Earthing and Bonding Explained

Earthing and bonding are fundamental safety measures in every electrical installation. They work together to protect people from electric shock by ensuring that, in the event of a fault, dangerous voltages are limited and protective devices operate quickly to disconnect the supply.

Earthing provides a path for fault current to flow safely to the general mass of earth, which causes protective devices (circuit breakers, fuses, or RCDs) to operate and disconnect the faulty circuit. Without effective earthing, a fault on an appliance or wiring could result in the metalwork becoming live at mains voltage, presenting a lethal shock hazard. The consumer unit houses these protective devices and is the starting point for the earthing arrangement.

Bonding ensures that all metalwork that could potentially carry a dangerous voltage is connected together and to earth, so that no significant voltage difference can exist between them. This prevents a person from receiving a shock by simultaneously touching two metal parts that are at different voltages.

In the UK, there are three main types of earthing system. The type of system installed in a property depends on how the electricity supply is provided and how the earth connection is made. Understanding these systems is essential for any electrician because they affect the design, testing, and safety of the installation. All three systems are covered in detail in the 18th Edition wiring regulations.

TN-S (Separate Earth)

In a TN-S system, the earth is provided by the supply through a separate conductor, which is typically the lead or aluminium sheath of the underground supply cable. The earth conductor is separate from the neutral throughout the supply network.

• ✓Earth provided via the metal sheath of the supply cable
• ✓The earth and neutral conductors are separate throughout
• ✓Common in older urban installations with underground cable supplies
• ✓Generally provides a very low earth fault loop impedance
• ✓Considered a reliable earthing arrangement

TN-C-S (PME — Protective Multiple Earthing)

In a TN-C-S system, the neutral and earth are combined in a single conductor (known as a PEN conductor) in the supply cable. At the property, the distribution network operator (DNO) provides an earth terminal from which the consumer's earth is derived. This is the most common earthing system for modern domestic installations in the UK.

• ✓Also known as PME (Protective Multiple Earthing)
• ✓Neutral and earth combined in the supply cable (PEN conductor)
• ✓Earth terminal provided by the DNO at the meter position
• ✓The most common system for new and modern domestic supplies
• ✓Has specific requirements for main bonding due to the PME risks
• ✓Special considerations for installations in locations with increased risk (bathrooms, swimming pools, caravans)

TT (Earth Electrode)

In a TT system, no earth is provided by the supply. The consumer must install their own earth connection, typically using an earth rod (electrode) driven into the ground. TT systems are common in rural areas where the supply is provided by overhead lines.

• ✓No earth provided by the electricity supply
• ✓Earth connection via a local earth electrode (rod) installed by the consumer
• ✓Common in rural areas with overhead line supplies
• ✓Requires RCD protection on all circuits (higher earth fault loop impedance)
• ✓Earth electrode resistance must be checked and documented
• ✓The earth rod must be in good contact with the soil

Related Course

18th Edition (2382)

The 18th Edition covers earthing systems in detail, including design requirements for each type.

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Main bonding (also called main protective bonding or main equipotential bonding) connects extraneous conductive parts — metalwork that enters the building from outside and could carry a voltage from the general mass of earth — to the main earthing terminal of the installation. This ensures that all metalwork in the building is at the same potential, eliminating dangerous voltage differences.

What Must Be Main Bonded

Main bonding is required to the following services where they enter the building:

• ✓Gas supply pipe (connected within 600mm of the gas meter)
• ✓Water supply pipe (connected as near as practicable to the point of entry)
• ✓Oil supply pipe where applicable
• ✓Any other metallic service that enters the building (such as structural steelwork)
• ✓Central heating and air conditioning systems where they include metallic connections to outside

Main Bonding Conductor Sizes

BS 7671 specifies minimum conductor sizes for main bonding. For a typical domestic installation:

• ✓TN systems: minimum 10mm squared copper
• ✓TN-C-S (PME) with large neutral: may require 16mm squared copper
• ✓TT systems: minimum 10mm squared copper
• ✓The main bonding conductor must not be less than half the size of the earthing conductor, subject to a minimum of 6mm squared

Supplementary bonding provides an additional layer of protection in specific locations where the risk of electric shock is higher, such as bathrooms. It connects exposed conductive parts (metalwork of electrical equipment) and extraneous conductive parts (non-electrical metalwork such as pipes) within the room to create a local equipotential zone.

