Supplementary Bonding - my questionable conclusions?

[EFLImpudence]

That's good to hear - and I presume that, by implication, that means that you also agree with all I said in my preceding posts?

Yes.

 

[JohnW2]

I have very varied experiences of asking the IET about issues relating to BS7671 - varying from very sensible/helpful replies (albeit 'covered in caveats/disclaimers') to the total ignoring of my queries, but I think I'll have a go in relation to this matter. Watch this space!

@EFLImpudence (and anyone else interested) - I hope you have been 'watching this space' with bated breath.

This is one of the former scenarios mentioned above, because I got a very rapid, detailed and to some extent helpful response from the IET!

I initially made a comment about my surprise that 701.415.2 imposed requirements which (with one exception) were only applicable if the installation was already non-compliant with current regs for other reasons (inadequacy/absence of main bonding, ADS or RCD protection in a bathroom).

The response is interesting (particularly the acknowledgement of past "bonding-beserk" behaviour!) and surprisingly sensible - acknowledging that new work may be done (e.g adding new circuits) in a bathroom which is (and will continue to be) otherwise 'non-compliant' with current regs (e.g. due to lack of RCD protection), so that SB is then required to partially mitigate that other (and continuing) non-compliance (such as absent RCD protection). With all the caveats and disclaimers removed, they wrote...

The requirement for supplementary protective bonding (SB) in bathrooms, has evolved and changed over the years, since it was first introduced by the 15th Edition IEE Wiring Regulations back in 1981. The requirements you refer to in BS 7671:2018+Amd1:2020 (the current 18th edition), have remained technically unchanged however since 2008 - when the current requirements appeared in the 17th edition.

To firstly clarify your initial 'observation' as to the current wording of regulation 701.415.2, it is necessary to consider evolution of installation practice, materials and workmanship as well as the regulations themselves since 1981, together with a bit of associated history:

• In 1981, the additional shock protection offered by 30mA RCD protection was largely unheard of - and certainly not a requirement of the then wiring regulations. Indeed, such RCD protection, has gradually increased throughout all installations in the intervening years, as the regulations have evolved, particularly onwards since 2008. Indeed, it is now required under many different regulations in the majority of buildings.
• In 1981, plastic pipework - especially for hot and cold water pipework was in its infancy - and the majority of pipework entering rooms containing a bath or shower would have been metallic.
• In light of this, the 1981 regulations, as having to follow the technical intent of International Standards, introduced the need for supplementary bonding of such locations to minimise the shock risk from simultaneously accessible exposed conductive parts (pieces of electrical equipment) and/or extraneous conductive parts (non-electrical equipment that could introduce a potential under fault conditions)
• This resulted in the wide-spread practice of the industry going 'bonding-berserk' especially in such locations - I was there ! Unfortunately, the practice in some quarters is still be applied (wrongly in many cases) by certain practitioners.
• With the increasing use of plastic pipework - the malpractice of sometimes now unnecessary supplementary protective bonding, often resulted paradoxically in certain metal work parts actually posing more of a shock risk, than had they been left un-bonded and sitting on the end of plastic (insulating) pipework.

With this background, the current wording of regulation 701.415.2 was therefore introduced in 2008. This encouraged the industry to consider more thoroughly the question of whether to provide SB or no

Under bullets (iv), (v) and (vi) of that regulation if all three are met (or even applicable - in the case of an all plastic piped room), then SB of the location can now be omitted.

However - whilst these requirements should be being met, if such a room was being subject to a new build or complete refurbishment, there are often instances where only certain electrical jobs have been instructed in such a room - leaving other circuits untouched. A classic example would be the installation of a new electric shower circuit only, where the client may instruct all other aspects or the room are left untouched. In this example, there may be a non-RCD protected lighting circuit, wall heater, fan, etc.

The wording of the regulation as it is now, therefore gives the option by providing SB, for a compliant installation - without extensive works to other circuits that may otherwise not be possible.

.... to which I have briefly responded, as follows ...

