Earthing and Bonding: The Part Nobody Sees

Protective devices depend on a low-impedance path back to source. Without it, a fault current may be too small to trip a breaker quickly, leaving exposed metalwork live.

Two different functions

Earthing connects exposed conductive parts to earth, providing the return path that allows a protective device to operate.

Bonding connects metallic parts together so they sit at the same potential. Its purpose is to prevent a dangerous voltage difference between two metal items a person could touch simultaneously — a tap and a radiator, for example.

They are related but not interchangeable, and specifications call for both.

The distinction matters because the two are judged differently. Earthing is judged by impedance: is the loop low enough that the protective device operates within the time the standard allows for that circuit? Bonding is judged by equalisation: is the potential difference between two touchable metal parts small enough to be harmless during a fault, even if current is still flowing? A building can have a perfectly good earth electrode and still be dangerous because a metallic riser was never bonded.

The path is only as good as its weakest joint

An earth fault loop runs through the circuit protective conductor, the earth bar in the distribution board, the submain, the main earth terminal, and back through the supply. Each is a joint, and impedance is cumulative. A single loose terminal, a painted surface under a lug, or a protective conductor sized generously but terminated badly can undo the whole chain.

This is why the design intent has to survive installation. Conductor cross-section is set by calculation against the circuits it protects, and the cable chosen has to be terminated with the right lug, onto clean metal, at the right tightness. Torque discipline is not ceremony here — an under-tightened bolt on an earth bar is a high-resistance joint that heats, oxidises and gets worse.

Where problems appear

• Bonding conductors omitted to incoming metallic services
• Connections made to painted or corroded surfaces, giving high resistance
• Earth continuity broken when a section of metallic containment is later replaced with plastic
• Undersized conductors relative to the circuits they protect
• Connections that were never re-tightened after initial installation
• Flexible connections and equipment doors left without a bonding strap
• An earth bar so crowded that two conductors share one hole and neither is properly clamped

Metallic services in a modern building

Bonding requirements were written when incoming water and gas services were metal throughout. Many buildings now have plastic incoming pipework and metallic distribution internally, or the reverse, so a bonding schedule copied from an old project bonds pipes that no longer need it and misses risers that do. Walk the actual services: incoming water, gas, structural steel, lift guides, metallic ducting and containment, and any extraneous conductive part entering the building. Decide each on what it is, not on what the standard drawing says.

Testing tells you what inspection cannot

Continuity and loop impedance tests reveal whether the path is genuinely low impedance. A connection can look perfect and still measure poorly if the contact surface is contaminated or the joint is loose. Record test results at commissioning so future comparisons are meaningful, and record them per circuit rather than as a single pass or fail for the board.

Three measurements do different jobs and none substitutes for another. Continuity confirms the conductor is unbroken end to end. Loop impedance confirms the whole path, including the supply, is low enough for the protective device to operate. Electrode resistance confirms the connection to the mass of earth itself. Add residual current device testing where RCDs or RCBOs form part of the protection, because an RCD provides protection that does not depend on loop impedance at all — precisely why it is valuable where the loop is marginal, and why it must be proven to operate.

Soil, salt and the Gulf

Electrode resistance is a function of soil resistivity, and dry sand is a poor conductor. An electrode arrangement that measures well after installation in damp backfill can measure very differently after a summer, so the season and conditions at the time of test belong in the record.

Corrosion is the second regional factor. Buried and coastal connections sit in a chloride-rich environment, and a copper conductor clamped to steel is a bimetallic couple that corrodes at the joint. Exothermic or properly rated compression connections, correct material pairing, and protection of the joint all matter more here than inland. Earth pits must remain accessible and identifiable — a test point paved over during landscaping is a test point that will never be used again.

Coordination with the rest of the installation

Earthing is rarely a standalone package. Lightning protection, generator and UPS neutral arrangements, and the earthing requirements of IT and telecom equipment all interact with the building earth, and the interfaces are usually where responsibility is unclear between subcontractors. Settle in writing who provides the main earth terminal, who bonds the risers, who bonds the containment, and who issues the test certificate — before the electrical installation starts going into the building.

Maintenance reality

Earthing is rarely revisited after handover. Corrosion at outdoor connections, damage during subsequent building works, and alterations that break continuity all accumulate silently. Periodic verification is worthwhile, particularly in coastal locations where corrosion is faster. Fit-out works are the usual culprit: a partition contractor cuts a section of metallic trunking, replaces it with plastic, and severs a continuity path nobody was told about.

Practical points

Keep connections accessible for inspection rather than burying them behind finishes, label bonding connections clearly, and re-test after any alteration that touches metallic services. Keep a set of measuring and inspection tools on site so verification happens when the work is done rather than months later. Requirements for conductor sizes, testing intervals and documentation follow the applicable wiring standard and the local authority; confirm which standard and which authority govern the project before writing the specification, because that decision drives everything else.