18th 544.1.2

[JohnW2]

The 'bond everything' nonsense probably originated in a publication from one of the electrical organisations such as NICEIC, or more likely in some condensed guide type book published by an electrical union or similar organisation.

That certainly sounds very possible. As I implied in a previous post, I'm sure that, in the dim and distant past, I actually read about this 'requirement' (and, knowing my hoarding traits, I probably still have it somewhere!), but have yet to remember where it was.

Kind Regards, John

 

[EFLImpudence]

See above - it is clear that I really should not have let myself mention the MET. Please mentally substitute 'local CPC' or 'exposed-c-p whenever I wrote 'MET'

Yes ok. It just made me wonder what you had in mind.

- but, as above, it again begs the question as to whether the regs really are requiring SB in the absence of any exposed-c-ps (i.e. just joining extraneous-c-ps together with G/Ys, and to nothing else - aka 'cross-bonding'. As we have both said, there seems to be no point (ever), so maybe we have been misunderstanding the potential ambiguity ion 415.2.1?

I would say it probably does apply to extraneous-c-p to extraneous-c-p, for SB in a location. Although, as you say the resistance between them might be low enough for SB not to be required anyway.
Does this mean that the Ia used should be that of the highest rated OPD in the installation, not just of the location (albeit there is none in this example)?

This might also apply to exposed-cp to exposed cp in the absence of extraneous-c-ps, and be more relevant as the resistance of small CPCs will be greater.
Which Ia should be used for this? Just those in the location?

Bear in mind, unlike most people, the 'simultaneously accessible' part of the regulation.

 

[JohnW2]

I would say it probably does apply to extraneous-c-p to extraneous-c-p, for SB in a location. Although, as you say the resistance between them might be low enough for SB not to be required anyway.

Well, for a start, I still think that the wording of the reg is far from totally clear (i.e. potentially ambiguous).

However, is there not somewhat of a circular argument here. It obviously depends upon what criteria/parameters one uses when testing (as you often mention) to determine whether something is (or is not) an extraneous-c-p - but is it not likely that if your test did indicate that it was an extraneous-c-p that the same data would also indicate that SB was not required?

This might also apply to exposed-cp to exposed cp in the absence of extraneous-c-ps, and be more relevant as the resistance of small CPCs will be greater.

I did almost mention that, too. However, I considered the fact that it is unusual enough (at least in my experience) to have even one exposed-c-p in a bathroom (electric towel rails being one possibility), so that two or more simultaneously-touchable ones would probably be very rare. Also, are we meant to be considering that the protective measures associated with other aspects of the installation could be unsatisfactory/non-compliant? If not, then if the CPCs connected individually to two or more (simultaneously touchable) exposed-c-ps were providing acceptable fault protection, would that not again (as above) also mean that SB was not needed.

Bear in mind, unlike most people, the 'simultaneously accessible' part of the regulation.

I don't fully understand that sentence but, yes, 'simultaneously touchable' (as above, particularly simultaneously-touchable exposed-c-ps) would certainly seem to rule out the need for SB in a lot of cases.

Kind Regards, John

 

[EFLImpudence]

However, is there not somewhat of a circular argument here. It obviously depends upon what criteria/parameters one uses when testing (as you often mention) to determine whether something is (or is not) an extraneous-c-p - but is it not likely that if your test did indicate that it was an extraneous-c-p that the same data would also indicate that SB was not required?

No. Determining if it is extraneous involves a measurement to the MET.
Determining if it requires SB involves a measurement to other exrtaneous and exposed-c-ps in the location.

Also, are we meant to be considering that the protective measures associated with other aspects of the installation could be unsatisfactory/non-compliant?

Not sure what you mean.

If not, then if the CPCs connected individually to two or more (simultaneously touchable) exposed-c-ps were providing acceptable fault protection, would that not again (as above) also mean that SB was not needed.

Not necessarily. Two CPCs might have considerable resistance from one ecp to the CU and back to the other.
Isn't that why it says they should be linked by SB?

I don't fully understand that sentence but, yes, 'simultaneously touchable' (as above, particularly simultaneously-touchable exposed-c-ps) would certainly seem to rule out the need for SB in a lot of cases.

I just meant that most people don't consider the "simultaneously accessible" part - just bonding everything in the room - accessible or not, let alone simultaneously.

