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What earthing and bonding are, why they are essential for electrical safety, and how to check they are correct.
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.
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.
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.
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.
PME and Open PEN Conductor Risk
The main risk with TN-C-S (PME) systems is that if the PEN conductor in the supply cable is broken or disconnected (an open PEN fault), the neutral voltage can appear on all the earthed metalwork in the installation. This is why main bonding requirements are more stringent for PME supplies, and why some special locations have restrictions on PME earthing.
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.
Related Course
18th Edition (2382)
The 18th Edition covers earthing systems in detail, including design requirements for each type.
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.
Main bonding is required to the following services where they enter the building:
BS 7671 specifies minimum conductor sizes for main bonding. For a typical domestic installation:
Connection Points
Main bonding connections to gas and water pipes should be made using proprietary bonding clamps that comply with BS 951. The clamp must be labelled with a permanent notice stating "Safety Electrical Connection — Do Not Remove." Connections must be made on the consumer side of the meter for gas, and as close as practicable to the point of entry for water.
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 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:
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.
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:
Related Course
Level 2 Diploma (2365)
Earthing and bonding principles are covered from Level 2, forming the foundation of installation safety knowledge.
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.
The continuity of all protective conductors (CPCs, main bonding, supplementary bonding) must be tested using a low-resistance ohmmeter. 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 (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).
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. Here are the most common issues:
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