As regards my initial observation, yes, I am familiar with the history of supplementary bonding (and the introduction of RCDs), in particular the 'bonding-beserk' period to which you refer - and which, as you say, still sometimes happens today (albeit mainly at the hands of trades other than electricians!).

What you say is pragmatically very sensible, as in the sort of examples you cite - e.g. if some circuits in a bathroom are non-compliant with the current edition of BS7671 because of the absence of RCD protection then, if new work (such as installation of a shower circuit) is undertaken, it makes sense to mitigate the risk due to the absence of RCD protection by requiring SB.

I was just a little surprised that a set of regulations would recognise a situation in which some aspects were (and continued to be) non-compliant by requiring some other steps to be taken to mitigate risks due to the other continuing non-compliance - although, as I've said, that is pragmatically sensible. Given that 'surprise', I suppose I should be able to say what I would have expected as an alternative approach, and that is not too easy. The only thing I can really think of would have been for new work (such as the installation of a new circuit) not to be 'allowed' unless all final circuits in the room were RCD protected - but that might well be considered to be too 'Draconian', and very unwelcome by some practitioners!

.... see continuation in next post (10,000 character maximum, it seems ) ....

 

[JohnW2]

.... continued from previous post ....

I then moved on to my real question - about my serious difficulty in understanding the thinking behind 701.415.2(vi). The response I received isn't really all that helpful (i.e. I'm not convinced that the guy really understood my point, since his reply doesn't really say anything which we didn't already know)

The response seems to be majoring on the protection provided by RCDs but, as I've said (here) before, that might be a reason for never requiring SB, but does not help me to understand the thinking specifically behind 701.415.2(vi) - so you will see my further attempt to quiz him below ..

Regarding your second point of how SB can reduce the shock risk:

SB is considered as an addition to fault protection under BS 7671 in certain situations and locations. By connecting together exposed conductive parts and extraneous conductive parts, it can minimise the touch voltage between them under fault conditions. It will have the effect of re-establishing the equipotential references at that location. Such bonding is not required to be wired back to the main earthing terminal, although in reality locally bonded parts will be connected to this terminal, by virtue of one or more circuit protective conductors in final circuits and /or extraneous conductive parts.

In AC systems, regulation 415.2.2 requires the resistance (in reality impedance) R, of the supplementary bonding to be:

R to be less than or equal to 50V/Ia (ohms)

Where, Ia for an overcurrent device is the minimum current that disconnects the circuit in question (supplying the fault current) within 5 seconds. Where RCD protection is employed, it is the rated residual operating current.

Where 30mA RCD protection is employed on the circuit, R will be 1667 ohms, and by applying the graphs of Fig.3A4 of BS 7671:2018+Amd1:2020, relating to Type B MCBs, it will be in the range 1.67 ohms down to 0.2 ohms for typical MCBs rated between 6 and 50 amp.

Where these values can be met, either by existing connection paths (as referred to in reg. 701.415.2 (vi) - or by providing supplementary bonding to give a suitably low impedance path, suitably fast automatic disconnection can be achieved by the protective device in question that is supplying the fault in question. This satisfies BS 7671 with regards protection against shock hazard.

Supplementary bonding in this instance, particularly where the additional protection of an RCD may not apply, therefore not only minimises touch voltages between simultaneously accessible parts, but can also assist by providing a low impedance earth fault path, which in turn can give sufficiently fast operation of the relevant protective device.

I trust this information may be of some use to you and aid your understanding.

... to which I have responded, at some length ...

Anyway, as you will realise, the above was not my main query, which related specifically to 701.415.2(vi) and, despite your detailed response, I am still having difficulty in understanding this.

This discussion can be simplified, since 701.415.2(vi) only becomes relevant if all the other conditions for omission of SB are satisfied, in particular that all circuits supplying the room are (30 mA) RCD protected and that all exposed-c-ps are satisfactorily connected to CPCs. That means that, in that situation, the regulation is saying that SB is required if the resistance/impedance from the exposed-c-p to the MET is greater than 1667 ohms, but not required if the resistance/impedance is less than that figure.

