Cross bonding in bathroom?

[jcp]

I'm not getting much response on the plumbing forum, so trying here instead.

While replacing a bathroom suite, I've discovered there is no cross bonding in place. The house is 1990 vintage, and the consumer unit has MCBs and an RCD on just the socket circuits (lights, cooker, etc not protected).

Should I add cross-bonding at the basin and (plastic) bath?

There is a heated towel-rail, but it is isolated by a short length (about 30cms) of plastic pipe being used to take the heating pipes through a seriously contorted series of bends as it goes through the bathroom wall.

There is also a ceiling light with cord switch, and a ceiling extractor fan, but no shaver socket.

Any advice would be much appreciated.

 

[EFLImpudence]

There isn't a yes or no answer. It will depend on the measurements.

It is 'supplementary bonding' not 'cross bonding' because it is supplementary to the main bonding which must be in place.
You seem to understand that isolated parts must not be bonded.



I wrote this for another recent thread.


An Insulation Test should be carried out first between parts and the MET.
If MegaOhm reading recorded then MUST NOT BE BONDED whether RCD protected or not.

If low IR, that is 0.00MΩ, then a continuity test.
If more than 23 kΩ then MUST NOT BE BONDED whether RCD protected or not.

Then testing BETWEEN simultaneously accessible parts NOT excluded by the above is required to determine if supplementary bonding is required.

If R < 50 / Ia then supplementary bonding is not required anyway (but will not hurt).
(Ia = 5 x the highest circuit OPD rating of the location)

 

[JohnW2]

I wrote this for another recent thread ...
An Insulation Test should be carried out first between parts and the MET.
If MegaOhm reading recorded then MUST NOT BE BONDED whether RCD protected or not.
If low IR, that is 0.00M&#937;, then a continuity test.
If more than 23 k&#937; then MUST NOT BE BONDED whether RCD protected or not.

I've been meaning to ask you - what is the point in the initial IR test you are telling people is required? All one is wanting to ascertain is whether or not the resistance between the part and MET is greater than 23 k&#937;, and one can get that answer by going straight to the standard resistance (what you call 'continuity') test.

Kind Regards, John

 

[jcp]

Thanks for that helpful reply.

AFAIU, supplementary bonding is a failsafe in case the earth is continuous on one extraneous metallic feature but not on another - that could go ‘live’ due to some fault elsewhere.

I know that the incoming water main to the house is plastic, but I believe there is a bond from the main brass stop cock to the consumer unit. There is supplementary bonding at the kitchen sink and the washing machine. Gas and a combi boiler were installed about 3 years ago; the gas main is bonded back to the consumer unit, and there is supplementary bonding between the gas, heating and hot water pipes. All these bonding points are in the kitchen, and within 6 feet of each other!

All the water pipes inside the building are copper, with either solder or compression joints - apart from that short section of plastic pipe to the bathroom towel rail.

Given the age of the property, I was surprised, when installing isolation valves for the new bathroom suite, to find there was currently no supplementary bonding. I have not yet tried to measure the resistance (using my Fluke) between the bathroom and MET, but I would be surprised if it were not very small indeed.

There is really no circuit wiring into the bathroom – the light fitting, switch, and extractor fan are enclosed (but I suppose the light shade can be unscrewed to change the light bulb).

In principal, should I just leave the pipes without supplementary bonding or not?

*****
p.s. I have just realised that the bath and basin taps are both ‘mixers’, so there will be default supplementary bonding between the pipes - through the tap bodies (though the tap tails are braided flexible pipe).

 

[EFLImpudence]

AFAIU, supplementary bonding is a failsafe in case the earth is continuous on one extraneous metallic feature but not on another - that could go ‘live’ due to some fault elsewhere.

No, it just ensures the potential difference is limited to 50V maximum in a fault situation.

I know that the incoming water main to the house is plastic, but I believe there is a bond from the main brass stop cock to the consumer unit.

