Supplementary Bonding - my questionable conclusions?

[JohnW2]

I think this gets us back to square one. Is there an impedance between the extraneous-c-ps below which you would consider that SB is not required - and how would that figure be determined?

Have 'you accidentally', or deliberately, 'moved the goalposts'? ....

.... We have never previously discussed the "impedance between the extraneous-c-ps". What we have discussed is (a) the impedance/resistance between extraneous-c-ps and exposed-c-ps [per 415.2.2] and (b) the impedance/resistance between extraneous-c-ps and MET [per 701.415.2(vi)].

In terms of those latter two measurements, as I have repeatedly said, no, I do NOT think that there is a value of that impedance/resistance "below which I would consider that SB is not required". On the contrary, as I have said, at least in the type of fault scenarios I have described, the only effect of that impedance being lower I can see is that the current flowing through the victim would be greater (for the duration of the fault).

That does not mean that I am saying that SB is 'always required' (since RCD protection alone may well be considered adequate), but it does mean that I do not think it remotely sensible that the magnitude of that impedance should have any bearing on the decision as to whether or not SB is required (if it ever is).

Kind Regards, John

 

[JohnW2]

I am not quite sure how a separate CPC is suppose to enter the room.

I haven't suggested that it would.

However, it's far from impossible that there would be, say simultaneously-touchable metal shaver socket (on a lighting circuit) and (Class I) electrically-heated towel rails (on a 'power' circuit), each with it's circuit's CPC (as well as L an N). If the exposed-c-p of either of those items became 'live' as a result of a fault, the (exposed-c-p of) the other one would represent exactly the same danger as would a simultaneously-touchable extraneous-c-p, even if there were no actual ('traditional') extraneous-c-ps in the room.

Kind Regards, John

 

[EFLImpudence]

Have 'you accidentally', or deliberately, 'moved the goalposts'? ....

I thought you had.

.... We have never previously discussed the "impedance between the extraneous-c-ps". What we have discussed is (a) the impedance/resistance between extraneous-c-ps and exposed-c-ps [per 415.2.2] and (b) the impedance/resistance between extraneous-c-ps and MET [per 701.415.2(vi)].

Yes, but you have introduced the proposal that an extraneous-c-p might introduce potential such that a shock might be had by touching it and another.
Presumably if the resistance/impedance between them is negligible without SB then SB would not be needed nor achieve an advantage, so above what value would necessitate SB.

In terms of those latter two measurements, as I have repeatedly said, no, I do NOT think that there is a value of that impedance/resistance "below which I would consider that SB is not required". On the contrary, as I have said, at least in the type of fault scenarios I have described, the only effect of that impedance being lower I can see is that the current flowing through the victim would be greater (for the duration of the fault).

Yeah but, no but, now you are saying that one extraneous-c-p would be acting the same as an exposed-c-p in a fault situation.
I think we really are back to square one with an extraneous-c-p acting as an exposed-c-p during a fault.
If there were negligible impedance between the two, then why will they not both be at the same potential.

That does not mean that I am saying that SB is 'always required' (since RCD protection alone may well be considered adequate), but it does mean that I do not think it remotely sensible that the magnitude of that impedance should have any bearing on the decision as to whether or not SB is required (if it ever is).

I think you are losing me again. Duration not magnitude?

 

[EFLImpudence]

I haven't suggested that it would.

However, it's far from impossible that there would be, say simultaneously-touchable metal shaver socket (on a lighting circuit) and (Class I) electrically-heated towel rails (on a 'power' circuit), each with it's circuit's CPC (as well as L an N). If the exposed-c-p of either of those items became 'live' as a result of a fault, the (exposed-c-p of) the other one would represent exactly the same danger as would a simultaneously-touchable extraneous-c-p, even if there were no actual ('traditional') extraneous-c-ps in the room.

Yes, but that is just the normal/usual situation for a bathroom against which the bonding is supposed to protect.



If there are NO exposed-c-ps, do you think it necessary to bond extraneous-c-ps?

If you do, below what resistance between them would you consider it unnecessary? 1Ω, 0.1Ω, 0.01Ω, 0.001Ω.
A metre of 4mm² being 0.004Ω

 

[JohnW2]

Goodness A very rapid further response and I'm not sure whether this means that he still doesn't understand my point or if he does now understand and thinks it may be something worth raising with JPE/64! ...