 

[JohnW2]

No. Determining if it is extraneous involves a measurement to the MET. ... Determining if it requires SB involves a measurement to other exrtaneous and exposed-c-ps in the location.

Yes, I obviously realise that, and was not suggesting that exactly the same measurements would be undertaken - but I was talking about what, in practice, is almost always (maybe just always!) going to be the situation.

In practice, if a pipe is an extraneous-c-p (to the location) that is going to be because it is in electrical continuity with a bonded pipe and/or because it is in electrical continuity with something (boiler, CH bits, immersion etc.) which is earthed. In either case, the resistance to the MET will be very low ('a few ohms' at the very most), hence the resistance between any two of those extraneous-c-ps will be 'twice very low' (essentially, still 'very low'). As for exposed-c-ps, to satisfy fault protection requirements, the resistance from any of them to the MET is, again, necessarily 'very low' - so, again, the resistance between any two exposed-c-ps will be 'twice very low' (essentially, still 'very low). Finally, it follows from the above that the resistance between any extraneous-c-p and any exposed-c-p will, yet again, be 'twice very low' (essentially, still 'very low).

Not sure what you mean.

As I went on to say, I'm not sure that (in deciding whether SB is needed) we have to take into account the possibility that the exposed-c-ps may have inadequate fault protection.

Not necessarily. Two CPCs might have considerable resistance from one ecp to the CU and back to the other.

Unless (as above) you think that one has to consider the possibility that one or both exposed-cps has pitifully unsatisfactory fault protection, I'm not sure where your 'considerable resistance' comes from. The Zs for each of the circuits concerned should be less than the 'max Zs' for the OPD concerned, so the max permissible resistance to MET would be Zs(max)-Ze. The maximum resistance between two exposed-c-ps which both have adequate fault protection would therefore be Zs(mas)1 + Zs(max)2 - (2 x Ze) - which, again, is 'very small'.

I just meant that most people don't consider the "simultaneously accessible" part - just bonding everything in the room - accessible or not, let alone simultaneously.

Oh, fair enough - in which case, as I said, I agree.

I've just thought of another possible terminological quandary. If a metal pipe emerging from (hence in electrical continuity with) something earthed (boiler, CH pump etc.) enters a bathroom, does it count as an extraneous-c-p or an exposed-c-p. If, as I suspect, your answer is the former, what would you say if, hypothetically (and for some bizarre reason), a bit of cable (rather than a pipe) were connected to the earth terminal of the boiler/pump and entered the bathroom with a bare end (or it's bare end were connected to something metal). That's obviously a silly example/analogy, but it illustrates the problem that the pipe can just be considered as a pipe (which could be an extraneous-c-p) or as an 'electrical extension' of the exposed-c-p of the boiler/pump/whatever!

Kind Regards, John

 

[EFLImpudence]

Yes, I obviously realise that, and was not suggesting that exactly the same measurements would be undertaken - but I was talking about what, in practice, is almost always (maybe just always!) going to be the situation.

In practice, if a pipe is an extraneous-c-p (to the location) that is going to be because it is in electrical continuity with a bonded pipe and/or because it is in electrical continuity with something (boiler, CH bits, immersion etc.) which is earthed. In either case, the resistance to the MET will be very low ('a few ohms' at the very most), hence the resistance between any two of those extraneous-c-ps will be 'twice very low' (essentially, still 'very low'). As for exposed-c-ps, to satisfy fault protection requirements, the resistance from any of them to the MET is, again, necessarily 'very low' - so, again, the resistance between any two exposed-c-ps will be 'twice very low' (essentially, still 'very low). Finally, it follows from the above that the resistance between any extraneous-c-p and any exposed-c-p will, yet again, be 'twice very low' (essentially, still 'very low).

When, then is supplementary bonding ever going to be required? What are all the regulations for?

As I went on to say, I'm not sure that (in deciding whether SB is needed) we have to take into account the possibility that the exposed-c-ps may have inadequate fault protection.

I don't see why you have brought up inadequate fault protection.
Well, I do but I don't see why you have unless you think SB is never required. That is the 'two very lows' are always low enough.

Unless (as above) you think that one has to consider the possibility that one or both exposed-cps has pitifully unsatisfactory fault protection, I'm not sure where your 'considerable resistance' comes from.

