6.3 Main & Supplementary Protective Bonding Continuity

Main Protective Bonding

Main protective bonding connects the main earthing terminal (MET) of the installation to extraneous-conductive-parts that enter the building — metallic water service pipes, gas service pipes, oil supply pipes, structural steelwork, and lightning protection system down-conductors. Its purpose is to minimise dangerous potential differences between exposed-conductive-parts of the installation and extraneous metalwork under fault conditions, so that anyone touching two metal parts simultaneously does not receive a shock from a voltage appearing between them.

The key word is extraneous — a part that is conductive, enters the building from outside, and can introduce a potential (including earth potential). A metal pipe that enters the building is extraneous; a metal trunking system entirely contained within the installation is not extraneous (it is an exposed-conductive-part and is earthed, not bonded, to the MET). Deciding what is extraneous is an inspection judgement made before the continuity test.

What to Test

Main bonding continuity is verified from the MET to each extraneous-conductive-part where the bonding conductor terminates. Every main bonding conductor must be tested:

The Long-Lead (Wander-Lead) Method

You cannot link both ends of a bonding conductor the way you link phase and CPC for an R1+R2 test — the "far end" is a buried metal pipe or a structural frame, not an accessory you can get back into. The standard method is the long-lead (wander-lead) method:

1. Null the long lead — connect the long lead to the standard lead and record or zero its resistance. A 10 m wander lead can add 0.3 to 0.5 Ω; not nulling it makes every reading falsely high.
2. Connect one instrument lead to the bonding conductor at the MET.
3. Run the long lead to the far end of the extraneous-conductive-part and clip or probe it there.
4. Measure — the reading is the resistance of the bonding conductor plus any connection resistance at the far end.

The same low-resistance ohmmeter is used as for protective-conductor continuity (test current ≥ 0.2 A, resolution ≤ 0.01 Ω).

Acceptance Value

GN3 10th Edition gives a practical target of 0.05 Ω for main bonding continuity — a low value confirming a sound, low-resistance connection. This is not a fixed BS 7671 pass/fail figure (there is no single tabulated maximum for bonding conductor resistance); the requirement is that the bonding connection is continuous and of low resistance, and 0.05 Ω is the recognised on-site benchmark. A reading significantly above 0.05 Ω suggests a loose connection, corrosion, or an undersized conductor and must be investigated before the installation is certified.

Main Bonding Conductor Sizes

Two different rules apply, and confusing them is a classic exam trap.

Non-PME supplies (TN-S, TT) are sized by Regulation 544.1.1: the main protective bonding conductor must have a cross-sectional area of not less than half that of the earthing conductor, subject to a minimum of 6 mm², and it need not exceed 25 mm² where the conductor is copper.

PME (TN-C-S) supplies are sized from the copper-equivalent cross-sectional area of the supply neutral (PEN) conductor, using Table 54.8:

The conductor never has to exceed 50 mm² copper or copper-equivalent, and the copper-equivalent conversion must be done first where the supply neutral is aluminium.

Note which conductor drives the PME calculation: it is the neutral of the supply, not the line conductor and not the consumer tails you can see most easily. Do not confuse Table 54.8 with Table 54.7, which gives the minimum size of a buried earthing conductor according to whether it is protected against corrosion and mechanical damage.

For a typical domestic PME (TN-C-S) supply with a 25 mm² neutral, the bonding conductor must therefore be at least 10 mm² copper (or copper-equivalent) because the PME earthed-neutral can carry diverted neutral currents, and the bonding must be robust enough to handle them safely. Undersized PME bonding is a serious defect and a common EICR observation, typically coded C2 (potentially dangerous). Always confirm the system type (TN-S, TN-C-S, or TT) from the supply characteristic before selecting or verifying bonding conductor sizes.

Supplementary Equipotential Bonding

Supplementary equipotential bonding connects exposed-conductive-parts and extraneous-conductive-parts within a particular location — most commonly a bathroom, shower room, swimming pool, or other special location. Its purpose is to reduce touch voltage further within that local zone, supplementing the main bonding where the disconnection time cannot be met or where the location adds extra risk (e.g. wet skin lowers body resistance).

Supplementary bonding is tested for continuity between the bonded parts within the location: for example, between the exposed metalwork of a bathroom luminaire and the bonded metallic pipework, or between the shower tray waste and any exposed structural metal. The same low-resistance ohmmeter is used, with leads nulled, and the same 0.05 Ω practical target applies.

Main Bonding vs Supplementary Bonding

Why Bonding Continuity Matters

Both main and supplementary bonding exist to hold an equipotential zone — a volume in which all conductive parts that a person could touch simultaneously are at substantially the same potential. If bonding continuity is lost, a fault can raise one metal part to a dangerous potential while a nearby part remains at earth potential, and the touch voltage between them can be lethal. The continuity test verifies that the equipotential zone is intact across every bonded part.

Key Points to Remember

• Main bonding connects the MET to extraneous-conductive-parts entering the building; supplementary bonding is local to a location.
• Use the long-lead method; null the long lead first — its resistance is much higher than a standard lead.
• 0.05 Ω is the GN3 practical target for bonding continuity — low, but not a fixed BS 7671 pass/fail.
• Reg 544.1.1 gives 6 mm² minimum for non-PME; Table 54.8 gives 10 mm² minimum for PME (TN-C-S) with a supply neutral of 35 mm² or less.
• Undersized or discontinuous PME bonding is a serious defect, typically coded C2 on an EICR.