Main protective bonding: what it is and why it matters

Helpful video reference. Artisan Electrics' walkthrough "Consumer Unit Change | Earth Bonding | extra sockets" shows a working consumer unit replacement on a UK property, including the earth bonding connections being made and labelled. It is a practical demonstration of the points covered below.

1. Understand what main protective bonding does

The wiring in your home is connected to earth. If a live conductor touches the metal case of an appliance, the fault current flows to earth and trips the protective device. That system works well for faults within your wiring.

The problem is that metal services — gas pipes, water pipes, central heating pipes — run into your home from outside. In a fault condition, one of those pipes could become live at a different voltage from everything else in the property. If you were touching a gas pipe and a live fitting at the same moment, the voltage difference between them would pass through you.

Main protective bonding solves this by connecting all those metal services together and to the same earthing point. In a fault, everything rises to the same potential. There is no voltage difference between the services, so no current path through you.

2. Identify which services need bonding

BS 7671 Regulation 411.3.1.2 lists the extraneous-conductive-parts that must be bonded in a domestic property. These are:

• The incoming gas service pipe
• The incoming water service pipe
• Any oil supply pipe
• Structural steelwork that enters the building from outside
• Central heating or air conditioning pipework where it enters the building

Plastic pipes and plastic fittings do not conduct electricity, so bonding a plastic section achieves nothing. However, if there is metal pipework on the building side of a plastic meter, stopcock or water company isolation valve, that metal section is still an extraneous-conductive-part and still needs a bonding conductor.

3. Know the conductor size requirement

BS 7671 Table 54.8 sets the minimum main bonding conductor size as half the cross-sectional area of the supply neutral, with a floor of 6 mm² and a ceiling of 25 mm².

In practice, the standard UK domestic supply uses a 35 mm² neutral, which gives a minimum bonding conductor of 10 mm² copper. This is what you should see in any installation updated to the current edition of the wiring regulations. In older properties you may find 4 mm² or 6 mm² conductors that met earlier editions. These are typically recorded as a C3 (improvement recommended) or C2 (potentially dangerous) item on an EICR, depending on the assessor's judgement and the earthing system.

The conductor must be:

• Green-and-yellow insulated throughout its length
• Fitted with a bonding clamp at the service pipe end
• Terminated at the main earthing terminal (MET) inside the consumer unit
• Labelled at both ends with a bonding label reading "Safety electrical connection — do not remove"

4. Locate the clamps and the main earthing terminal

The bonding clamp on the gas pipe should be within 600 mm of the gas meter on the consumer side (or as close as practicable). On the water supply, the clamp goes on the metal pipework within 600 mm of the point where the pipe enters the property — on the consumer side of the main stopcock.

From the clamp, the green-and-yellow conductor runs back to the main earthing terminal. The MET is a brass block or busbar, usually fixed to the back or side of the consumer unit enclosure. Every earth and bonding conductor in the installation — including the incoming earth from the supply, the circuit protective conductors from each cable, and the bonding conductors from each service — terminates here.

On a well-installed consumer unit the MET is clearly labelled, each conductor is identified, and the connections are tight. On older fuseboards the MET is sometimes a single earth block on the back of an open-backed enclosure, and bonding conductors may not be labelled at all.

5. Spot signs of missing or disturbed bonding

Missing main bonding is one of the most frequently noted deficiencies on domestic EICRs. Common causes include:

• Kitchen or bathroom renovations where plumbers replaced metal pipework with plastic push-fit and cut the bonding clamp off with the old pipe
• Gas or water meter replacements where the bonding clamp was removed and not reinstated
• Consumer unit replacements by unqualified workers who omitted bonding connections
• Original bonding clamps on plastic isolation valves, which are non-conductive and therefore ineffective

You can check for main bonding yourself without touching anything. Trace the gas and water pipes from their entry points and look for a green-and-yellow conductor clamped within the first 600 mm. If you cannot find one — or if you find a green-and-yellow conductor that runs to nowhere or is clamped to a plastic fitting — note it down for your electrician.

6. Understand what changes during a consumer unit upgrade

A consumer unit replacement is the standard trigger for a full bonding review. Part P requires the work to comply with BS 7671, which means the electrician must inspect every bonding connection, check conductor sizes and remake or upgrade anything that does not comply.

In practice this means checking both the gas and water bonding clamps, confirming they are on metal pipework within 600 mm of the entry point, verifying the conductor size is at least 10 mm², and fitting bonding labels at both ends. On a property with oil supply pipework or structural steelwork, those services are included too.

If the property has a TT earthing system (an earth rod rather than a supply earth), the requirements are the same but the consequences of missing bonding are more severe because the earth rod impedance is higher and the prospective fault current is lower — meaning protective devices may be slower to operate.

When to call us

If your EICR has noted missing or undersized bonding, or if you have had pipework replaced and are not sure the bonding was reinstated, Richard can inspect and upgrade the bonding conductors to current BS 7671 requirements. This work is straightforward and is often done at the same time as a consumer unit upgrade.