What is main bonding and earthing, and does my house have it?

These two words get used interchangeably and they do different jobs. Earthing is about making faults disconnect. Bonding is about making sure that while a fault exists, there is no dangerous voltage between two things you might touch at once. Both are invisible in normal use and both are commonly compromised by later plumbing and kitchen work.

Step by step

1. Understand earthing.Every circuit has a circuit protective conductor running back to the main earthing terminal. If a live conductor touches an earthed metal part, current flows through that path, and because the path has low resistance the current is large enough to operate the MCB or RCD almost immediately. Without it, the metalwork simply sits live.
2. Understand main protective bonding.Separate green-and-yellow conductors run from the main earthing terminal to the incoming gas pipe and the incoming water pipe, usually clamped within about 600mm of the meter or the point of entry. They ensure the pipework cannot be at a different potential from the electrical earth.
3. Look for the bonding conductors.Follow the incoming water pipe from the stopcock and the gas pipe from the meter. You are looking for a green-and-yellow cable clamped on with a brass clamp carrying a label saying "Safety Electrical Connection — Do Not Remove". Their absence is a common and significant finding.
4. Note the size, but do not judge it yourself.Modern requirements commonly call for 10mm bonding conductors on a typical domestic supply, and older houses often have 6mm or 4mm, which may now be inadequate. What is required depends on the supply type and the size of the incoming tails, so it is a judgement for the electrician.
5. Identify your earthing arrangement if you can.TN-C-S or PME, where the earth comes from the supply neutral, is now the most common. TN-S has a separate earth from the supply cable's sheath. TT has no supply earth at all and relies on an earth rod at the property, and requires RCD protection throughout. Your EICR states which you have, and it changes what is required elsewhere.
6. Watch for the plastic pipe problem.Where copper has been replaced with plastic, sections of metal pipework can end up electrically isolated. Bonding to a metal pipe that is no longer continuous back to the incoming service does nothing, and an isolated metal section can float. This is a growing cause of tingles from taps and radiators.
7. Watch for bonding removed during work.Kitchen refits, bathroom refits, boiler replacements and meter moves all involve disturbing pipework near where bonding clamps live. Conductors get cut off and not replaced. If work has been done, check the clamps are still there.
8. Get it verified rather than assumed.Presence is visible; adequacy is not. Continuity and resistance of the bonding, the earthing arrangement and the earth fault loop impedance are all measured at an EICR. That is the only way to know it works.

Tips

• Photograph the bonding clamps at the gas and water entries. If a tradesperson later cuts one off, you have evidence of what was there.
• Never remove or loosen a conductor with that yellow label on it, even temporarily. If work requires it to be moved, an electrician does it and reinstates it.
• If your house has an earth rod outside and no supply earth, you have a TT system, and RCD protection throughout is not optional — it is what makes the installation safe.

Common mistakes

• Assuming an earth wire on a radiator or a tap counts as bonding — Main bonding connects the incoming services near their point of entry, not individual fittings. A wire clamped to a radiator is usually leftover supplementary bonding and proves nothing about the main bonding.
• Removing a bonding clamp during plumbing work and not reinstating it — It leaves the pipework unbonded, which is exactly the condition that produces a potential difference between a tap and the floor. It is one of the commonest serious findings on an EICR.
• Judging bonding adequacy by looking at it — A conductor can be present, correctly sized and still ineffective because of a corroded clamp or a break somewhere along its run. Only testing proves continuity.

If it doesn't work

Questions people ask