Yes there should be

There is supplementary bonding at the kitchen sink and the washing machine.

That is unnecessary and may make the sink more hazardous.
Supplementary bonding is only required in special locations - bathroom etc.

Gas and a combi boiler were installed about 3 years ago; the gas main is bonded back to the consumer unit,

Correct.

and there is supplementary bonding between the gas, heating and hot water pipes. All these bonding points are in the kitchen, and within 6 feet of each other!

All unnecessary but plumbers like doing it.
That is what they normally call 'cross bonding'

All the water pipes inside the building are copper, with either solder or compression joints - apart from that short section of plastic pipe to the bathroom towel rail.

Given the age of the property, I was surprised, when installing isolation valves for the new bathroom suite, to find there was currently no supplementary bonding. I have not yet tried to measure the resistance (using my Fluke) between the bathroom and MET, but I would be surprised if it were not very small indeed.

That is very likely.
The measurement BETWEEN parts may be so low that it is not required.

There is really no circuit wiring into the bathroom – the light fitting, switch, and extractor fan are enclosed (but I suppose the light shade can be unscrewed to change the light bulb).
In principal, should I just leave the pipes without supplementary bonding or not?

If the ( 6A ? ) lighting circuit is the only one in the bathroom then a resistance of under 1.67&#937; ( 50/(6x5) ) between simultaneously accessible extraneous parts means it is not required.

p.s. I have just realised that the bath and basin taps are both ‘mixers’, so there will be default supplementary bonding between the pipes - through the tap bodies (though the tap tails are braided flexible pipe)

Yes. so it is extremely unlikely that resistance between the pipes will be over 1.67&#937;.

 

[EFLImpudence]

I've been meaning to ask you - what is the point in the initial IR test you are telling people is required? All one is wanting to ascertain is whether or not the resistance between the part and MET is greater than 23 k&#937;, and one can get that answer by going straight to the standard resistance (what you call 'continuity') test.

This has come a lot later than I expected.

What you say makes sense but that is the best way to test.
It's good to know that it is >1000M&#937;.

As I keep saying 23k&#937; is just the figure derived from 10mA and, in a way, irrelevant as readings will usually be a few ohms or many megaohms.
However, should the measurement be 0.00M&#937; a check is obviously required.

You may think telling people who don't know to do it may be a bit pointless but it's not really.

 

[EFLImpudence]

jcp

I should add that if you remove any of the 'unnecessary' bonding at the boiler it may affect the measurements in the bathroom if it is nearby.
The bond to the kitchen sink should definitely be removed.

Supplementary bonding FOR the bathroom doesn't have to be IN the bathroom.

I very much doubt this is the case but you never know.

 

[JohnW2]

I've been meaning to ask you - what is the point in the initial IR test you are telling people is required? All one is wanting to ascertain is whether or not the resistance between the part and MET is greater than 23 k&#937;, and one can get that answer by going straight to the standard resistance (what you call 'continuity') test.

This has come a lot later than I expected.

... and I lot later than I intended, but I kept forgetting to ask!

What you say makes sense but that is the best way to test. It's good to know that it is >1000M&#937;. As I keep saying 23k&#937; is just the figure derived from 10mA and, in a way, irrelevant as readings will usually be a few ohms or many megaohms. However, should the measurement be 0.00M&#937; a check is obviously required.

I'm thinking aloud (and perhaps turning my question/comment on its head!), so please bear with me ....

Firstly, what I wrote is wrong if one is using an MFT like yours and mine (and probably many/most others). Its 'continuity' range cannot indicate whether resistance is above or below 23k, since the highest result it can give is ">2000&#937;".

