT systems). Usually a copper rod driven into the ground outside the property. Check the electrode is in good condition, the connection is clean and tight, and the cable from the electrode to the MET is intact and protected from mechanical damage.
Bonding labels. All bonding connections should have a warning label stating "Safety Electrical Connection — Do Not Remove." Missing labels are a C3 observation on an EICR.

07 · Electrical Basics

Earthing on the EICR: What Gets Tested and Recorded

During an EICR, the electrician carries out several tests and checks related to the earthing system:

Ze measurement. The external earth fault loop impedance is measured at the origin of the installation. This confirms the earthing arrangement and verifies the supply earth is intact. Unusually high Ze may indicate a deteriorating supply earth.
Zs measurement on each circuit. The earth fault loop impedance at the furthest point of each circuit confirms the CPC is intact and the total loop impedance allows the protective device to trip within the required time.
R1+R2 continuity test. Measures the combined resistance of the line and protective conductors end-to-end. This is the installation's contribution to Zs (Zs = Ze + R1+R2).
Main bonding continuity. The resistance of each main bonding conductor is measured to confirm it is continuous and of low resistance. A reading above 0.05 ohms may indicate a poor connection.
Earth electrode resistance (TT systems). On TT systems, the resistance of the earth electrode is measured using the fall-of-potential method or the loop impedance method. See earth electrode testing.

Common earthing-related defects found during EICRs include: missing or disconnected main bonding (C1), undersized bonding conductors (C2), loose earthing connections (C2), missing bonding labels (C3), and high Ze readings indicating a deteriorating supply earth (C2 or FI for further investigation).

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Frequently Asked Questions About Earthing

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What Is Earthing? Why Electrical Earthing Matters

Key Takeaways

What Is Earthing?

Get the BS 7671 A4:2026 Cheat Sheet — free

Why Earthing Matters: The Danger Without It

How Earthing Provides Safety

Earthing Arrangements: TN-S, TN-C-S (PME), and TT

TN-S — Separate Earth

TN-C-S — Combined Earth and Neutral (PME)

TT — Earth Electrode

Record earthing arrangements on your EICR

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Bonding: Main Protective and Supplementary

Main Protective Bonding

Supplementary Bonding

Visual Checks for Earthing: What to Look For

Earthing on the EICR: What Gets Tested and Recorded

Complete EICR certificates on your phone

Frequently Asked Questions About Earthing

What is the purpose of earthing in an electrical installation?

What is the difference between earthing and bonding?

How do I know what earthing arrangement my property has?

What is an open PEN fault and why is it dangerous?

When is a TT earthing system used?

What size should main bonding conductors be?

Continue reading

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How Electricity Works

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Supplementary Bonding Guide

Earth Fault Loop Explained

Earth Electrode Test

Electrical Acronyms Glossary

What electricians say

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