Earthing & Bonding FAQ

Protective bonding connects all accessible metalwork — water pipes, gas pipes, structural steel, cable trays, and equipment enclosures — to the main earthing terminal. This creates an equipotential zone where the voltage difference between any two touchable metal surfaces is minimised during a fault. Main bonding uses conductors of at least 6mm² copper or 10mm² aluminium as specified by BS 7671.

Use the fall-of-potential method with a dedicated earth resistance tester. Drive an auxiliary current electrode and a potential electrode into the ground at distances of 62% and 38% respectively of the current electrode distance from the electrode under test. Take readings at several potential electrode positions to verify a flat resistance curve. Test during dry conditions for worst-case values.

For TT systems, the earth electrode resistance RA must satisfy RA × IΔn ≤ 50V, where IΔn is the RCD rated residual current. With a 30mA RCD, maximum RA is 1,667Ω — easily achievable. For TN systems, the total loop impedance Zs must enable protective device operation within 0.4 seconds. Substations typically require below 1Ω. General commercial installations target below 20Ω.

PME (Protective Multiple Earthing) uses the supply neutral as the earth reference in a TN-C-S system. The risk arises if the combined neutral-earth (PEN) conductor breaks: all connected metalwork rises to dangerous potential because the earth return path is lost. BS 7671 imposes restrictions on PME use near swimming pools, construction sites, caravan parks, and EV charging with outdoor cables.

The minimum protective conductor size depends on the associated phase conductor size: for phase conductors up to 16mm², the protective conductor must equal the phase size; from 16–35mm², it must be at least 16mm²; above 35mm², it must be at least half the phase size. Alternatively, the adiabatic equation k²S² ≥ I²t provides an exact calculation based on actual fault current and clearing time.

Soil type and moisture content are the primary factors. Sandy or rocky soil has high resistivity (1,000–5,000 Ω·m), requiring extensive electrode systems. Dry conditions increase resistance significantly. Solutions include driving longer rods to reach moister soil layers, using multiple rods in parallel with adequate spacing, installing horizontal strip electrodes, or using conductive concrete (Ufer ground) for a reliable low-resistance connection.

BS 7671 requires supplementary bonding in locations containing a bath or shower only if the automatic disconnection conditions cannot be verified — specifically, if the earth fault loop impedance Zs cannot be confirmed to meet the required values. When the protective device and installation comply with the disconnection requirements, supplementary bonding in the bathroom is not required.

Earthing connects the electrical installation to the general mass of earth via an electrode, providing a fault current return path to the supply transformer. Bonding connects exposed and extraneous metalwork together to create an equipotential zone, minimising voltage differences between touchable surfaces during faults. Both are essential — earthing enables fault clearance; bonding prevents electric shock.

Lightning protection systems require a low-impedance path to earth to safely dissipate lightning current, typically below 10Ω total for the earthing system. The lightning protection earth should be bonded to the installation earth at the main earthing terminal to prevent dangerous potential differences between systems during a strike. Separation distance calculations prevent flashover between conductors.