This is probably best discussed in the context of an example. Consider (in the absence of SB) a fault (of 'negligible impedance') between the line conductor and an exposed-c-p of an item which is supplied via 2.5/1.5mm^2 T+E cable in a TN installation, with the supply voltage being 230V during the fault. The potential of the exposed-c-p will then, during the duration of the fault, rise to approximately 144V (5/8 x 230V) above MET potential. The RCD will clear that fault within a maximum of 40 ms [since the fault current will obviously be a lot more than 150 mA (30 mA x 5)] but will obviously have no effect on the magnitude of current flowing through any victim during that period.

It would then seem that if a person simultaneously touches the 'live' exposed-c-p and an extraneous-c-p, then the only effect of the resistance/impedance from the extraneous-c-p to MET would be to determine the magnitude of the current through that person until the fault was cleared (by the RCD).

If one ignores the (insignificant, in context) impedance of the line conductor of the circuit, and assumes that the impedance of the path through the victim is, say, 1,000 ohms, then ..

• SB is not required by the regulation (because that resistance/impedance is less than 1,667 ohms) when, in the absence of SB, the current through the victim would be at least about 54 mA [ 144V / (1,667 + 1,000) ] mA, the current approaching about 144 mA [ 144 / (0 + 1,000) ] when the resistance/impedance becomes very small (as it usually would be).
• SB is required by the regulation (because that resistance/impedance is more than 1,667 ohms) when, in the absence of Sb, the current through the victim would be less than about 54 mA (as above), with that current becoming increasingly small as the resistance/impedance rises (i.e. getting further away from the figure that would enable SB to be omitted).

If, in the fault scenario I have described, the only effect of the resistance/impedance (from extraneous-c-p to MET) is to determine the magnitude of the current through the victim (for the duration of the fault), I struggle to understand why the regulation should require SB to be installed when, without SB, the current through the victim would be lower than in the situation in which the regulation does not require SB.

I think I must be missing something. Is the regulation perhaps contemplating some fault scenarios other than that which I have described?

In considering other possible fault scenarios, if there were a fault from line conductor to an exposed-c-p but without that exposed-c-p being effectively connected to the CPC, then the 'touch voltage' between that 'live' exposed-c-p and anything connected to the MET would be 230V, but that would remain the case regardless of the resistance/impedance of the path from that 'something' to the MET.

In that situation, the fault, per se, would not result in the RCD (or ADS) operating. However, if someone simultaneously touched the 'live' exposed-c-p and an extraneous-c-p, the current flowing through them would be dependent upon that impedance between extraneous-c-p and MET. However, again, the current through the victim would decrease as that resistance/impedance increased. If the current through the victim exceeds the trip threshold of the RCD (I-delta-n), usually 30 mA, the device will obviously operate, even though it did not do so in response to the onset of the fault.

So, (again assuming a body resistance of 1,000 ohms)...

• SB is not required by the regulation (because that resistance/impedance is less than 1,667 ohms) when, in the absence of SB, the current through the victim would be at least about 86 mA [ 230 / (1,667 + 1,000) ], with the current approaching about 230 mA [ 230 / (0 + 1,000) ] when the resistance/impedance becomes very small.
• SB is required by the regulation (because that resistance/impedance is more than 1,667 ohms) when, in the absence of SB, the current through the victim will be less than about 86 mA (as above), with the current becoming increasingly small as the resistance/impedance rises (i.e. getting further away from the figure that would enable SB to be omitted), and if the resistance/impedance rises to more than about 6,667 ohms [ (230/0.03) - 1,000], current through the victim would fall to below 30 mA, and the RCD may not operate - should definitely not operate if resistance/impedance is more than about14,333 ohms [ (230 / 0.015) - 1,000 ].

So, again, I struggle (for the same reasons as above) to see how/why the regulation is appropriate in the context of that (more complex, 'double') type of fault.

I therefore wonder whether you may possibly be able help me further in understanding the thinking behind 701.415.2(vi), maybe by pointing out fault scenarios which I have not considered?

Many thanks for your assistance.

