Supplementary Bonding - my questionable conclusions?

[JohnW2]

In a current thread ( see here), I have been discussing some issues about Supplementary Bonding (SB) which, after years of not thinking so deeply about these things, have recently occurred to me. Since my conclusions seem to be considerably at variance with what BS7671 seems to imply, I’m not all that comfortable, and therefore would welcome and comments which may reveal flaws in my thinking.

In the meantime, my main current (tentative!) conclusions appear to be as follows:

• IF one wants to ensure that a situation cannot arise in which a dangerous potential difference can arise between simultaneously-touchable extraneous-c-ps and exposed-c-ps, or between simultaneously-touchable exposed-c-ps on different circuits, then:

• The only way of achieving that appears to be to install Supplementary Bonding, locally joining together all simultaneously-touchable extraneous- and exposed-c-ps

• Although BS7671 only (occasionally) requires SB in bathrooms, the above applies more-or-less equally to any room. The only small difference is that the severity of electric shock may be somewhat greater in a bathroom because of ‘wet skin’, but that is equally true in any room which has water (kitchens, utility rooms, bedrooms, toilets, bedrooms with hand-wash basins etc.).. Even with dry skin (hence in any room) there is a risk of severe electric shock if dangerous PDs can arise between simultaneously-touchable parts.

• 701.415.2(vi), which (via a Note) invokes the test criteria described in 415.2.2, implies that risk reduces (and hence SB may be omitted if all other conditions are satisfied) if the resistance from extraneous-c-p to MET is low. However, by my reasoning this is NOT correct. Indeed, on the contrary, it seems to me that the severity of shock will actually tend to (slightly) increase as that resistance decreases - with the risk of shock being as high as ever, and the severity of the shock (if it happens) being at its highest if that resistance became (hypothetically) 'zero'.

• IF, on the other hand, one decides that when there is RCD protection, there is no need to worry about the possibility of a dangerous potential difference arising between simultaneously-touchable extraneous-c-ps and exposed-c-ps, or between simultaneously-touchable exposed-c-ps on different circuits, then:

• there would seem to be minimal reason for requiring SB anywhere when there is RCD protection, thereby questioning the need to single out bathrooms as requiring SB (under certain circumstances).

• Other regulations in BS7671 already require that circuits supply bathrooms should be RCD protected and, indeed, that in the absence of RCD protection, SB must be installed in bathrooms.

Some of the above seems to be considerably at variance with the thinking (and requirements) of BS7671, so I would be very interested to hear any thoughts about the above, particularly if they reveal any flaws in my reasoning as discussed in the other thread.

My greatest concern is probably the implications of 701.415.2(vi) - which seems to be suggesting that (all other conditions being satisfied) SB can be omitted in the situation in which (by my reasoning) the risk from electric shock of the type I have described is at its greatest (‘severity’-wise).

I therefore have to question the validity of my reasoning, since I would not presume to suggest that I ‘know better’ than the many experts (or ‘experts’) who must have given their blessing to the regulation!

Kind Regards, John

 

[EFLImpudence]

Something I cannot get out of my mind:

In the event of an L to exp-c-p fault at an appliance, are you sure the MET is still at 0V during the fault?

After all, it is not really THE earth point (the planet), is it? In TN-S and TN-C-S (not PME), that will be at the sub-station.
So, R1 and R2 is a continuous conductor from the supply to the MET and back to the sub-station.
Are your calculations and diagrams ignoring the effect of Ze?

If the CPC were disconnected from the MET the Voltage on it would be ~230V, wouldn't it? Yes.

Does that not mean that the Voltage between the exp-c-p and the MET during the fault is If x R2?

 

[JohnW2]

Something I cannot get out of my mind: ... In the event of an L to exp-c-p fault at an appliance, are you sure the MET is still at 0V during the fault? ... After all, it is not really THE earth point (the planet), is it?

Sure, but I have said nothing about the potential difference between the MET and 'the planet' ('true earth'), since that is irrelevant.

Everything I've said would be equally true if the MET were at 5,000V above true earth potential. The whole point of (hopefully) having a property constituted as an equipotential zone, is that all that then matters (to people within that property) are potential differences relative to the potential of the MET. Hence, everything I've said relates to 'potentials relative to the MET', and the potential difference of the MET 'relative to the MET' is obviously always 0V.

In practice, of course, during a high current L-E fault the potential of the MET will rise relative to 'true earth' potential, due to the voltage drop in the 'earth component' of the Ze. However, as above, that is irrelevant to these discussions, since all that matters to a person within the property are potentials relative to MET potential (regardless of what MET potential is relative to 'true earth' potential).