Bathrooms

Bathrooms are the most common location where supplementary bonding is considered. Under the current BS 7671, supplementary bonding in a bathroom can be omitted if all of the following conditions are met:

• ✓All circuits in the bathroom are protected by a 30mA RCD
• ✓Main protective bonding is in place and effective
• ✓All extraneous conductive parts in the room are effectively connected to the main bonding via the building structure

If any of these conditions cannot be confirmed, supplementary bonding must be installed. This typically involves connecting metallic pipework (hot water, cold water, central heating, waste pipes if metallic) and any exposed conductive parts of electrical equipment (such as a towel rail or extractor fan body) using 4mm squared copper conductors.

Other Locations

Supplementary bonding may also be required in other special locations as defined by BS 7671, including swimming pools, saunas, and locations with earth-connected medical equipment. The requirements vary by location and should be checked in the relevant section of the regulations.

Chapter 54 of BS 7671 (the 18th Edition Wiring Regulations) sets out the requirements for earthing arrangements and protective conductors. Key regulations include:

Key Regulations

• ✓Regulation 542 — Earthing arrangements: requirements for the earthing conductor, its connection to the means of earthing, and the earth electrode (for TT systems)
• ✓Regulation 543 — Protective conductors: sizing requirements for circuit protective conductors (CPCs)
• ✓Regulation 544 — Protective bonding conductors: sizing requirements for main bonding and supplementary bonding conductors
• ✓Table 54.7 — Minimum cross-sectional area of protective conductors in relation to line conductors
• ✓Table 54.8 — Minimum cross-sectional area of main protective bonding conductors

Related Course

Level 2 Diploma (2365)

Earthing and bonding principles are covered from Level 2, forming the foundation of installation safety knowledge.

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After installing or checking earthing and bonding, specific tests must be carried out to confirm effectiveness. These tests form part of the standard initial verification or periodic inspection process.

Continuity of Protective Conductors

The continuity of all protective conductors (CPCs, main bonding, supplementary bonding) must be tested using a low-resistance ohmmeter as part of standard electrical testing procedures. The test confirms that the conductor provides a continuous, low-resistance path from the point of use back to the main earthing terminal. Results are measured in ohms and should be very low (typically under 1 ohm for domestic circuits).

Earth Fault Loop Impedance

Earth fault loop impedance (Zs) testing measures the total impedance of the earth fault current path. This determines whether the protective device will operate quickly enough in the event of a fault. The measured Zs must be low enough to ensure the circuit breaker or fuse trips within the required disconnection time (0.4 seconds for final circuits, 5 seconds for distribution circuits in TN systems). Understanding how protective devices operate is essential for interpreting these results.

Earth Electrode Resistance (TT Systems)

For TT systems, the resistance of the earth electrode must be tested. This is typically done using a dedicated earth electrode resistance tester or by calculation from the measured external earth fault loop impedance. The resistance must be low enough to ensure the RCD operates within the required time.

During periodic inspections, electricians frequently encounter earthing and bonding defects. Correctly following safe isolation procedures before investigating these issues is essential. Here are the most common issues:

• ✓Missing main bonding to gas or water — often found in older properties where plastic replacement pipes have been installed without reconnecting the bond
• ✓Bonding connected on the wrong side of the gas meter (should be on the consumer side)
• ✓Undersized bonding conductors — 6mm copper used where 10mm is required
• ✓Missing or damaged bonding labels
• ✓Corroded or loose bonding connections
• ✓Plastic water main installed without removing the redundant bond (or vice versa — plastic main installed and bond removed when internal metallic pipework still needs bonding)
• ✓Missing supplementary bonding in bathrooms where conditions for omission are not met
• ✓Degraded earth electrodes in TT systems (high resistance due to dry soil or corroded rod)