wiring
Earthing and main protective bonding
Bringing bonding up to standard costs £150 to £400. It is the least visible work an electrician does and among the most important: bonding is what makes the metalwork in a house sit at the same voltage, so a fault has nothing to push a current through you to reach.
What this covers
What it costs. Bonding brought up to standard is £150 to £400. Full cost breakdown.
What the job covers
- Running 10mm bonding to incoming gas and water
- Fitting an earth rod on a TT installation
- Supplementary bonding in bathrooms where required
- Correcting a missing or undersized main earthing conductor
- Checking earth loop impedance across the installation
What is involved
Two separate things get confused. Main protective bonding is a heavy conductor, usually 10mm, from the main earthing terminal to the incoming gas and water services, clamped within a short distance of where they enter the building. Supplementary bonding is smaller conductors tying metalwork together within a room, historically in bathrooms. The first is required on essentially every installation. The second can be left out where the installation meets a set of conditions, which is why a modern bathroom often has none and an older one has it everywhere.
The earthing arrangement decides a good deal else. Most UK supplies are PME, where the earth comes from the supply neutral, and it is reliable and cheap and it has one consequence: a fault on the supply neutral can put a voltage on anything earthed. That is why an EV charger needs open PEN detection or an earth rod, and why bonding sizes on a PME supply are larger. A TT installation has no supply earth at all and relies on its own rod, which makes residual current protection not a refinement but the only thing standing between a fault and the person who finds it.
This is the commonest C2 on an electrical installation condition report, and it is usually cheap to fix. Bonding missing at the gas meter, bonding present but undersized because it was run when 6mm was acceptable, or a clamp fitted to a pipe that has since been replaced with plastic and now bonds nothing. None of it is visible from the middle of a room and all of it takes an afternoon.
Choosing someone for the job
Ask what earthing arrangement the property has, because it changes the answer to almost everything else. Ask whether the bonding conductors are sized for that arrangement rather than for what was acceptable when they went in. If the gas or water pipework has been replaced in plastic, say so, because a clamp on a plastic pipe is bonding nothing.
Common questions
What is the difference between earthing and bonding?
Earthing gives a fault current a path back so a protective device operates. Bonding ties metalwork together so that during a fault everything you can touch is at the same voltage. They are different jobs and both are needed.
My bathroom has no supplementary bonding. Is that wrong?
Not necessarily. It can be left out where the installation meets a set of conditions, which most modern ones do. An older installation that does not meet them needs it, which is why some bathrooms have it and some do not.