Steel Bath - Equipotential Bonding versus RDBOs

[Richard Conway]

Hi all,

On Friday I will be fitting a new bath which for reasons unrelated to this thread (well, not entirely I suppose) will be made of steel.

Our bathroom currently has no equipotential bonding, and I am aware that in order to comply with the regs I will either need to install equipotential bonding or upgrade my installation to meet the 17th edition regs.

Now, I'm aware that there is probably more to the 17th Edition than simply sticking some RCBOs in my CU, but I was wondering if that would be enough to nagate the need for me to install equipotential bonding.

At the moment, my CU is a split load that consists of the following (please bear in mind it's a small terraced house):

RCD Side Whole house sockets (ring) Kitchen Sockets (ring) Oven (radial) Immersion Heater (radial)

Non-RCD side Upstairs lighting (including bathroom fan) Downstairs lighting

Now, I understand the dangers of putting either of the lighting circuits on the RCD side, but what if I replced them with RCBOs? Would this be enough to not require equipotential bonding as there would be no circuits in the house without RCD protection.

Any thoughts/opinions welcomed.

Thanks, Richard.

[BruceB]

If you are installing a bath and not touching the electrics then there is no

*requirement* to add supplementary equipotential bonding.

If you are also modifying electrics then the modifications will have to be to the 17th edition (which requires rcds in bathrooms); there is no option to do it to 16th standards instead, assuming this has not being in progress for a few years. But even so, it is unlikely that a steel bath would need to be bonded as it is unlikely to be an extraneous conductive part. The water pipes supplying it or waste away from it if either are metal might be extraneous and need bonding.

Why do you think it needs to be bonded?

Regards Bruce

[John Rumm]

A new metal bath in unlikely to make the situation worse...

As long as you main equipotential bonding is up to scratch then you could replace the lighting MCB with a 30mA/6A trip RCBO. (I am presuming the downstairs lighting circuit does not enter the bathroom).

[Richard Conway]

The packaging the bath came in had several largely printed warnings that it must be earthed.

[Richard Conway]

The downstairs lighting circuit does not enter the bathroom, but it does travel in proximity to the pipework in some places, which I thought was one of the reasons behind equipotential bonding in the bathroom - i.e. to prevent a large potential difference occuring if, say, a tap were to become live as a result of it's supplying pipework coming into contact with a faulty cable etc.

[george (dicegeorge)]

. there should be a bonding wire onto the plumbing somewhere, i think there's a problem when some of the metal pipe is replaced with plastic, so the bonding doesnt reach the bath..

[g]

[BruceB]

Good answer! But I would not consider a bath manufacturer as definitive on electrical matters!

I have re-read 701.415.2. There is no specific requirement for a metal bath to have supplementary equipotential bonding. But if it was an extraneous conductive part it would need to be bonded. Metallic pipes coming into the location need to be bonded if they are extraneous.

But the regulation goes on to say supplementary equipotential bonding can be omitted provided a number of conditions are met:

- main protective equipotential bonding is in place (ie water, gas etc)

- all final circuits in bathroom meet appropriate disconnection times

- all final circuits in bathroom have rcd protection

(I have simplified a bit)

If you are not altering the electrics then my inclination would be not to start adding new bonding.

Regards Bruce

[John Rumm]

The bonding is intended to limit the potential difference you can experience as a result of conductive components introducing dangerous voltages into the zone. Normally one would only include the CPCs of a circuit that actually enters the bathroom within the bonding.

However, if you think the layout of the pipework/wiring is such that there would be a real possibility of the pipework making contact with the downstairs lighting circuit, then you would be better off installing supplementary bonding in the bathroom. (in reality the main bonding would hopefully ensure that the existing MCB on the downstairs lighting circuit would open should such a fault occur)

[John Rumm]

There is no requirement to bond the bath since it can't by itself bring a voltage into the zone. Typically the pipes feeding the taps, the waste connection (if metal), basin taps, CH pipework, and the CPCs of any circuits used in the room would be bonded.

[Richard Conway]

The main bonding is another issue I've meant to ask about in the past. The incoming gas pipework is connected just after the meter, but the water supply pipework isn't connected at all - this appears to be because the pipework immediately before and after the stopcock is plastic and goes all the way up to the loft to serve the header tanks. The original copper pipework is then T'd off this plastic and comes back down to serve the bathroom/kitchen.

The most sensible way I can see of bonding this would be in the loft - but that isn't really feasible as there's no easy route back to the meter.

Any thoughts?

[John Rumm]

Two points: Firstly the rules regarding bonding of baths (and metal shower trays) have changed. The 16th edition used to list them as something requiring bonding (this I expect was actually in error since a bath, can't bring a potential into a zone). The 17th edition has removed this anomaly.

Secondly, the requirement that existed previously was for supplementary equipotential bonding and NOT earthing[1].

So the instructions from the bath manufacturer are not only out of date but at best were sloppily worded.

[1] The purposes of which, and the function of each are different: