Is supplementary bonding still required in bathrooms?

Not always. Since the 17th Edition of BS 7671, supplementary bonding in a bathroom can be omitted provided all circuits in the bathroom are protected by a 30 mA RCD and the main protective bonding of the incoming gas and water services is already correctly in place. However, if you cannot confirm both those conditions exist, supplementary bonding is still required.

What size cable is used for supplementary bonding in a bathroom?

Typically a 4 mm2 green/yellow earth conductor. Where the supplementary bonding connects directly between two extraneous-conductive-parts (not via the main earth terminal), BS 7671 requires it to be mechanically protected or at least 4 mm2.

Will an EICR flag missing supplementary bonding?

Yes. An EICR inspector will look for supplementary bonding in the bathroom. If it is absent and the circuits do not meet the RCD omission criteria, it will be coded C2 (potentially dangerous). If the omission criteria are met, the inspector should note it as satisfactory.

Can I install supplementary bonding myself?

The physical work of fitting earth clamps and running a green/yellow conductor between metal parts is not notifiable under Part P. However, the decision about whether bonding is required and confirming the RCD criteria are correctly met is best made by a qualified electrician, especially if you are preparing for a landlord certificate.

Bathroom Supplementary Bonding Explained (UK)

Helpful video reference. We use John Ward's clear explanation "Earthing & Bonding — Part 3: Supplementary Protective Equipotential Bonding" as the video reference for this guide. John is a UK electrician who covers the regulations precisely, including when supplementary bonding is and is not required. If you want to understand the theory before calling anyone out, his video is the best starting point.

Before you start. Checking existing bonding conductors is low-risk work, but any repair or installation of bonding should be done with the relevant circuits isolated. If you are doing continuity tests between parts, you do not need to isolate — just ensure no live parts are exposed. Do not disconnect a bonding conductor without understanding what it is connected to.

1. What supplementary bonding actually does

In a bathroom, the risk of electric shock is higher because wet skin has much lower resistance than dry skin, and metal pipework, taps, and fittings can introduce voltages into the space from external sources — a fault on the heating system, a fault in a neighbouring property's plumbing, static build-up, and similar. Supplementary bonding links all these metal parts together so they stay at the same electrical potential. If everything is at the same potential, there is no voltage difference between parts and therefore no path for current through a person.

This is separate from the main protective bonding at the consumer unit, which bonds the incoming water and gas services to the main earth. Supplementary bonding works within the room itself.

2. What counts as an extraneous-conductive-part in a bathroom

An extraneous-conductive-part is a metal part that is not part of the electrical installation but could carry a voltage into the room. In a typical UK bathroom that includes:

Metal water supply pipes (hot and cold)
Metal waste pipes
A metal bath frame or cast iron bath
A radiator or heated towel rail connected to the central heating pipework
Metal structural elements that pass through the floor or wall

Plastic pipes, plastic baths, and plastic waste systems do not need bonding — they cannot carry a voltage. This is one reason why modern bathrooms with plastic plumbing are often simpler to assess.

3. When supplementary bonding can be omitted

Since the 17th Edition of BS 7671 (2008), regulation 701.415.2 allows supplementary bonding to be omitted if both of the following conditions are met:

All circuits supplying the bathroom (lighting, shaver sockets, heated towel rail, extractor fan) are protected by a 30 mA RCD.
Main protective bonding of the incoming services (gas pipe and water main where they enter the building) has been correctly installed.

The key word is both. If even one circuit in the bathroom is not RCD-protected, or if the incoming services are not bonded at the consumer unit, supplementary bonding is still required. An older property with a split-load board where the bathroom lighting is on the non-RCD side would need supplementary bonding, for example.

If you are unsure which condition applies to your home, an EICR will clarify it.

4. What existing supplementary bonding looks like

Look for 4 mm² green/yellow conductors, typically clipped along the floor or skirting, connecting the pipes, radiator, and bath frame to each other. The connections are made with earth clamps, which bite through the pipe surface to make metal-to-metal contact. Clamps painted over, clamps on plastic inserts, or conductors hidden under floorboards are all things an EICR inspector looks for.

In older installations the conductors may run back to a local earth point rather than between the parts directly — both arrangements are acceptable as long as the connection is continuous.

5. Testing the bonding yourself

If you have a digital multimeter, set it to the lowest resistance range. Touch one probe to a metal pipe and the other to the metal bath frame. A reading close to zero ohms confirms they are bonded. Move the probes to cover all combinations: hot pipe to cold pipe, pipe to radiator, radiator to bath. Any high reading indicates a broken or corroded bond that needs making good.

Professional electricians use a low-resistance ohmmeter rather than a general multimeter, which is more accurate at very low resistances. But for a basic check, a multimeter is sufficient to spot an obvious open circuit.

6. Making good failed bonding connections

Corroded or loose earth clamps can be replaced without specialist tools. The new clamp should bite firmly onto bare metal — clean any paint or scale from the bonding point first. Use the correct clamp size for the pipe diameter, and ensure the 4 mm² conductor is correctly terminated in the clamp with no strand damage.

If you are tracing bonding and find the conductor is cut, hidden in a wall, or missing entirely, call an electrician. Running new bonding in an existing bathroom can mean lifting floorboards or drilling through joists, and it is worth having it done properly and documented.

Stop and call an electrician if: you cannot confirm which circuits in your bathroom are RCD-protected, the bonding conductors are missing entirely, you have a landlord property and the EICR has coded the bonding as C2 (potentially dangerous), or you are not confident interpreting the multimeter readings you are getting.

When to call us

Supplementary bonding queries come up regularly with landlord EICRs and property sales in east Kent. Richard can inspect the bathroom, confirm whether bonding is required or can be omitted, install or remediate bonding conductors, and issue the relevant Minor Works Certificate or Electrical Installation Certificate. It is usually a short visit.

EICR remedials in Sandwich and east Kent

Richard handles supplementary bonding installation and other EICR remedial work throughout east Kent, with clear documentation on completion.

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Bathroom supplementary bonding explained