Bathroom supplementary bonding

Helpful video reference. John Ward (jwflame), a UK electrician based in Dorset, covers the supplementary bonding question in his video "Supplementary Bonding In Bathrooms - Is It Required?". His explanation of the exemption conditions in BS 7671 Section 701 is the clearest available on YouTube.

1. What supplementary bonding is

Supplementary bonding connects metallic parts within a bathroom — the bath, basin taps, exposed water pipes, metal shower fittings, heating pipes — to each other and to the protective earth. The purpose is to equalise potential between those parts so that if a fault occurs, there is no dangerous voltage difference between things a person might touch simultaneously.

It is separate from main protective bonding (the conductors connecting gas and water incoming services to the consumer unit earth). Supplementary bonding is local to the bathroom.

2. When it can be omitted (the modern position)

BS 7671:2018 Section 701.415.2 states that supplementary bonding may be omitted where all of the following apply:

• All circuits in the bathroom are protected by a 30 mA RCD
• All final circuits meet the disconnection time requirements of BS 7671
• All metal parts are connected to the protective earthing system via their own circuit earth conductors (for example, an electric towel rail is earthed via its supply cable)

In a modern bathroom with RCBO-protected circuits throughout, supplementary bonding is almost always unnecessary. The RCBO provides RCD protection on each individual circuit, and all circuits satisfy the disconnection requirement.

3. When it is still required

Supplementary bonding remains required if any circuit in the bathroom does NOT have 30 mA RCD protection. This includes:

• Older split-load boards where some circuits sit behind a main switch but not an RCD
• Circuits protected only by an MCB with no RCD upstream
• Any new circuit added without appropriate RCD protection

In these cases, the bonding conductors must connect all simultaneously accessible metallic parts.

4. What the bonding conductor looks like

Supplementary bonding conductors are 4 mm² green-and-yellow sleeved copper wire (or 2.5 mm² if mechanically protected). They connect from a clamp on the metallic part to a local earth terminal or directly to another bonded part. The connections must be accessible for inspection — they cannot be buried without a maintenance-free enclosure.

During an EICR, the inspector will check whether bonding is present and test its continuity with a low-resistance ohmmeter. A missing bonding conductor where one is required is a C2 observation (potentially dangerous).

5. Plastic pipes and non-metallic fittings

Plastic water pipes do not need bonding — they cannot conduct electricity. If all your water pipework entering the bathroom is plastic, there may be nothing to bond even if the circuit protection does not meet the exemption conditions. However, if a metal bath, metal basin or metal shower tray is present, those are still extraneous-conductive-parts that must be assessed.