The CPC on a lighting circuit plus another circuit CPC can be relatively high, It might not be low enough for 50/Ia

I've just thought of another possible terminological quandary. If a metal pipe emerging from (hence in electrical continuity with) something earthed (boiler, CH pump etc.) enters a bathroom, does it count as an extraneous-c-p or an exposed-c-p. If, as I suspect, your answer is the former,

It would, dare I say obviously.
Even if you consider it exposed where the fault could occur, it is still only an extraneous-c-p to a bathroom.

what would you say if, hypothetically (and for some bizarre reason), a bit of cable (rather than a pipe) were connected to the earth terminal of the boiler/pump and entered the bathroom with a bare end (or it's bare end were connected to something metal).

I would say "Hello Bernard".

That's obviously a silly example/analogy, but it illustrates the problem that the pipe can just be considered as a pipe (which could be an extraneous-c-p) or as an 'electrical extension' of the exposed-c-p of the boiler/pump/whatever!

It is silly and no different than the pipe becoming live because of a fault in the boiler to which it is connected.

It will still only be an extraneous-c-p to a bathroom.
It is not only earth potential that can be a reason for it being an extraneous-c-p.

 

[ban-all-sheds]

If a metal pipe emerging from (hence in electrical continuity with) something earthed (boiler, CH pump etc.) enters a bathroom, does it count as an extraneous-c-p or an exposed-c-p.

Do you really not know?

Try looking in Part 2.

 

[JohnW2]

When, then is supplementary bonding ever going to be required? What are all the regulations for?

Interesting questions, but I think my reasoning was somewhat awry .....

I don't see why you have brought up inadequate fault protection. Well, I do but I don't se e why you have unless you think SB is never required. That is the 'two very lows' are always low enough.

OK, on reflection, the 'two very lows' may not necessarily be low enough. What I had overlooked was that fault protection requires Zs ≤218.5/Ia which means that, even with a fairly high Ze (and R1), the R2 (even of one circuit) could be a fair bit higher than 50/Ia.

Indeed, reflecting even more, I'm not even sure that ≤50/Ia (between an extraneous- and an exposed-c-p - see below), namely about ≤0.321Ω, is often going to be achievable with a typical 32A circuit.

Of course, the path between the extraneous and exposed-c-ps would only carry the entire fault current if the CPC to the exposed-c-p were 'absent'. If a (connected!) CPC is present, then the fault current will be shared between it and the extraneous-exposed path, so that a resistance appreciably higher than 50/Ia would be needed before the 'touch voltage' (pd between extraneous and exposed) would reach 50V.

The CPC on a lighting circuit plus another circuit CPC can be relatively high, It might not be low enough for 50/Ia

Interestingly, whilst I think that 415.2.1 is potentially ambiguous, it appears that 415.2.2 is not, in that it seems to be clear in saying that the ≤50/Ia relates to the resistance between an extraneous-c-p and an exposed-c-p, not between two exposed-c-p.s (or two extraneous-c-ps).

It would, dare I say obviously. ... Even if you consider it exposed where the fault could occur, it is still only an extraneous-c-p to a bathroom. .... It is silly and no different than the pipe becoming live because of a fault in the boiler to which it is connected. .... It will still only be an extraneous-c-p to a bathroom. It is not only earth potential that can be a reason for it being an extraneous-c-p.

It's probably of no real relevance to this discussion but it just suddenly occurred to me that the distinction between extraneous- and exposed-c-ps is not always necessarily as clear cut as one might think - and I don't think it is (technically speaking) necessarily as simple as BAS and yourself are suggesting ...

... As you say, the potential which an extraneous-c-p may be 'liable to introduce' is not necessarily (although nearly always is) earth potential. as you say, there is clearly no doubt that a conductive part which enters a bathroom and 'is liable to introduce a potential' is an extraneous-c-p - but I think it could be argued that there are situations in which it would also satisfy the Part 2 definition of an exposed-c-p.

However, as I said, I don't think it is relevant to this discussion, so probably not worth pursuing.

Kind Regards, John

 

[EFLImpudence]

Interestingly, whilst I think that 415.2.1 is potentially ambiguous, it appears that 415.2.2 is not, in that it seems to be clear in saying that the ≤50/Ia relates to the resistance between an extraneous-c-p and an exposed-c-p, not between two exposed-c-p.s (or two extraneous-c-ps).

I do not see how 415.2.2 could be written to make that indisputable other than specifically saying it does not apply to {whichever}.
After all, as it is written it could be read as meaning the resistance between simultaneously accessible exposed-c-ps and the resistance between simultaneously accessible extraneous-c-ps.