The reply I provided yesterday, whilst summarising as succinctly as possible (under the constraints of technical helpline reply) the issues being queried, may be complemented by further study of the two additional IET BS 7671-related publications, both of which go into a little more detail on the particular concerns you have:
https://shop.theiet.org/guidance-note-8-earthing-bonding-4th-edition
https://shop.theiet.org/guidance-note-7-special-locations-6th-edition

Should you believe however, the requirements of BS 7671 to be technically incorrect in any way, as detailed on page 2 of the publication,
It is the constant aim of the IET and BSI to improve the quality of our products and services. We should be grateful if anyone finding an inaccuracy or ambiguity while using this British Standard would inform the Secretary, G D Cronshaw {[email protected]}, The IET, Six Hills Way, Stevenage, SG1 2AY, United Kingdom
....you can raise the matter with the secretary of JPEL/64, the committee responsible for its technical content. A suitably-composed and worded technical paper, covering concerns, and any proposed re-wording/amendment that you feel is necessary will normally elicit detailed consideration and debate by the relevant sub-committees - in the publication's continual evolution process. These committees meet regularly and comprise typically representatives from the organisations listed on pages 9 and 10 of the standard.

Whether this is worth pursuing is, however, a different matter. Even if the regulation is 'incorrect', it's pretty moot, since, as we've discussed in the past, it is almost never going to happen that 701.415.2(vi) will not be satisfied - the measurement in question will almost inevitably be either very low (just a few ohms at most) or so high as to mean that the part does not really qualify as an extraneous-c-p in the first place. It just feels wrong that they should have a condition which doesn't make sense (at least, to me), even if it will always be satisfied!

Kind Regards, John

 

[JohnW2]

Yes, but you have introduced the proposal that an extraneous-c-p might introduce potential such that a shock might be had by touching it and another.

OK, but that only arose because, in 'correcting your error', you introduced the concept of a room with no exposed-c-ps and only extraneous-c-ps. However, since (to the best of my knowledge) there is no regulation which says anything about impedance/resistance between two (or more) extraneous-c-ps, I would suggest that we ignore that potential confusion to the main discussion.

Yeah but, no but, now you are saying that one extraneous-c-p would be acting the same as an exposed-c-p in a fault situation. I think we really are back to square one with an extraneous-c-p acting as an exposed-c-p during a fault.

I'm a bit confused, since I have been saying the opposite - that, an exposed-c-p on a circuit without a fault (i.e. at MET potential) acts as if it were an extraneous-c-p in relation to a fault on some other circuit.

If there were negligible impedance between the two, then why will they not both be at the same potential.

No. You need to remember the diagrams that we both drew ...

... consider the exposed-c-p of something on 'circuit A' connected to the MET by a CPC of 1Ω. Consider also an extraneous-c-p with a connection to the MET also of 1Ω (OR the exposed-c-p of something on 'Circuit B', also connected to MET via CPC of 1Ω). The measured impedance/resistance between the exposed-c-p on Circuit A and the extraneous-c-p (OR the exposed-c-p on circuit B) would then be just 2Ω - which I imagine you would accept (in context) as fairly 'negligible'.

Now imagine a fault from L to the exposed-c-p on Circuit A which resulted in, say, 150A flowing though the CPC (to the MET). The potential of that exposed-c-p would therefore rise to 150V above MET potential. However, since no current is flowing through its connection to the MET (hence no 'VD'), the extraneous-c-p (OR the exposed-c-p on Circuit B) would still be at MET potential.

There would therefore be a potential difference of 150V between the exposed-c-p with the fault ('Circuit A') and the extraneous-c-p (or exposed-c-p on Circuit B), despite the resistance between the two being only 2Ω. Large faulkt currents resulkt in large ';voltage drops', even when impedances are very low

I think you are losing me again. Duration not magnitude?

The whole concept of 'protecting people with RCDs' relies on the assumption that to limit duration without limiting magnitude is 'enough' - which, up to a point, is roughly correct.

If one wanted to have a regulation allowing omission of SB which was based on the magnitude of a potential shock, then (as I have explained) the condition would have to specify a minimum, not maximum, impedance/resistance between extraneous-c-p and MET - and that would be silly, since that condition would virtually never be satisfied, such that SB would virtually always be required.

Kind Regards, John

 

[EFLImpudence]

I think you are making this too complicated by introducing different specific scenarios from the original bonding situations.

Anyway:

OK, but that only arose because, in 'correcting your error', you introduced the concept of a room with no exposed-c-ps and only extraneous-c-ps. However, since (to the best of my knowledge) there is no regulation which says anything about impedance/resistance between two (or more) extraneous-c-ps, I would suggest that we ignore that potential confusion to the main discussion.

Ok, but you introduced the situation where an extraneous-c-p might become live - presumably by something like a fault at a boiler.

I'm a bit confused, since I have been saying the opposite - that, an exposed-c-p on a circuit without a fault (i.e. at MET potential) acts as if it were an extraneous-c-p in relation to a fault on some other circuit.

Ok, like the extraneous-c-p pipe with a fault at a boiler.
However, there won't be any amps flowing in it, will there?, because it is a dead-end - until you touch it and something else.

