Bonding / Earthing System

In everyday speech, earthing and bonding are used as if they meant the same thing, but technically they differ. Earthing connects the electrical installation to the ground through an earth electrode, for example a foundation earth in the concrete or a driven earth rod. Bonding means connecting all the metal parts in a building to each other and to that earth. When people talk about earthing the heating, they almost always mean bonding.

Why does it matter? Imagine the insulation fails inside a washing machine or a water heater and a live wire touches the metal casing. If the metal parts in the house are properly bonded, the fault current has a path to flow along. The circuit breaker or residual current device (RCD) switches the circuit off quickly. No voltage difference builds up between a radiator and a tap that could pass through a person touching both.

There are two levels. Main protective bonding connects the incoming water and gas services, the heating and any metal structures to the main earthing terminal near the consumer unit, at the point where they enter the building. Supplementary bonding is added in higher-risk rooms, typically a bathroom or boiler room, where metal parts close to each other are linked together.

This is work on electrical protection, so it is done by a qualified electrician, often together with the heating engineer. The process usually looks roughly like this:

1. Survey: the electrician checks whether the house has a main earthing terminal, where any existing bonding runs and which parts of the heating are metal.
2. Planning: they decide which parts need connecting and the route the conductors will take. In a new build, the electrical design covers this.
3. Fitting clamps: earth clamps are fixed onto cleaned metal pipework. They must sit firmly on bare metal.
4. Running conductors: a green and yellow conductor of the correct size runs from the clamps to the bonding terminal. It is fixed along its route and protected wherever it could be damaged.
5. Connecting the boiler and equipment: the boiler, hot water cylinder or manifold is connected according to the manufacturer's instructions.
6. Testing: the electrician tests the continuity of the protective conductors and the bonding. The results go into the test certificate or report.

Some older houses have no proper earth at all. If a new earth electrode is needed, digging and an earth resistance test are added. That is a separate electrical job.

These parts of a heating system are usually connected to the bonding:

• Metal central heating pipework where it enters the boiler room or near the manifold
• The metal frame or casing of the boiler, heat pump or electric heater
• A metal hot water cylinder and its connections
• Metal parts in the bathroom, if the design or the electrician's assessment requires it

Connections are made with earth clamps or straps made of a material that does not corrode against the pipe. Copper needs a different clamp from galvanised steel. With the wrong pairing, corrosion sets in and over time the joint stops conducting. The conductor has green and yellow insulation and a cross-section set by the local wiring regulations, which in most countries are based on the international IEC 60364 series. The electrician chooses the exact size depending on whether it is main or supplementary bonding.

Gas pipework has its own rules. The gas service often has an insulating joint that separates the outside section from the inside. Bonding is done after it, on the inside of the building, usually close to the meter. Never work on any part of the gas installation yourself. That is a job for a qualified, registered gas engineer. If you smell gas during the work, close the main gas valve, open windows and call the gas emergency line or the local emergency number, 112 in many countries.

It is simplest in a new build or during a full heating replacement. The pipes are exposed and the main earthing terminal is often near the boiler room. The electrician then fits a few clamps and runs short conductors.

More work is involved with:

• Old pipework that first has to be cleaned of paint and rust
• Pipes hidden in walls or floors that have to be reached
• A long distance between the boiler room and the consumer unit
• A house without a working earth, where an earth electrode has to be added
• A block of flats, where the risers are shared parts of the building and the work has to be agreed with the building management or owners' association

A common situation is a refit in which part of the steel pipework is replaced with plastic. Plastic does not conduct, so the original bonding path is broken. The metal sections on either side are then left unconnected, and the bonding has to be added again. If the bonding is part of a larger renovation, check with your local building authority what the work as a whole requires under local building regulations.

• A clamp fitted over paint or rust, so there is no conductive contact
• Bonding connected to the neutral conductor instead of the protective earth
• Nobody restores the broken bonding after pipework is changed to plastic
• An unsuitable pairing of metals at the joint, which leads to corrosion
• A conductor run loosely through a space where it can be knocked or damaged by damp
• Joints covered by tiling or boxing, so they cannot be checked during inspection
• A heating engineer bonding the system alone, without an electrician and without testing

Keep in mind that heating or water pipes must never be used as a substitute for the electrical earth. The heating is connected to the protective system, but it does not create that protection on its own.

Only a test confirms that the bonding works. You cannot judge it by eye. When the work is handed over, you can still check a few things yourself:

• Clamps are tightened firmly and sit on clean metal
• Conductors have green and yellow insulation and are fixed along their whole route
• The boiler, the cylinder and all agreed metal pipework are connected
• Joints remain accessible and are not built into walls
• Conductors at the bonding terminal are labelled or recorded
• The electrician has carried out a continuity test
• You have received a test certificate or report showing the electrician's name and qualification