Bathroom Supplementary Bonding back to Main Earth Terminal

[asterisk]

Recently:

The bonding from the bathroom shouldn't go to the main CU. It should be contained with in the bathroom connecting pipework and electrical points together.

and

But in the UK, you don't earth it as such, the cable must not go back to the main earth terminal by the meter directly, rather it must only go to the earth conductors of all circuits that enter the bathroom, to introduce a separate earth brings problems of its own.
see the nic note on bathrooms linked in the reference section

(my emphasis)

However the NICEIC link from the reference section:
http://www.niceic.org.uk/downloads/NL139supp.pdf

The NICEIC is regularly asked whether BS 7671 requires a separately-run, independent, protective conductor to link the supplementary equipotential bonding in a bathroom or shower room with the Main Earthing Terminal (MET) of the installation. The answer is that no such independent protective conductor connection is required by BS 7671.

Why do these two electricians say shouldn't and must not, where as the NICEIC say is not required?
Is this just a case of Chinese Whispers where "is not required" becomes "don't" which becomes "mustn't"?

Also what are the extra problems associated with having this seperate earth?

Regards to all
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[Damocles]

In theory anyway, it is possible for some fault to develop where some piece of metalwork going into the bathroom becomes live. It would then be bringing voltage into the bathroom onto the metalwork you might be touching. Despite the bonding in the bathroom this might somehow cause a shock risk. So the theory goes that you should not have any extra connections into the bathroom at all, only ones which are absolutely necessary. Any power going in should already have its own earth associated with it.

Once upon a time bonding back to CU was recomended. No one has banned it as far as I know.

 

[plugwash]

adding a connection from the bathroom bonding to the main earth may make you slighly safer or slightly less safe depending on the situation.

the main advantagage to doing it is to reduce the shock voltage on pipes leaving the bathroom relative to earthed metalwork that is not connected to the bathroom bonding

the main disadvantage to doing so is that it may slightly increase the voltage between items in the bathroom by providing a better earth path and therefore allowing more current to flow. (though this extra current flow can also be an advantage wehn it comes to making sure breakers trip fast)

in reality it will almost certainly make negligabile difference either way

another situation in which it might be a good idea is if you have a high current feed (say for a shower) which happens to take an unusually long path to the bathroom and your lighting feed to the bathroom is unusually short. In this case it may be an idea to run a bonding core back to the CU to reduce fault currents down the lighting earth in the event of a fault on the high current cuircuit. (if the lighting cuircuit is the same length of longer than the shower cuircuit then its share of teh fault current will be kept to safe values by ohms law)

 

[mapj1]

If you must you can, and as stated the risk of doing it or not is so small as to make the exercise futile, but the theoretical risk is that of introducing a 'good' earth, say to the pipes bonded to the MET, and a poor one to a defective applience that in fault could be 50 or even 100V above true ground, and having a significant step voltage between them. The theory goes it is better to have it all at the same voltage, even if this is not so close to ground.
And no I don't read NIC guidence like gospel, the IEE regs are far less restrictive in general, the NIC are a bit recipe driven.
Also it is wrong (in my book) to perform un-necessarry actions that serve no clear beneficial purpose, they may add confuion to future users/maintainers, and introduce further connections that may later require maintanance or inspection, or be mis-used if their purpose was not realised .

 

[felix]

The theory goes it is better to have it all at the same voltage, even if this is not so close to ground.

OK, that certainly makes sense.

the theoretical risk is that of introducing a 'good' earth, say to the pipes bonded to the MET, and a poor one to a defective applience that in fault could be 50 or even 100V above true ground

And so does that.

Now for the contentious bit. The incoming water main is bonded to the MET with a short fat wire. That water main runs straight up into the bathroom. It's copper pipe with soldered joints all the way. That's got to be a very good earth!

I see only two ways out:

1) All the cross bonding connections within the bathroom must be very good indeed, much better in fact than that pipe. This will send most fault current straight down the pipe while also maintaining a low PD between the faulty appliance and the exposed pipe.

2) Cut a perfectly good copper pipe and put a plastic coupling into it. This will allow the pipe to float at the fault voltage while the fault current takes the more conventional route down the earth wire of the faulty appliance. The combination of cold, hot and CH pipes entering a typical bathroom will need a carefully worked out set of these deliberate breaks.

A third option, bonding the pipework in the bathroom only, fails dismally when their are two bathrooms. Which one gets the earth? And what about the kitchen?

If anybody cares to pick holes in this I'm all ears.

Also it is wrong (in my book) to perform un-necessarry actions that serve no clear beneficial purpose, they may add confuion to future users/maintainers, and introduce further connections that may later require maintanance or inspection, or be mis-used if their purpose was not realised .

Redoubled in No Trumps!

 

[ban-all-sheds]

Now for the contentious bit. The incoming water main is bonded to the MET with a short fat wire. That water main runs straight up into the bathroom. It's copper pipe with soldered joints all the way. That's got to be a very good earth!

Why is that contentious? You're right - it is a good earth, but that's not the point. Firstly, it might not always remain a good earth, and secondly the purpose of supplementary equipotential bonding is not to earth everything, per se, it is to ensure that all extraneous-conductive-parts and the cpc of all circuits supplying appliances in the bathroom are at the same potential. That's why those are the items which are bonded and why there is no requirement to take the supplementary equipotential bonding back to the main earth terminal.

