What is Bonding
in Electrical Systems?
UK bonding explained. The difference between main protective bonding plus supplementary bonding, what BS 7671 requires plus where each conductor is fitted in 2026.
Bonding is the practice of connecting metallic parts that are not part of the electrical installation (gas pipes, water pipes, structural metalwork) to the earthing system using dedicated conductors. The goal is to keep all metalwork in the property at the same electrical potential during a fault so that nobody touching two metal items at once gets a shock. UK bonding splits into main protective bonding (the gas plus water bonds at the MET, BS 7671 411.3.1.2) plus supplementary bonding (used in specific locations like older bathrooms, BS 7671 415.2). The two have different cable sizes, different rules plus different test requirements.
Three figures
worth knowing
Main Bonding
The standard CSA for main protective bonding to gas plus water on a UK domestic install with a TN-C-S supply.
Supplementary Bonding
Typical CSA for supplementary bonding conductors. Mechanical protection (clipped or in conduit) reduces the requirement to 2.5mm sq.
BS 7671 Section
Section 411.3.1.2 covers main protective bonding. Section 415.2 covers supplementary bonding for additional protection.
Main bonding plus
supplementary bonding
Both types share a common goal of keeping metalwork at equal potential. The where, when plus how differ significantly between them.
Main Protective Bonding
BS 7671 Section 411.3.1.2What it is. A dedicated conductor running from the Main Earthing Terminal to each "extraneous-conductive-part" entering the building. The classic targets are the incoming gas service pipe plus the incoming water service pipe. Where structural metalwork (steel beams, oil tank piping, lightning protection downconductors) is exposed plus accessible, it must also be bonded.
Why it exists. Without main bonding the metallic services would float at an indeterminate potential during a fault. A person touching the gas tap plus the kitchen sink simultaneously could feel a shock voltage. Main bonding ties everything to the same reference at the MET so that even during a fault every metal object in the property sits at the same potential as every other.
Where it terminates. One end at the MET via a bolted connection. The other end on the gas or water pipe via a BS 951 earthing clamp within 600mm of the gas meter or stop tap, on the customer side of any insulating section.
Supplementary Bonding
BS 7671 Section 415.2What it is. Additional bonding conductors connecting metallic parts within a specific location (typically a bathroom or shower room) to each other plus to the CPC of any circuit serving that location. Common targets include taps, radiator pipes, structural steel that enters the room plus any exposed earth-bonded metalwork.
When it is needed. Required by 415.2 only where one of two conditions applies: (1) automatic disconnection of supply cannot be achieved within the BS 7671 disconnection times for the circuits serving the location; (2) all final circuits do not have 30mA RCD protection. On modern UK installs where every circuit is RCD or RCBO protected, supplementary bonding is normally not required for new bathrooms. Older bathrooms wired before RCD protection was universal almost always needed it.
Where it terminates. Each end on a metallic part within the location. The conductor itself is normally clipped or run in conduit for mechanical protection. No separate clamp standard required because it is not part of the main earthing system.
Equal potential is the safety goal
The whole rationale for bonding sits on a simple physics principle. An electric shock requires a potential difference. If two metal objects touched simultaneously by the same person are at the same voltage, no current flows through that person regardless of how high the voltage is above earth. Bonding works by deliberately tying metalwork together so any rise in potential affects every metal item equally.
The earth fault scenario
To see why bonding matters, picture an earth fault inside an appliance. A live conductor inside a washing machine has worn through its insulation plus is now touching the metal casing. The casing is connected to earth via the CPC (the green-yellow wire) which runs back to the consumer unit plus eventually to the MET. Earth fault current flows through that path. The MCB upstream detects the overcurrent plus trips. So far so good.
Now consider what happens during the few milliseconds before the MCB trips. The earth fault loop has some impedance which means there is a voltage drop along it. The MET sits at a slightly elevated potential above true earth. Anything connected to the MET (the consumer unit casing, the earth bar, every CPC in the property) shares that elevated potential.
The metallic gas pipe in the kitchen is not connected to the electrical installation. Without main bonding it sits at true earth potential. A person touching the washing machine casing plus the gas tap at the same moment briefly bridges the voltage difference between the two. With main bonding the gas pipe shares the elevated potential of the MET. The voltage difference disappears. The shock disappears with it.
Why supplementary bonding rules have changed
Older UK installs (pre-17th Edition) could not always rely on RCD protection. The fault disconnection time on a TN-C-S system without an RCD depends on the earth fault loop impedance which is harder to control in a wet environment like a bathroom. The historic answer was supplementary bonding within the room itself. By tying the taps, pipes, radiators plus light fittings together, the room became its own equipotential zone. Even if disconnection was slow, no significant voltage difference existed within reach of a wet person.
