What is earthing in electrical installations?

Earthing is the connection of all exposed metal parts of an electrical installation - appliance casings, metal conduit, steel enclosures, pipework - to a common earth terminal, and from there to the earth electrode (a metal rod buried in the ground or the electricity supplier's earth connection). If a fault makes a live conductor touch exposed metal, the low-resistance earth path allows a large fault current to flow, tripping the MCB or fuse and disconnecting the circuit before the voltage on the metal surface can cause a dangerous shock.

What is the difference between earthing and bonding?

Earthing connects the installation's exposed metalwork to earth so fault currents flow safely. Bonding (main protective bonding and supplementary bonding) connects all metallic services entering a building - gas pipes, water pipes, central heating pipework - to the main earth terminal, ensuring they are all at the same potential and that no dangerous voltage difference can exist between them in a fault condition. Bonding is especially important in bathrooms where multiple metallic surfaces are in close proximity and could be touched simultaneously.

What are TN-S, TN-C-S, and TT earthing systems?

These are different methods by which the electricity distributor provides an earth reference. TN-S uses a separate earth conductor in the supply cable (common in older urban areas with lead-sheathed cables). TN-C-S (PME - Protective Multiple Earthing) is the most common modern system - the distributor's neutral conductor is also used as the earth, with multiple connections to ground along the distribution network. TT uses no earth from the distributor - the installation must provide its own earth electrode driven into the ground. TT systems require RCD protection on all circuits as fault loop impedance is high.

Electrical noun

Earthing

/ ˈɜːθɪŋ /

Also known as: earth connection, protective earth (PE), grounding

Definition

Earthing is the connection of all exposed conductive parts (metal casings of appliances, metal conduit, structural steelwork, etc.) to a common earth terminal and from there to the general mass of earth. It ensures that if a fault causes a live conductor to contact metal, the fault current flows to earth through the low-impedance earth path rather than through a person who touches the metalwork - and the large fault current trips the overcurrent protective device (MCB or fuse) to disconnect the circuit.

In practice

In domestic installations, the earth terminal at the consumer unit connects to the electricity distributor's earth (in TN-C-S and TN-S systems) via the electricity meter tails. From the consumer unit, a green-and-yellow earth conductor runs with every circuit cable to provide earth continuity at every socket, switch, and appliance connection. The earth conductor is sized to carry the maximum prospective fault current for the disconnection time required by BS 7671.

Main protective bonding connects incoming services - gas pipe, water pipe, oil pipe - to the main earth terminal near the consumer unit using 10mm² green-and-yellow cable. This ensures all metallic services entering the building are at the same potential as the electrical earth. Supplementary bonding in bathrooms (now less commonly required in new installations) connects metallic pipework, baths, and shower trays together locally.

Building Regulations

Relevant PartPart P - Electrical safety in dwellings

StandardBS 7671:2018+A2:2022 Chapter 54 - Earthing and bonding

Main bonding conductorMinimum 10 mm² for TN systems, 4 mm² for TT

Earth colour codeGreen-and-yellow stripes (since 2006)

BS 7671 Chapter 54 covers earthing arrangements and protective conductors. Every installation must be earthed - if the distributor cannot provide a PME earth, a local earth rod must be installed. The earthing conductor from the meter to the consumer unit is typically 16mm² or 25mm². All metalwork (enclosures, conduit, containment) must be bonded to earth. An EICR (Electrical Installation Condition Report) checks the integrity of earthing and bonding as a priority item.

Full Building Regulations guidance

Earthing

Just so you know...