It is therefore my breath which is now bated. It will be interesting to see what (if any!) further response I get from them and I will, of course, report back s appropriate - so keep watching this space!


Kind Regards, John

 

[EFLImpudence]

Interesting, but obvious that your 'discovery' has not been considered by them either - unless, as you say, we are missing something.

I disagree with the first statement from the IET regarding the 'bonding berserk' period.
We have previously deduced (thanks, I think, to Securespark's old copy of the 15th) that the requirements of the 15th were virtually identical to the 18th. Any 'berserkness' being due to misinterpretation - as it still is.

Also the replier's wording in this passage:

"In light of this, the 1981 regulations, as having to follow the technical intent of International Standards, introduced the need for supplementary bonding of such locations to minimise the shock risk from simultaneously accessible exposed conductive parts (pieces of electrical equipment) and/or extraneous conductive parts (non-electrical equipment that could introduce a potential under fault conditions)"

The regulation just uses 'and'. I think the addition of '/or' significantly changes the requirements in that the SB would then be required when there are no exposed-c-ps in the room; just between exposed-c-ps.

I may be wrong about that but that is what it means to me.

 

[JohnW2]

Interesting, but obvious that your 'discovery' has not been considered by them either - unless, as you say, we are missing something.

Quite- as I said, not only don't they seem to have considered my point in the past, but I don't think that they actually understood what 'point' I was trying to make. I obviously tried to keep my initial query fairly brief, and I don't think that he has yet "got it". Hopefully, my more detailed subsequent e-mail ('with numbers') might help them to understand my point - do you think it is fairly clear?

As I have (now) said to them, as I see it the only effect of having a low resistance from extraneous-c-p to MET is that it will tend to increase the current through a victim (until the fault is cleared) - which hardly seems a good reason for allowing SB to be omitted

I disagree with the first statement from the IET regarding the 'bonding berserk' period.
We have previously deduced (thanks, I think, to Securespark's old copy of the 15th) that the requirements of the 15th were virtually identical to the 18th. Any 'berserkness' being due to misinterpretation - as it still is.

I'm not sure that he was necessarily meaning to imply otherwise, but maybe I misunderstood him.

Also the replier's wording in this passage:
"In light of this, the 1981 regulations, as having to follow the technical intent of International Standards, introduced the need for supplementary bonding of such locations to minimise the shock risk from simultaneously accessible exposed conductive parts (pieces of electrical equipment) and/or extraneous conductive parts (non-electrical equipment that could introduce a potential under fault conditions)"
The regulation just uses 'and'. I think the addition of '/or' significantly changes the requirements in that the SB would then be required when there are no exposed-c-ps in the room; just between exposed-c-ps. I may be wrong about that but that is what it means to me.

Having just thought about this (probably for the first time!), I think you may be wrong, since, even in the absence of any extraneous-c-ps, the existence (albeit rare) of exposed-c-ps on two different circuits (hence possible large PDs between them under fault conditions) in the room is just as potentially dangerous as having an exposed-c-ps and extraneous-c-ps, isn't it?

In other words, if a fault results in one exposed-c-p becoming 'live', the existence of a simultaneously-touchable exposed-c-p on a different circuit (hence at MET potential) is just as 'dangerous' as is an extraneous-c-p, isn't it?

However, yet again, this is a brand new 'discovery' on my part which I've never thought of before. Until you (just) raised the issue, I would have said exactly the same as you - i.e. that SB cannot be required if there are no extraneous-c-ps!

As for your reason for your comment, I'm not actually convinced that "and/or" necessarily has any different meaning from "and" in this context. Although 701.415.2 says ...

701.415.2 Supplementary protective equipotential bonding
Local supplementary protective equipotential bonding according to Regulation 415.2 shall be established connecting together the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to the accessible extraneous-conductive-parts ...

... which might be taken to imply that it only applies if there are extraneous-c-ps, it refers to 415.2, and that says ...

415.2.1 Supplementary protective equipotential bonding shall include all simultaneously accessible exposed conductive-parts of fixed equipment and extraneous-conductive-parts ...