... Are your calculations and diagrams ignoring the effect of Ze?

As per the second 'disclaimer' at the start of my most recent post in the other thread, yes, I am ignoring Ze (hence ignoring the rise in MET potential {relative to true earth} during a fault) - but, as above, that is irrelevant to these discussions - since (again!) " all that matters to a person within the property are potentials relative to MET potential (regardless of what MET potential is relative to 'true earth' potential)".

If the CPC were disconnected from the MET the Voltage on it would be ~230V, wouldn't it? ...

If the CPC were disconnected from the MET then, yes, the (disconnected) CPC would then be at full supply voltage, since there would then be no 'fault current' flowing, hence no VDs anywhere. However, I'm not sure what your point is!

Does that not mean that the Voltage between the exp-c-p and the MET during the fault is If x R2?

The short answer is No

I'm not sure what "exp"-c-p you are referring to but, as above, if you disconnected the CPC from the MET, then the potential of the exposed-c-p (and disconnected CPC) would rise to full supply potential (since no current is flowing anywhere). However, when the CPC is connected to the MET, then If flows through R1 and R2, forming a 'potential divider, such that the PD between exposed-c-p and MET (i.e. the voltage across R2) becomes ~230 x R2 / (R1+R2) - i.e. my ~142V with 2.5/1.5 mm² cable.

Kind Regards, John

 

[JohnW2]

... just to clarify/simplify ...

... imagine yourself permanently locked up inside a house, with all doors and windows closed and locked. If any/all required main bonding were in place, then you wouldn't know anything, nor care, about the potential of your house's MET (and everything connected to it) relative to true earth - the MET might be at true earth potential, 50V above true earth potential or even 5,000V above true earth potential, and you would neither know nor care. All that would matter to you would be potential differences within your house relative to the potential of your MET.

Kind Regards, John

 

[EFLImpudence]

Didn't mean to post yet.

 

[JohnW2]

Didn't mean to post yet.

Fair enough - my breath remains bated

Kind Regards, John

 

[EFLImpudence]

Please answer this for me.

Imagine the Line conductor of a circuit is a bare conductor.
One could touch that Line conductor at any two points and not get a shock - apart from the Volt drop created by the current and resistance of the conductor between the two points of contact.
E.g. 10A x 0.1Ω = 0.1V

Is that correct?

So, with an earth fault at an appliance, in effect the line and CPC are one continuous conductor.
Why then, cannot one touch the CPC at two points (exposed-c-p and MET) and be subject to the same conditions?
E.g. 160A x 0.1Ω = 16V

Why is it not the 16V but 142V you calculated?


P.S. person not earthed, of course.

 

[JohnW2]

Please answer this for me. Imagine the Line conductor of a circuit is a bare conductor.
One could touch that Line conductor at any two points and not get a shock - apart from the Volt drop created by the current and resistance of the conductor between the two points of contact. E.g. 10A x 0.1Ω = 0.1V Is that correct?

Yes, that is correct, other than for your arithmetical error (10A * 0.1Ω = 1V, not 0.1V ).

However, in terms of the fault scenarios we are talking about, it could well be, say, 600A X 0.3Ω = 180V !

So, with an earth fault at an appliance, in effect the line and CPC are one continuous conductor. Why then, cannot one touch the CPC at two points (exposed-c-p and MET) and be subject to the same conditions?
E.g. 160A x 0.1Ω = 16V ... Why is it not the 16V but 142V you calculated?

At least your arithmetic is correct this time

Your application of Ohm's Law is obviously correct - i.e. if there were 160A flowing through the path (consisting of the L-conductor {R1}and CPC {R2} in series, hence a total resistance of R1+R2), then if you touched two parts of that path which has a resistance of 0.1Ω between them, then you would indeed experience a 16V potential difference.

However, your figures are impossible (unless one had a cable in which the CPC was massive in comparison with the L-conductor). With a 230V supply, a current of 160A implies that R1+R2 = ~1.44Ω. With a real-word cable, the resistance between the exposed-c-p and the MET (i.e. the resistance of the CPC, R2) would then be an awful lot more than 0.1Ω - for example, as I've said, with a 2.5/1.5mm² cable, if R1+R2 = 1.44Ω, then R2 would be about 0.9Ω (1.44 x 5/8), not 0.1Ω. Hence the PD between exposed-c-p and MET (i.e. across R2, the CPC) would be about 144V (160A x 0.9Ω)

Does that help/make sense?

Kind Regards, John

 

[JohnW2]

Arrrgh! ...

 

[EFLImpudence]

Ah, sorry about the error (partial edit)
Yes, it could be more - but it shows that the Ze has an effect.

