Supplementary Bonding: When Is It Required and When Can It Be Omitted?

Supplementary bonding (also called supplementary equipotential bonding) is the practice of connecting exposed-conductive-parts and extraneous-conductive-parts within a specific location to each other, creating a local equipotential zone. The purpose is to reduce the potential difference (voltage) between metallic parts that a person might simultaneously touch, particularly in locations where the risk of electric shock is increased.

The most common location where supplementary bonding is discussed is the bathroom. In a bathroom, a person is often wet or standing on a wet floor — both of which significantly reduce body resistance and increase the severity of an electric shock. If a person simultaneously touches two metallic parts that are at different electrical potentials (for example, a metal tap connected to a water pipe and a metal towel rail connected to an earthed circuit), the potential difference between them could cause a dangerous current to flow through the body.

Supplementary bonding eliminates this risk by ensuring that all metallic parts within the room are at the same potential. If everything is at the same voltage, there can be no current flow between them — regardless of what voltage that might be relative to true earth.

This is different from main protective bonding, which connects services at the point of entry into the building. Supplementary bonding is additional, local bonding within a specific room. Under certain conditions in BS 7671, supplementary bonding in bathrooms can be omitted — but understanding when and why is essential.

Regulation 415.2 of BS 7671:2018 is the general regulation for supplementary equipotential bonding. It states that where the conditions for automatic disconnection of supply cannot be met (specifically, where disconnection times under Regulation 411.3.2.2 or 411.3.2.4 cannot be achieved), supplementary equipotential bonding must be provided.

In the context of bathrooms, the specific regulation is 701.415.2, which provides a modified set of rules for bathroom locations. The interaction between these regulations determines whether supplementary bonding is required in any given bathroom.

The 50V/Ia formula from Regulation 415.2 provides a way to verify the effectiveness of supplementary bonding by testing. If the resistance between two simultaneously accessible bonded parts is low enough that the touch voltage cannot exceed 50V, the bonding is effective. In practice, this means the resistance measured between bonded parts should be very low — typically less than 0.05 ohms.

Supplementary bonding is required in the following situations:

In practice, most modern domestic installations with RCBO consumer units or split-load boards with RCD protection on all circuits will meet the omission conditions for bathrooms. However, older installations with rewireable fuseboards or MCB-only boards without RCD protection will almost always require supplementary bonding in the bathroom.

Under Regulation 701.415.2, supplementary bonding in a bathroom can be omitted if all three of the following conditions are met:

If any one of these conditions is not met, supplementary bonding must be provided. During a periodic inspection (EICR), you must assess all three conditions before deciding whether supplementary bonding is required or can be validly omitted. If supplementary bonding has been omitted but the conditions for omission are not all met, record this as a C2 observation.

A common mistake is assuming that RCD protection alone is sufficient to omit supplementary bonding. All three conditions must be met — not just the RCD condition. Check the main bonding carefully before deciding that supplementary bonding can be left out.

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Bathrooms are where supplementary bonding questions arise most frequently. Here is the practical decision process for a typical domestic bathroom:

During periodic inspections, check the bathroom bonding as part of the standard inspection schedule. Record whether supplementary bonding is present, absent, or not required (with the reason). If bonding is required but absent, record as C2 (Potentially Dangerous).

The conductor size for supplementary bonding is set out in Regulation 544.2 of BS 7671. The rules are:

The simplest approach is to use 4 mm green/yellow single-core cable for all supplementary bonding connections. This meets the requirement for both extraneous-to-extraneous and exposed-to-extraneous connections, regardless of mechanical protection. It avoids any need to calculate conductor sizes or assess mechanical protection — just use 4 mm and it complies.

The 4 mm cable is readily available, inexpensive, and easy to terminate. Use BS 951 bonding clamps on pipes and standard screw terminals on electrical equipment earth terminals. Where the cable is surface-run, clip it neatly using green/yellow saddle clips.

Installing supplementary bonding in a bathroom is a straightforward task. Here is the step-by-step process:

The entire installation for a typical bathroom takes 30 to 60 minutes and uses a few metres of 4 mm cable plus a handful of bonding clamps. It is a small job but a critical one — supplementary bonding can be the difference between a safe bathroom and a lethal one.

Testing supplementary bonding is part of the standard inspection and testing sequence, both for initial verification of new work and for periodic inspection (EICR).

Record the supplementary bonding test results on the EICR or EIC. Note which parts are bonded, the measured resistance, and the conductor size. If supplementary bonding is absent where required, record it as C2. If the bonding is present but deteriorated (corroded clamps, loose connections), record the specific defect with the appropriate classification.

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