An earthing network is the complete system of conductors that connects a building’s electrical installation to the earth. Its purpose is to provide a safe path for fault current to flow to ground, enabling protective devices such as residual current devices (RCDs) to disconnect the supply and prevent electric shock or fire.
Application
The earthing network starts at the main earthing terminal in the consumer unit and extends through circuit protective conductors (CPCs) to every socket, switch, and fixed appliance in the building. Each connection point has a green-and-yellow earth conductor that links back to the main earthing terminal. Metallic pipes, radiators, and other extraneous conductive parts are connected to the earthing network through supplementary bonding conductors.
Different earthing systems are used depending on the electricity supply arrangement. In a TN system, the supply authority provides the earth connection through the combined neutral and earth conductor. In a TT system, the building owner must install a local earth electrode, typically a driven earth rod or a foundation earth electrode. BS 7671 in the UK and the National Electrical Code in the US specify requirements for each system type.
Earth resistance is a critical performance indicator. If the resistance is too high, fault current cannot flow quickly enough to trip the protective device, leaving people at risk. For TT systems, a maximum earth resistance of around 200 ohms is typically required when used with a 30 mA RCD.
In new construction, a foundation earth electrode is often installed as a copper or galvanised steel conductor embedded in or beneath the concrete foundation. This provides a consistently low earth resistance. In existing buildings, the earthing network can be improved by adding supplementary earth rods or by inspecting and renewing deteriorated connections.
Related terms
- Earthing
- Residual current device
- Consumer unit
- Foundation
- Bonding
- Insulation
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