That's good. I thought I was going mad.

Yes, but contrary to what I said before, perhaps your method of using ratios of conductors was not ideal as there are different permutations.
There could be 600A but that's unlikely.

In some circumstances it could mean that the touch voltage was low enough without supplementary bonding such as just a metal light with circuit and water pipes connected to the CU on the other side of the bathroom wall.
Perhaps not a frequent occurrence but it is possible.

There still seems to be nothing related to R < 50/Ia.

In the above diagram it would be 50/30 = 1.666 which would be way too high.

 

[JohnW2]

Yes, it could be more - but it shows that the Ze has an effect.

Yes, but not an effect on what we are considering, which is potential differences relative to the MET.

Yes, but contrary to what I said before, perhaps your method of using ratios of conductors was not ideal as there are different permutations.

I don't understand. As I explained and illustrated in my recent post in the other thread, "using the ratio of conductors" is simply arithmetically slightly simpler that marking out the current and then multiplying by the resistance - both will give identical answers.

There could be 600A but that's unlikely.

The current doesn't affect anything. If it were 600A going through R1 (L-conductor) and R2 (CPC in series), then the voltage across R2 would be (600 x R2) and the voltage across R1 would be (600 x R1) so the ratio of voltages across the resistors (i.e. R2/R1) would be (600 x R2) / (600 x R1) = R2/R1.

If we consider, say, 2.5/1.5 mm² cable, then R2/R1 is always 5/3, regardless of the length of the cable. The voltage across R2 (CPC) will therefore always be 5/3 times the voltage across R1 (L-conductor) - which means that the voltage across R2 (CPC) will always be 5/8 of the voltage across R1+R2 (i.e. the full supply voltage during fault) - and all that regardless of the actual value of the resistances (hence current) - it is only the ratio of the resistances (which will always be the same for any length of any particular cable) that matters.

In some circumstances it could mean that the touch voltage was low enough without supplementary bonding such as just a metal light with circuit and water pipes connected to the CU on the other side of the bathroom wall. Perhaps not a frequent occurrence but it is possible.

For a start, in terms of T+E, the figures in your diagram would only work for 1.0mm cable, since that is the only T+E for which R1=R2, so I will assume that such is the cable we're talking about (i.e. R2/R1 = 1).

You have somewhat moved the goalposts. As I said, I was 'ignoring' Ze - or, at least, was considering a situation in which the voltage at transformer and the Ze were such that the voltage at the origin of the installation was 230V during the fault. You have calculated a low 'touch voltage' by postulating a situation in which Ze was (50%) higher than (R1+R2), which means that the voltage at the original of the installation would fall to only 92V during the fault.

My calculation would therefore still work. With a supply voltage (at origin of installation) during fault of 92V and R2/R1=1, my calculation would lead to the same answer (92V/2 = 46V) as you got.

If you want to include the added complication of supply voltage falling during the fault (with supply voltage of 230V {assuming no other loads} before onset of the fault), try repeating your calculation with (probably more realistic, with 1.0mm² cable) Ze = 0.2Ω and R1=R2=1.0Ω. Using 'my approach', I would determine that the supply voltage (at origin) would fall to about 209V during the fault - so that, with R1=R2, the 'touch voltage' (voltage across R2) would be about 104.5V. I presume (hope) you will get the same.

If one used my 'simplification' of assuming that the voltage at the transformer and the Ze were such that voltage at the origin was 230V during the fault then, of course, with 1.0mm² cable (hence R2=R1) I would get a touch voltage of 115V (230v/2) - or, if it were 2.5mm cable [R2 = (R1 x 5/3], I would get the 'now infamous' 144V .

Kind Regards, John

 

[JohnW2]

.... There still seems to be nothing related to R < 50/Ia.

I should have added (but didn't, because it probably seemed obvious) ....

... I agree. Quite apart from discussions about the magnitude of the 'touch voltage' (pd between extraneous- and exposed-cps) (which I hope I have clarified in my previous post), what you (and your diagram/calculations) seem to indicate is that you now agree that the 'touch voltage' is in no way dependent upon the resistance from extraneous-c-p to MET - hence making it very difficult to understand 701.415.2(vi).

On the contrary, as I've said, the only effect of that resistance is that, for any given magnitude of 'live' voltage (relative to MET) one is touching (as well as touching the extraneous-c-p), the current through a victim increases as that resistance decreases. 701.415.2 therefore seems to be, if anything, "erring on the side of being dangerous", by implying that 'lower is better' for that resistance

Kind Regards, John

 

[JohnW2]

I seem to have 'silenced' EFLI, at least temporarily, and no-one else has yet to get involved, all of which is a bit of a pity.