No. You need to remember the diagrams that we both drew ...

I do but it is not the same, is it?

... consider the exposed-c-p of something on 'circuit A' connected to the MET by a CPC of 1Ω. Consider also an extraneous-c-p with a connection to the MET also of 1Ω (OR the exposed-c-p of something on 'Circuit B', also connected to MET via CPC of 1Ω). The measured impedance/resistance between the exposed-c-p on Circuit A and the extraneous-c-p (OR the exposed-c-p on circuit B) would then be just 2Ω - which I imagine you would accept (in context) as fairly 'negligible'.

Yes, but I meant an actually negligible value - as in the recognised value of 0.05Ω.
If the pipes were such that the impedance between them were 0.05Ω then SB would not be necessary.

Now imagine a fault from L to the exposed-c-p on Circuit A which resulted in, say, 150A flowing though the CPC (to the MET). The potential of that exposed-c-p would therefore rise to 150V above MET potential. However, since no current is flowing through its connection to the MET (hence no 'VD'), the extraneous-c-p (OR the exposed-c-p on Circuit B) would still be at MET potential.

There would therefore be a potential difference of 150V between the exposed-c-p with the fault ('Circuit A') and the extraneous-c-p (or exposed-c-p on Circuit B), despite the resistance between the two being only 2Ω. Large fault currents resulkt in large ';voltage drops', even when impedances are very low

But surely there would be no current in the dead-end CPC/exposed-c-p - until you touched it and something earthed.
That would be just like receiving a shock from a live conductor while earthed; not really what we are talking about, is it?
In the case of a pipe enlivened by a boiler fault then surely all earthed parts and extraneous-c-ps in the room will be similarly enlivened to a similar voltage.

If one wanted to have a regulation allowing omission of SB which was based on the magnitude of a potential shock, then (as I have explained) the condition would have to specify a minimum, not maximum, impedance/resistance between extraneous-c-p and MET - and that would be silly, since that condition would virtually never be satisfied, such that SB would virtually always be required.

Yes, I think we agree on that now since your 'discovery'

 

[EFLImpudence]

...

 

[JohnW2]

Just testing - EFLI says that he can't post on this thread, so I'm seeing if I can

 

[EFLImpudence]

I think you are making this too complicated by introducing different specific scenarios from the original bonding situations.

Anyway:

OK, but that only arose because, in 'correcting your error', you introduced the concept of a room with no exposed-c-ps and only extraneous-c-ps. However, since (to the best of my knowledge) there is no regulation which says anything about impedance/resistance between two (or more) extraneous-c-ps, I would suggest that we ignore that potential confusion to the main discussion.

Ok, but you introduced the situation where an extraneous-c-p might become live - presumably by something like a fault at a boiler.

I'm a bit confused, since I have been saying the opposite - that, an exposed-c-p on a circuit without a fault (i.e. at MET potential) acts as if it were an extraneous-c-p in relation to a fault on some other circuit.

Ok, like the extraneous-c-p pipe with a fault at a boiler.
However, there won't be any amps flowing in it, will there?, because it is a dead-end - until you touch it and something else.

No. You need to remember the diagrams that we both drew ...

I do but it is not the same, is it?

... consider the exposed-c-p of something on 'circuit A' connected to the MET by a CPC of 1Ω. Consider also an extraneous-c-p with a connection to the MET also of 1Ω (OR the exposed-c-p of something on 'Circuit B', also connected to MET via CPC of 1Ω). The measured impedance/resistance between the exposed-c-p on Circuit A and the extraneous-c-p (OR the exposed-c-p on circuit B) would then be just 2Ω - which I imagine you would accept (in context) as fairly 'negligible'.

Yes, but I meant an actually negligible value - as in the recognised value of 0.05Ω.
If the pipes were such that the impedance between them were 0.05Ω then SB would not be necessary.

Now imagine a fault from L to the exposed-c-p on Circuit A which resulted in, say, 150A flowing though the CPC (to the MET). The potential of that exposed-c-p would therefore rise to 150V above MET potential. However, since no current is flowing through its connection to the MET (hence no 'VD'), the extraneous-c-p (OR the exposed-c-p on Circuit B) would still be at MET potential.

There would therefore be a potential difference of 150V between the exposed-c-p with the fault ('Circuit A') and the extraneous-c-p (or exposed-c-p on Circuit B), despite the resistance between the two being only 2Ω. Large fault currents resulkt in large ';voltage drops', even when impedances are very low

But surely there would be no current in the dead-end CPC/exposed-c-p - until you touched it and something earthed.
That would be just like receiving a shock from a live conductor while earthed; not really what we are talking about, is it?
In the case of a pipe enlivened by a boiler fault then surely all earthed parts and extraneous-c-ps in the room will be similarly enlivened to a similar voltage.

