Earth Rod
/ ɜːθ rɒd /
Also known as: earth electrode, earthing rod, ground rod, copper earth spike, driven rod
Definition
An earth rod is a copper-clad steel rod (typically 16mm diameter, 1.2m or 2.4m long) driven vertically into the ground outside a building to form a direct metallic connection between the electrical installation and the general mass of earth. The primary earthing means for TT systems (rural overhead supply with no distributor earth connection), and used as a supplementary earth electrode in other system types where supply earth impedance is high. Connected to the main earthing terminal (MET) with 16mm2 or 25mm2 green-and-yellow cable, protected mechanically above ground. The top of the rod must remain accessible in an inspection pit for periodic resistance testing. Resistance should be below 200 ohms; for a 30mA RCD, up to 1667 ohms remains compliant with the 50V touch voltage limit in BS 7671.
In practice
TT systems are far more common in rural UK than many electricians assume - any property on an overhead supply (wooden poles) rather than underground cables is almost certainly on a TT system. The absence of a PEN (combined protective earth and neutral) conductor in an overhead supply means there is no supply earth, and the installation must rely entirely on its own earth electrode. RCDs are mandatory on TT systems for all circuits, since the fault current through a TT system earth rod is typically too small to operate an MCB alone (the high impedance of the earth path limits the fault current).
Earth rod resistance increases in dry summer conditions as soil moisture content falls - an installation that tests satisfactorily in winter may have higher resistance in a dry summer. For installations in chalky, sandy, or well-drained soils, multiple rods in parallel may be needed to maintain acceptable resistance year-round. Routine inspection of the earth rod connection and resistance measurement should be included in every periodic inspection of a TT installation, as corrosion of the connection or drying of the surrounding soil can cause earth resistance to increase significantly between inspections.
See also