Q & A of the Day – How do I supplementary bond this bathroom?

Question: I'm trying to understand how to carry out supplementary bonding. If I use my domestic bathroom as an example, I have a steel bath, a washbasin, a radiator (run from an oil fired central heating system, a shaver point and ceiling lights.

My understanding of how to run supplementary bonding is that I need to run a 4mm cable continuously between all the copper pipes and the bath to the earth connection in my light fitting. I cannot just run this cable back to the CPC bar in the consumer unit. As I have two lights in the bathroom (on the same circuit), do I need to run a second continuous wire from the pipes to the earth connection in that fitting too? And, as I have a shaver point, do I need to run a third continuous wire to the earth connection in this fitting?

So now I have three sets of supplementary bonding wires from the radiator and bath pipes etc., each one connected separately to the earth connection of my electrical circuits in the bathroom. It seems an awful lot of wires attached to the pipes, and if I had an electric shower, would I need a fourth wire going to the earth on that?

Could I, in fact, run one a single continuous wire from the radiator and bath pipes to a brass CPC block and run a single wire from each of the electrical devices earth to this block? Everything is connected but now I only have one wire running from the pipes to the radiator and bath. Or have I totally misunderstood to carry this out?

Note that in my bathroom, none of the circuits going into it are RCD-protected; just MCBs. The wiring is about nine years old, so is pre-17th Edition. Also the overall system is a TN-C-S.

Answer: As this is a domestic installation, and assuming that all circuits in the bathroom have supplementary protection using a 30mA RCD (which you say is not the case currently), if main bonding is provided to the main water and gas pipes, if applicable, and earth fault loop impedance values are within acceptable limits, the supplementary bonding may be omitted in these areas. See Regulation 701.415.2 BS7671 - 17th Edition.

Supplementary bonding may be required, and should connect together the earth connections of exposed conductive parts (electrical fittings ) and extraneous conductive parts (metal parts, water pipes etc) together. 4mm cables are normally used. These bonding conductors are not installed from the consumer unit.

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