If one's only interest is in whether or not the resistance is, or is not, above 23k (we agree it's essentially arbitrary, but there has to be some specific threshold if one is to get a yes/no answer about bonding), then I could turn my question/comment on its head, since one could almost always achieve an answer to that question with the IR range of an MFT such as yours and mine. Any answer of 0.03M&#937; or above indicates that the resistance is at least 25k&#937; (hence >23k&#937. If the result is 0.01&#937; or 0.00&#937;, then one can be sure that the resistance is <23k&#937;. Only if it gives a result of 0.02&#937; is there uncertainty as to whether the resistance is above or below 23k&#937; - and you/I would then have a bit of a problem with our 1652's (and probably many other MFTs), since all the 'continuity' range could tell us would be whether the resistance was above or below 2k&#937; - so not a complete answer. If, as a separate question, one wants to know exactly how low a "<23k&#937;" resistance is, one would obvioulsy undertake a 'continuity' measurement (and will get an answer, provided it is less than 2k&#937.

If one were using a multimeter, even the cheapest and nastiest will give answers up to about 2 M&#937;, and most that cost any significant amount of money will measure up to at least 20M&#937;, maybe higher, and all will measure down to 1&#937;, more often 0.1&#937;. I would suggest that such a meter would really give us all the information one wants. If the resistence were 'low', it would certainly indicate whether it were above or below 23k&#937;, and if it were high, I personally would think that knowing it was, say, >20M&#937; was more than enough. You say that it would be 'good to know' that it was above 1000M&#937;, but would that really mean anything more to you than >20M&#937;??

As you say, in practice one is probably very unlikely to get answers between, say 5&#937; and a good few M&#937; - so it's almost a dichotomous answer one is looking for.

You may think telling people who don't know to do it may be a bit pointless but it's not really.

As above, I guess it depends upon what sort of meter they are going to be using. If a multimeter, then (IMO) they simply have to make a single measurement (using whatever range proves necessary). If they're using an MFT, then an IR measurement alone will usually suffice. In the one (extremely unlikley) inconclusive case (IR result = 0.02M&#937, and/or if one wants to know exactly how low a "<23k&#937;" resistance is [e.g. to ascertain whether it is below 50/(Ia*5)], then one undertakes a 'continuity' measurement - but that's a separate issue, and 'our' MFTs would only give an actual answer if it were <2k&#937; (which it virtually always would be).

Does any of this make sense?

Kind Regards, John

 

[EFLImpudence]

Oh yes of course it makes sense. You are right.

I am getting my decimal points in the wrong place aren't I?

In my defence - it shows the practicalities of the test in that the theoretical never happens.

When testing, - with MFT - many M&#937; or, if the result is 0.00M&#937;, turn to 'continuity' and get 5&#937; (or whatever). It's not likely to be between very low or very high.

However, the 'cut-off figure' has to be mentioned as that is the 'theoretical' limit of 'safe'.

So, as you say, above 0.03M&#937; - no bond
0.00 or 0.01M&#937; - bond required but I would still determine the actual resistance.

 

[JohnW2]

Oh yes of course it makes sense. You are right.

Glad you agree.

I am getting my decimal points in the wrong place aren't I?

If you have, I hadn't noticed - otherwise I would probably have mentioned it

In my defence - it shows the practicalities of the test in that the theoretical never happens. When testing, - with MFT - many M&#937; or, if the result is 0.00M&#937;, turn to 'continuity' and get 5&#937; (or whatever). It's not likely to be between very low or very high.

Yes, agreed. As we've both said, it virtually always a yes/no dichotomy, which usually makes precise measurements less important (for this purpose).

However, the 'cut-off figure' has to be mentioned as that is the 'theoretical' limit of 'safe'.

True, but (a) where does that figure actually come from (I realise that it equates to 10mA at 230V, but where is that criterion actually stated?) and (b) with our MFTs, we cannot necessarily determine whether that (incredibly unlikely) threshold has been exceeded, since all we'll know is that the resistance is between 15k&#937; and 25k&#937;.

So, as you say, above 0.03M&#937; - no bond ... 0.00 or 0.01M&#937; - bond required but I would still determine the actual resistance.