Although that says "and", I don't think it precludes the situation in which there are no extraneous-c-ps - i.e. I think that, as written, it probably means the same as "...Supplementary protective equipotential bonding shall include all simultaneously accessible exposed conductive-parts of fixed equipment and any extraneous-conductive-parts which exist in the room ..."

As I've said above, if it doesn't mean that then, electrically speaking, I think it probably should, for the reason I have explained

After all, the first of the above regs includes "... connecting together the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to ..." and I wouldn't imagine that you would suggest that the reg doesn't apply when there are not both Class I and Class II items present, would you? ... in other words, again, I think the "and" implies "and/or" - i.e. it would still apply if there were only Class I or only Class II items present?

Kind Regards, John

 

[JohnW2]

However, yet again, this is a brand new 'discovery' on my part which I've never thought of before. Until you (just) raised the issue, I would have said exactly the same as you - i.e. that SB cannot be required if there are no extraneous-c-ps!

It has occurred to me (again for the first time ever) that there is another way of looking at this ...

As far as a circuit (with its exposed-c-ps) in a room is concerned, the CPC of any other circuit which enters the room (and anything connected to that CPC) essentially/electrically/functionally IS an extraneous-c-p, since it is "liable to introduce a potential, generally Earth potential ....", even though it does not satisfy the BS7671 definition because of the last bit of that definition (as emboldened here), which says ".... liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation".

So, as I said, and despite the last clause of that definition, it would seem that the CPC (hence connected exposed-c-ps) of a different circuit presents just as much of a 'danger' as does a more 'conventional' sort of 'extraneous-c-p'.

Kind Regards, John

 

[EFLImpudence]

Having just thought about this (probably for the first time!), I think you may be wrong, since, even in the absence of any extraneous-c-ps, the existence (albeit rare) of exposed-c-ps on two different circuits (hence possible large PDs between them under fault conditions) in the room is just as potentially dangerous as having an exposed-c-ps and extraneous-c-ps, isn't it?

Ah I'm sorry, what I wrote didn't actually make sense. It should have read:

"The regulation just uses 'and'. I think the addition of '/or' significantly changes the requirements in that the SB would then be required when there are no exposed-c-ps in the room; just between extraneous-c-ps."

 

[EFLImpudence]

As far as a circuit (with its exposed-c-ps) in a room is concerned, the CPC of any other circuit which enters the room (and anything connected to that CPC) essentially/electrically/functionally IS an extraneous-c-p,

Well yes, that is what usually makes a pipe an extraneous-c-p.

since it is "liable to introduce a potential, generally Earth potential ....", even though it does not satisfy the BS7671 definition because of the last bit of that definition (as emboldened here), which says ".... liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation".

Yes, but generally Earth potential means it could be another potential.

Presumably something that IS part of the electrical installation is bound to be liable to introduce a potential.

 

[EFLImpudence]

After all, the first of the above regs includes "... connecting together the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to ..." and I wouldn't imagine that you would suggest that the reg doesn't apply when there are not both Class I and Class II items present, would you? ... in other words, again, I think the "and" implies "and/or" - i.e. it would still apply if there were only Class I or only Class II items present?

No, not in the way you have framed the question, However -
what is the point of bonding the CPCs of Class II items? I think the regulation is just to cover future possible need for a CPC (I'm sure it says that somewhere); not any electrical situation.

Bearing in mind my error above, I would ask it the other way round.

Do you think the CPCs of Class II items need to be bonded (either together or to extraneous-c-ps) if there were no Class I items in the room?

 

[JohnW2]

Ah I'm sorry, what I wrote didn't actually make sense. It should have read: .... "The regulation just uses 'and'. I think the addition of '/or' significantly changes the requirements in that the SB would then be required when there are no exposed-c-ps in the room; just between extraneous-c-ps."

This is getting quite complicated

For a start, as I have often observed, unless there are electrically-heated 'towel rails'/radiators, it's probably pretty rare that there any any exposed-c-ps in a bathroom.