View attachment 233598

If one wanted to have a regulation allowing omission of SB which was based on the magnitude of a potential shock, then (as I have explained) the condition would have to specify a minimum, not maximum, impedance/resistance between extraneous-c-p and MET - and that would be silly, since that condition would virtually never be satisfied, such that SB would virtually always be required.

Yes, I think we agree on that now since your 'discovery'

 

[EFLImpudence]

 

[EFLImpudence]

Very odd. Original post and third attempt still awaiting moderation.

Second post which I tried and deleted, indeed replacing with dots, has this note.

As you can see from the screenshot now posted - which would not originally work - contains very few symbols; certainly nothing out of the ordinary.

 

[JohnW2]

I think you are making this too complicated by introducing different specific scenarios from the original bonding situations.

Since it is all that matters to our discussion about the regulation under consideration, I have done all I can (and will continue to try) to restrict the discussion to the simplest situation, with just one exposed-c-p (with a fault from L to exposed-c-p) and one extraneous-c-p in the room. In particular, I have not knowingly/intentionally been responsible for introducing the concept of a room with no exposed-c-ps, or of rooms with multiple exposed- and/or extraneous-c-ps.

Yes, but I meant an actually negligible value - as in the recognised value of 0.05Ω.
If the pipes were such that the impedance between them were 0.05Ω then SB would not be necessary.

I'm not sure where that 'recognised value' comes from - I only recall that figure being mentioned (probably in OSG!) as 'guidance' in relation to main bonding conductors. In any event, the figure (no matter how low) makes absolutely no difference difference to the generality of what I was saying ...

... It is probably an unrealistic/unattainable situation but IF (without SB) the measured resistance from exposed-c-p to extraneous-c-p (via MET) were only 0.05Ω, then lets assume that is made up of exposed-MET=0.025 and extraneous-MET of 0.025Ω. If the former (CPC of circuit) were 0.025Ω, then, if it were 2.5/1.5mm² cable, the R1 would be 0.015Ω (0.025 x 3/5), hence R1+R2 would be 0.04Ω (0.025 + 0.015). If supply voltage during the fault were 230V, the fault current would be 5,750A (230V/0.04) . The voltage ('drop') across the CPC would, in that hypothetical situation, hence be ('the infamous') 143.75V (which I have been calling 144V) - so that the PD between exposed-c-p and MET will be about 144V.

Since no current is flowing through the extraneous-c-p, there will be no VD across is, so the PD between extraneous- and exposed-c-ps will, as always, be about 144V. As I've said many times before, that figure (the 'touch voltage') depends essentially only on the R1/R2 ratio of the cable (e.g. 5/3 for 2.5/1.5mm² cable), regardless of any measured resistances/impedances.

If you wanted to be pedantic and consider the current through a victim touching both parts, that current would be (assuming 'body resistance' of 1,000Ω) about 143.75/(1000+0.025+0.015) = ~143.74 mA, which would result in a VD of about 3.6 mV (0.14375 x 0.025) between extraneous-c-p and MET - so the touch voltage' (between extraneous- and exposed-c-ps) would be about 3.6 mV less than 143.75V

Of course, the above scenario couldn't happen in practice since, even if R1+R2 were (I think unrealistically) only 0.04Ω (which, as above, would be necessary to achieve your 'recognised' figure of 0.05Ω 'between parts'), Ze would mean that fault current would be much lower than I have suggested, since the installation's supply voltage during the fault would be a lot less than 230V. In fact, if R1+R2 were 0.04Ω and Ze were 0.35Ω, the the installation's supply voltage during the fault would fall to about 23.6 V [ 230 x (0.04) / (0.04+0.35) ], hence the touch voltage' only about 14.7 V (23.6 x 5/8)

Yes, I think we agree on that now since your 'discovery'

Indeed. My conclusion (as a result of the 'discovery') is pretty simple ...

• IF, in any room, one wants to avoid any possibility of dangerous 'touch voltages' arising between two 'parts' (exposed- and/or extraneous), for the very brief duration of a fault until cleared, then the only way of achieving that (regardless of anything else, and any measurements) is by having local SB conductors electrically connecting those parts.
• IF, in any room, one is satisfied that the degree of protection against injury/death provided by a (30mA) RCD's limitation of the duration of the fault (hence shock) (regardless of magnitude of shock) then, provided (30mA) RCD protection is present, SB is never required.
• IF, even in the presence of RCD protection, one wished to limit the magnitude, as well as duration, of possible shocks, one would have to have SB, joining all exposed-0 and extraneous-c-ps in every room.

Kind Regards, John

 

[JohnW2]

Here we go again (see next post for the 'continuation') ...

... continued in next post ....

 

[JohnW2]

.... continued ...