Indeed ... and 0.02M&#937; (not that it will ever happen) reach for your multimeter

Kind Regards, John

 

[EFLImpudence]

True, but (a) where does that figure actually come from (I realise that it equates to 10mA at 230V, but where is that criterion actually stated?)

I can't find anything British (as I have before) but it is the current above which a person can't let go.
http://engineering.dartmouth.edu/safety/electrical/TheFatalCurrent.html

and (b) with our MFTs, we cannot necessarily determine whether that (incredibly unlikely) threshold has been exceeded, since all we'll know is that the resistance is between 15k&#937; and 25k&#937;.

Not quite sure what you mean as if bonded the problem is negated.
Do you mean if the reading is 0.02M&#937; but may be between 23 and 25k&#937;?

Of course, if the voltage is actually higher than 230V then the figure will change.
Perhaps we should use 25k&#937;.
I don't see the point of allowing 1k&#937; for the body as people are different.

Indeed ... and 0.02M&#937; (not that it will ever happen) reach for your multimeter

I think the battery is flat.

 

[jcp]

Just got back to a PC and seen the discussion above. Thanks for your response to my posting, which was beautifully clear, and I also enjoyed your continuing discussion.
So really, thanks.

 

[JohnW2]

True, but (a) where does that figure actually come from (I realise that it equates to 10mA at 230V, but where is that criterion actually stated?)

I can't find anything British (as I have before) but it is the current above which a person can't let go.

I know about the physiological effects of the current - but I was asking where have you seen it written that such a criteria should be used for determining whether bonding is required.

and (b) with our MFTs, we cannot necessarily determine whether that (incredibly unlikely) threshold has been exceeded, since all we'll know is that the resistance is between 15k&#937; and 25k&#937;.

Not quite sure what you mean as if bonded the problem is negated. Do you mean if the reading is 0.02M&#937; but may be between 23 and 25k&#937;?

I mean that if our MFT (with 2 dp display/resolution) gives a reading of 0.02M&#937;, that means the resistance may be anything between 15k&#937; and 25k&#937; (well 24.9999999....&#937.

Of course, if the voltage is actually higher than 230V then the figure will change. Perhaps we should use 25k&#937;. I don't see the point of allowing 1k&#937; for the body as people are different.

It's all so vague/'arbitrary' that I really don't think it matters a jot as to whether one considers 230V or 250V, or whether one makes the 1k&#937; allowance for the human body. As we've both said, figures in the region of 23k&#937; really just shouldn't happen. If I got a reading of, say, 50k&#937;, I would not just say "doesn't need bonding". rather, I would 'investigate' - wouldn't you? ... and, similarly, I would probably want to investigate if I got answers of, say, 10k&#937; or 1k&#937;.

Kind Regards, John

 

[EFLImpudence]

I was asking where have you seen it written that such a criteria should be used for determining whether bonding is required.

Oh, sorry, it's in GN8.

If I got a reading of, say, 50k&#937;, I would not just say "doesn't need bonding". rather, I would 'investigate' - wouldn't you? ... and, similarly, I would probably want to investigate if I got answers of, say, 10k&#937; or 1k&#937;.

I would if I did.

 

[JohnW2]

I was asking where have you seen it written that such a criteria should be used for determining whether bonding is required.

Oh, sorry, it's in GN8.

Thanks. I probably wasn't clear enough about what I was asking!

If I got a reading of, say, 50k&#937;, I would not just say "doesn't need bonding". rather, I would 'investigate' - wouldn't you? ... and, similarly, I would probably want to investigate if I got answers of, say, 10k&#937; or 1k&#937;.

I would if I did.

So we are agreed. It's so unlikley that one would get figures which are not 'at the extremes' (one would expect, say, <5&#937; or >>1M&#937, that if I got other readings, I would want to try to find out the reason.

Kind Regards, John