However, 701.415.2 requires that SB be connected to (amongst other things) "the terminals of the protective conductor of each circuit supplying .... Class II equipment". Since Class II items do not have exposed-c-ps in the normal sense (and often literally will have no exposed conductive parts at all), they seem to be thinking that this requirement could event apply if there were no Class I items (hence no conventional exposed-c-ps) in the room at all?

As for a room with only extraneous-c-ps (no exposed-c-ps), I suppose it is not inevitable that the "potentials liable to be introduced" by different extraneous-c-ps would be the same - so, at least per definitions, it's not impossible that (in the absence of SB) dangerous PDs could arise between two or more different extraneous-c-ps.

As I've recently been saying, I do see a potential issue in rooms with only exposed-c-ps (no extraneous-c-ps) if more than one circuit is involved.

... so, as I said, a bit complicated, rather than 'Simples'

Kind Regards, John

 

[EFLImpudence]

However, 701.415.2 requires that SB be connected to (amongst other things) "the terminals of the protective conductor of each circuit supplying .... Class II equipment". Since Class II items do not have exposed-c-ps in the normal sense (and often literally will have no exposed conductive parts at all), they seem to be thinking that this requirement could event apply if there were no Class I items (hence no conventional exposed-c-ps) in the room at all?

I'm going to ignore the bonding of CPCs of Class II items for the reason I have said.

As for a room with only extraneous-c-ps (no exposed-c-ps), I suppose it is not inevitable that the "potentials liable to be introduced" by different extraneous-c-ps would be the same - so, at least per definitions, it's not impossible that (in the absence of SB) dangerous PDs could arise between two or more different extraneous-c-ps.

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?

 

[JohnW2]

Well yes, that is what usually makes a pipe an extraneous-c-p.

Possibly (if you are thinking about extraneous-c-ps which were 'earthed' as a result of attached 'electrical' things - like boilers, immersions, CH components etc.) but I was talking about the 'actual CPCs (bare copper conductors) in the cables serving other circuits.

Yes, but generally Earth potential means it could be another potential.

Indeed so - as I just wrote, that could be used as an argument for requiring SB connecting multiple extraneous-c-ps entering a room, even if there were no exposed-c-ps in the room (and, indeed, in theory even if there was no 'electricity supply' to the room at all!).

Presumably something that IS part of the electrical installation is bound to be liable to introduce a potential.

Quite - which is why I don't think it is necessarily rational that they have a blanket exclusion of such parts from their definition of an extraneous-c-p. As I see it, the CPC of, say, a nearby immersion circuit is just as 'dangerous' as, say, a conventional (pipe) extraneous-c-p, even though it is 'part of the electrical installation'.

Kind Regards, John

 

[JohnW2]

However - what is the point of bonding the CPCs of Class II items?

Obviously no electrical point at all.

Bearing in mind my error above, I would ask it the other way round. ... Do you think the CPCs of Class II items need to be bonded (either together or to extraneous-c-ps) if there were no Class I items in the room?

As above, there is obviously no electrical point in bonding the CPCs of Class II items at all - regardless of whatever else might be in the room.

We are talking about the regulations, not common sense. Are you forgetting that the regs are not necessarily meant to make sense?

Kind Regards, John

 

[EFLImpudence]

Quite - which is why I don't think it is necessarily rational that they have a blanket exclusion of such parts from their definition of an extraneous-c-p. As I see it, the CPC of, say, a nearby immersion circuit is just as 'dangerous' as, say, a conventional (pipe) extraneous-c-p, even though it is 'part of the electrical installation'.

Yes, but parts of the electrical installation are bound to present a danger.

A pipe, for example is not part of the electrical installation.


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

 

[EFLImpudence]

Obviously no electrical point at all.
As above, there is obviously no electrical point in bonding the CPCs of Class II items at all - regardless of whatever else might be in the room.
We are talking about the regulations, not common sense. Are you forgetting that the regs are not necessarily meant to make sense?

I realise that but if something is required just to cover future eventualities, it is not reasonable to mandate that it should be subject to all the regulations covering usual parts.


Don't forget that a Class II item likely only has a CPC in the cable because of a similar non-electrical requirement.