Earthing and bonding

Main protective bonding connects incoming metallic services — typically the gas and water pipes — to the main earthing terminal, so that everything in the property stays at a similar potential during a fault. You can usually see it as a green-and-yellow cable with a labelled clamp near the gas meter and stopcock.

Missing or undersized main bonding is one of the most frequently recorded EICR observations, and it is often required before other work can be considered complete.

TN-S takes the earth from the sheath of the supply cable. TN-C-S (PME) takes it from the supply neutral and is the most common arrangement in modern UK housing. TT has no earth from the network at all and relies on an earth electrode at the property, which is typical of rural and some outbuilding supplies.

The system determines bonding requirements, the protective devices needed and, notably, how an EV chargepoint must be arranged.

An earth electrode is required where the property is on a TT system, and is sometimes used for outbuildings or specific installations where the supply earth cannot be exported. Its effectiveness depends on ground conditions and it has to be tested, not just installed.

Modern EV chargepoints with integral open-PEN protection have largely removed the need for a rod at chargepoints, which used to be standard practice.

Bonding applies to metallic services that could introduce a potential into the building. Where the incoming service is entirely plastic, main bonding to that service may not be required — but if there is metal pipework beyond a plastic incomer, the assessment is more nuanced.

This is decided by inspection at the property, not by a general rule, and it is a common reason a report defers judgement until the incoming service can be seen.