Actually, I think 415.2.1. is less ambiguous (in fact - not ambiguous at all) because it says "SB shall include ALL simultaneously accessible exposed-c-ps and extraneous-c-ps".

 

[AlanE]

The 'bond everything' nonsense probably originated in a publication from one of the electrical organisations such as NICEIC, or more likely in some condensed guide type book published by an electrical union or similar organisation.

That certainly sounds very possible. As I implied in a previous post, I'm sure that, in the dim and distant past, I actually read about this 'requirement' (and, knowing my hoarding traits, I probably still have it somewhere!), but have yet to remember where it was.

Kind Regards, John

Both you you have admitted not reading what others have posted!

John you have recolections and may still have it somewhere and flameport your accusation that it was perhaps the NICEIC.

I quoted, as did another, a publication from the IEE that ADMITS it made it into the wiring regs at some point, may have been withdrawn later but they admit it had been included and that it was to have repercussions for many years to come.

 

[AlanE]

PME was not a forerunner of anything.
It is a botch caused by the supply companies not maintaining their TN-S cables which necessitated the connection of household CPCs (earthing) to the Neutral of the supply.

It came before CNE systems so doen't that in itself make it a forrunner?

 

[EFLImpudence]

I quoted, as did another, a publication from the IEE that ADMITS it made it into the wiring regs at some point, may have been withdrawn later but they admit it had been included and that it was to have repercussions for many years to come.

FFS Alan.

THERE IS NOTHING WRONG WITH THAT REGULATION. IT IS CORRECT; NOT WRONG.

It was people's inability to read that lead to the mistakes - and it still is.

Have you read anything I have written?

 

[JohnW2]

I do not see how 415.2.2 could be written to make that indisputable other than specifically saying it does not apply to {whichever}.

You're right - I must have been half asleep when I (mis)read it in the middle of the night - as you imply, the wording is, in fact, essentially the same as in 415.2.1 ....

After all, as it is written it could be read as meaning the resistance between simultaneously accessible exposed-c-ps and the resistance between simultaneously accessible extraneous-c-ps.

If, as I think I have always assumed (almost subconsciously), they were not intending to include those things, I think that they could have produced clearer wording for both 415.2.1 and 415.2.2 - something like ...

(415.2.1) " ... shall include bonding between exposed-conductive-parts and any accessible extraneous-conductive-parts with which they are simultaneously accessible(touchable) ... "

(415.2.2) "... between any exposed-conductive-parts and any extraneous-conductive-parts with which they are simultaneously accessible(touchable) ... "

I write "accessible(touchable)" because, although I presume that we all assume that 'simultaneously accessible' is meant to mean 'simultaneously touchable', I don't think that, strictly speaking, that's what 'accessible' means.

Don't forget the relevance of this part of the discussion (i.e. what I said that started it all!). IF, per above, it is not intended to require extraneous-extraneous bonds (in the absence of simultaneously touchable exposed-c-ps), then it would be clear that 'cross-bonding' (with no G/Y connection to CPCs or exposed-c-ps) was not "SB as required by BS7671".

Actually, I think 415.2.1. is less ambiguous (in fact - not ambiguous at all) because it says "SB shall include ALL simultaneously accessible exposed-c-ps and extraneous-c-ps".

If I may be allowed to 'think aloud' without being accused of introducing irrelevances, another issue regarding interpretation of "simultaneously accessible(touchable)" has come to my mind. How do you think "ALL simultaneously accessible(touchable) exposed-c-ps and extraneous-c-ps" is meant to be interpreted when several such parts are involved? ....

... In other words, if extraneous-c-p A can be simultaneously touched with exposed-c-p B, and extraneous-c-p C can be simultaneously touched with exposed-c-p D, but A and D cannot be simultaneously touched and B and C cannot be simultaneously touched, is it compliant to just bond (A and C) and (B and D), or is it necessary to bond together 'ALL' four of the parts?

Kind Regards, John

 

[JohnW2]

It came before CNE systems so doen't that in itself make it a forrunner?

Are you saying that PME came before TN-C-S (i.e. PME was implemented with TN-S)? If so, when and why?

Kind Regards, John

 

[EFLImpudence]

I shall still rely on the 'ALL' in 415.2.1.

As for A,B,C,D; I would say it applies to all of them. It would only NOT apply to one (or more) of them if it was not simultaneously accessible with any of the others.