I see only two ways out:

Two ways out of what?

1) All the cross bonding connections within the bathroom must be very good indeed, much better in fact than that pipe. This will send most fault current straight down the pipe while also maintaining a low PD between the faulty appliance and the exposed pipe.

They need to be good, but I'm not sure I understand the rest of that...

2) Cut a perfectly good copper pipe and put a plastic coupling into it. This will allow the pipe to float at the fault voltage while the fault current takes the more conventional route down the earth wire of the faulty appliance. The combination of cold, hot and CH pipes entering a typical bathroom will need a carefully worked out set of these deliberate breaks.

Depends what you mean by "coupling". A short length of plastic immediately makes the need for supplementary equipotential bonding much more acute, as you now have a poor path to earth (water conducts electricity, remember), which is the worst of all possible worlds. You need a length of at least 1m of plastic pipe, and preferably more to be sure, before you can consider the downstream pipes and radiators etc to no longer be extraneous-conductive-parts.

A third option, bonding the pipework in the bathroom only, fails dismally when their are two bathrooms. Which one gets the earth? And what about the kitchen?

Why does it fail dismally? Or at all? If you have 2 or 200 bathrooms the rules are exactly the same - in each one you install supplementary equipotential bonding between the extraneous-conductive-parts in the bathroom and the cpcs of circuits supplying appliances in the bathroom.

As for the kitchen, there is no requirement (any longer) to have supplementary equipotential bonding. Generally kitchens are not full of wet, naked people. If yours is then I'd like an invitation to your next party. I'll bring a bottle and a reel of 4mm² earth and some clamps....

 

[felix]

Actually BAS I think we're on the same side here, ie in favour of bonding in bathrooms, but I didn't put it all that well. I was trying to get my head around this one from Plugwash

the main disadvantage to doing so is that it may slightly increase the voltage between items in the bathroom by providing a better earth path and therefore allowing more current to flow.

which appears to be true. I then asked myself whether it's feasible to keep that voltage difference at a safe level when the pipe itself is almost a dead short back to the MET.

Doing nothing at all is clearly not an answer and what I meant by bonding the pipe in the bathroom only was to remove all bonds OUTSIDE the room. This is not going to solve the problem either but will create plenty more. Inserting a plastic coupling puts resistance into the pipe so that the bonding in the bathroom can pull the pipe voltage up to that of the faulty appliance while the fault current blasts its way down the earth wire.

I suppose my real question is this. Are we deluding ourselves by thinking that cross bonding will make our bathrooms safe or do we need those plastic bits as well?

 

[ban-all-sheds]

As well?

Instead of.

By far the safest environment is an earth-free one, so if you use lengths of plastic pipe (as I said, not just "couplers") to effectively remove the path from the exposed metalwork in the bathroom to earth (i.e. make it a high resistance), you can then remove the bonding, and you're better off.

As I've observed before, you wouldn't bond a metal toothbrush holder or towel hook, so why bond a radiator or a bath if it is not connected to an earth?

 

[felix]

By far the safest environment is an earth-free one, so if you use lengths of plastic pipe (as I said, not just "couplers") to effectively remove the path from the exposed metalwork in the bathroom to earth (i.e. make it a high resistance), you can then remove the bonding, and you're better off.

An (almost) fully floating bathroom. Yes, that makes the most sense of all.

 

[Lectrician]

I have only just read this topic.

NICEIC expand on the regs, and provide a very good technical reference. The regs do not say to link the supp bond to the MET under any section or any paragraph. They also do not say not to connect the live directly to the earthed back box of a socket

The term 'must not' and 'shouldn't' may be a little strong in the preceeding topics, but they are 100% correct - FACT.

If the regs where to print every 'do not' situation, then the print wouldn't cost £40, it would cost £4000

Just wire to the regs, dont go over the top, don't miss anything, and you will comply. Making stands on crap like this make the job ten times harder than it should be. No need to earth back to the MET, do it you wish - thats your look out, we have advised - end of.

 

[asterisk]

NICEIC expand on the regs, and provide a very good technical reference. The regs do not say to link the supp bond to the MET under any section or any paragraph. They also do not say not to connect the live directly to the earthed back box of a socket

No argument from me, nor anyone else I hope, on that score.

The term 'must not' and 'shouldn't' may be a little strong in the preceeding topics, but they are 100% correct - FACT.

There are certainly things to consider about doing this.

If the regs where to print every 'do not' situation, then the print wouldn't cost £40, it would cost £4000

Agreed. It would also make C&G 2381 a weight lifting course.

Just wire to the regs, dont go over the top, don't miss anything, and you will comply.

Agreed also.

Making stands on rubbish like this make the job ten times harder than it should be. No need to earth back to the MET, do it you wish - thats your look out, we have advised - end of.

I wasn't making a stand, nor was I being intentionally antagonistic to you or mapj1. I was curious at the disparity in language.

If NICEIC felt the need to comment as they get frequently asked this by their members then I'm not so sure it's rubbish.

The original question was even asked, though not by me, on the 2381 course I did, but the chap taking the course wasn't exactly sure why you shouldn't connect it back to the MET and would get back to us, but he didn't and we forgot to ask.

Anyway, I've been duly advised. Thanks all.

Regards
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