However, it has occurred to me that there is a related 'common misapprehension' which again seemingly leads to the conclusion that there are some potential risks within an electrical installation which could only be minimised by Supplementary Bonding (regardless of anything else), unless one feels that RCDs provide adequate protection (in which case Supplementary Bonding would never be deemed necessary).

Within a true 'equipotential zone', there is obviously no risk that anyone could be subjected to any significant potential difference if they touched two things (extraneous- and/or exposed-c-ps), since all such things would, by definition, be at the same potential.

What we do within a building is to ensure that all extraneous-c-ps and all exposed-c-ps are 'effectively' connected to the MET of the installation (via Main Bonding conductors and CPCs respectively). In the absence of faults, that does, indeed, ensure that no appreciable potential differences exist between any of those parts - since, because no significant current is normally flowing in either bonding conductors or CPCs (hence no voltage drop) all those parts are normally at essentially MET potential - hence all at the same potential.

However, what some people seem to overlook (since they think they have created something which is always an 'equipotential zone') is that everything changes if there is an "L->E" (L->exposed-c-p/CPC) fault in something connected to one of the circuits. A very high fault current then flows through the CPC leading to a high voltage drop in the CPC - which, as I have shown, will usually result in the potential of the exposed-c-p (and any other nearby exposed-c-ps on the same circuit) rising to well over 100V above MET potential for the duration of the fault, whilst all exposed-c-ps on other circuits, and all extraneous-c-ps will remain at MET potential. There will therefore be potentially dangerous PDs (during the duration of the fault) between one or more exposed-c-ps and all the other exposed- and extraneous-c-ps.

IF one is concerned about such potentially dangerous PDs (which exist only until the fault is cleared by an OPD or RCD), then:

• The only way to prevent that situation arising would be to ensure that all simultaneously touchable parts (exposed- and/or extraneous-) are joined 'locally' by Supplementary Bonding, regardless of any other considerations.
• In the absence of Supplementary Bonding, items with simultaneously touchable exposed-c-ps (e.g. Class I appliances in a kitchen) should ideally all be supplied by the same circuit (hence same CPC, hence roughly equal potentials, even during a fault)

IF, on the other hand, one is not concerned about these dangerous potential differences, on the basis that they would be rapidly terminated by operation of an RCD (assuming one exists), then one should never feel that Supplementary Bonding is required, in any room (unless as 'belt and braces' in case of RCD failure).

Kind Regards, John

 

[EFLImpudence]

I didn't know what else to say - and thought others may have had some input - Scousespark? .

I (now) agree with your latest post.

In view of what you have realised (discovered), when talking about a shock received by a person touching two conductive-parts during a fault who is not himself earthed (by the floor), is the danger actually any more in a bathroom than a kitchen?

 

[JohnW2]

I didn't know what else to say - and thought others may have had some input - Scousespark? . ...

Fair enough. As I said, I would certainly welcome input from others. Although I am as sure as I can be that everything I have said, illustrated and calculated is correct, the issue is so potentially 'important' that I would at like some third-party-reassurance that I haven't got it all wrong!

I (now) agree with your latest post.

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?

In view of what you have realised (discovered), when talking about a shock received by a person touching two conductive-parts during a fault who is not himself earthed (by the floor), is the danger actually any more in a bathroom than a kitchen?

Quite so. As I said early on, in terms of the sort of risk we have been discussing, the only sense in which the risk might sometimes be greater in a bathroom is that people are perhaps more likely to have wet hands (or whatever body part is involved) in a bathroom - but, as you imply, that is certainly a strong possibility in a kitchen (or a loo with basin) etc.

The only way I can (partially) attempt to rationalise the thinking that results in SB not being required ('everywhere') is on the basis that, as you have observed, all the 'touch voltages' (PDs between simultaneously touchable things) I have been talking about only persist for the durations of the fault - so if there is RCD protection to limit that duration to a 'safe' extent, then there is "no reason to worry about" (or try to prevent) those very brief (high) touch voltages. However, that argument would apply everywhere, including bathrooms - so, as above, could not explain the 'singling out' of bathrooms.

As I've probably observed before, one odd thing about 701.415.2 is that all of the conditions for omitting SB in bathrooms other than 701.415.2(vi) refer to situations in which failure to satisfy the conditions would already be a non-compliance with BS7671 (inadequate/absent main bonding, ADS or RCD protection in a bathroom) - and it seems very odd to have a regulation which is effectively 'requiring' something (SB) only in situations in which the installation was already non-compliant!

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!

Kind Regards, John