The 17th Edition (2008) made 30mA RCD protection mandatory on bathroom circuits. Once an RCD is present, disconnection happens within 40ms which is fast enough to prevent dangerous shock without the equipotential zone. BS 7671 415.2 now states supplementary bonding can be omitted where:
- All low voltage circuits in the location have automatic disconnection of supply meeting the times in Table 41.1.
- All low voltage circuits in the location have additional protection by a 30mA RCD.
- All extraneous-conductive-parts within the location are effectively connected to the main protective bonding.
On modern UK domestic installs all three conditions are normally met. Supplementary bonding in new bathrooms is therefore typically not required. Older bathrooms still have it from the original install. EICR engineers normally leave existing supplementary bonding in place because removal serves no benefit plus risks creating non-compliance.
Cable sizing fundamentals
BS 7671 Section 544 sets the cross-sectional areas for protective bonding:
- Main bonding for TN-S: half the CSA of the main earthing conductor with a minimum of 6mm sq plus a maximum of 25mm sq.
- Main bonding for TN-C-S (PME): based on the supply neutral CSA. Typically 10mm sq for a 25mm sq supply tail (the standard UK domestic case).
- Supplementary bonding: a function of the connected CPCs. Typically 2.5mm sq if mechanically protected, 4mm sq if not.
Five common locations
where bonding lives
Bonding conductors are normally tucked out of sight but follow predictable routes around UK domestic properties. These are the five most common locations.
Gas meter
A green-yellow conductor terminating on the incoming gas pipe via a BS 951 clamp. Within 600mm of the gas meter on the customer side. Travels back to the MET in conduit, capping or clipped along skirting. Always present on UK gas-supplied properties.
Water stop tap
A green-yellow conductor on the incoming water pipe via a BS 951 clamp. Within 600mm of the stop tap. Modern UK supplies often use plastic pipe at the entry which removes the bond requirement. Where copper or steel enters the building bonding is mandatory.
Structural steel
Where exposed accessible structural metalwork is present (steel beams in renovations, lightning protection downconductors on heritage buildings, external oil tank pipework) main bonding is required. Less common on standard UK domestic builds. Always required where present.
Older bathroom
A network of green-yellow conductors interconnecting taps, radiator pipes, exposed structural metal plus light fittings inside an older bathroom. Pre-2008 standard. Normally left in place during modern upgrades. Removal serves no benefit plus risks creating non-compliance.
The MET
The MET is where every main bonding conductor terminates. Inside the consumer unit on most modern installs or in the meter cabinet on older configurations. Look for the converging green-yellow conductors plus the SAFETY ELECTRICAL CONNECTION DO NOT REMOVE label.
Four things every homeowner
should know
Bonding is not earthing
Bonding ties metalwork to the earthing system. The earthing system itself is a separate concept that returns fault current to the supply.
Old main bonding often needs upgrading
Pre-2008 main bonding was sometimes 6mm sq. BS 7671 now requires 10mm sq on a UK domestic TN-C-S supply. Discovered routinely during board upgrades.
Plastic pipe removes the gas bond?
Only the section that is plastic. If the gas pipe is metal at the meter even on a plastic-supply property bonding is still required. Always check.
Existing bathroom bonds stay put
Modern bathrooms with full RCD protection do not need supplementary bonding. Existing bonds in older bathrooms should be left in place.
Earthing plus Bonding Upgrades in Milton Keynes
C-Lec Electrical inspects, replaces plus certifies main protective bonding to BS 7671 across Milton Keynes, Bedford plus the surrounding Bedfordshire area. Full EIC plus Building Control notification on completion.
For the wider context on consumer units, RCBOs, AFDDs plus the regulations behind UK distribution boards, head back to our full guide to consumer units where every common question is answered in one place.
Back to the Consumer Units Guide
This article sits inside our complete Consumer Units knowledge base. The hub covers everything from board types plus RCBOs through to landlord requirements plus BS 7671 wiring regulations.
More on consumer
unit anatomy
To understand the central earthing point that every main bonding conductor terminates at, head to what is the main earthing terminal for the practical breakdown of MET configurations plus cable sizes. To understand the BS 7671 framework around earthing plus bonding, see consumer unit wiring regulations. To understand how RCDs interact with bonding requirements (especially in bathrooms), see are RCDs legally required. If you need bonding work or a board upgrade in Milton Keynes or Bedford, our consumer unit upgrades service page is the fastest route to a quote.