SWA - Consumer unit or external box?

[JohnW2]

Well they may not actually say that but that is the outcome.
As in the house, the earthing conductor is sized by the adiabatic equation and bonding conductors half of that - subject to the minimum, of course - but is that for electrical reasons?.

Who knows - but I suspect that the minimum may well be in relation to 'mechanical ruggedness', much as maybe also the reason for the minimum for a single CPC being 4mm².

Yes, sorry, I should have said CPC - although it will be the earthing conductor for the out-building, won't it?

I wouldn't have thought so - unless you're going to suggest that the outbuilding is a separate installation (!!) (with it's own MET). As I said, I have always thought that the 'earthing conductor is only/just the conductor from the earth (TN or local electrode) to the MET, and I presume that one installation can only have one MET (the clue being the 'M').

It is correct. However, if a bonding conductor of, say 3mm², were all that were actually required (apart from the minimum specification) then, surely a copper equivalent of that would be required; not the 6mm² minimum.

I would have thought that there would still be a minimum CSA for whatever material was being used, that minimum being equivalent to 6mm² of copper - in other words, if the minimum for copper is 6mm², the minimum for steel would presumably be a lot greater than 6mm².

I haven't looked it up, but if a 2.5mm² CPC (earthing conductor) is adequate for the supply in the out building then the equivalent of 1.25mm² is likely - if it needs to be used instead or as well as the CPC (EC).

You seem to again be assuming that the CPC to the outbuilding can be regarded as an 'earthing conductor' and that a bonding conductor is therefore only required to have a CPC of half the CSA of that CPC, rather than half of the required CSA of the (house's) installation's true earthing conductor (from DNO/rod earth to the {house's} MET). I'm not at all sure that one can make those assumptions.

Kind Regards, John

 

[EFLImpudence]

Who knows - but I suspect that the minimum may well be in relation to 'mechanical ruggedness', much as maybe also the reason for the minimum for a single CPC being 4mm².

Quite, so 6mm² steel - possibly less.

I wouldn't have thought so - unless you're going to suggest that the outbuilding is a separate installation (!!) (with it's own MET). As I said, I have always thought that the 'earthing conductor is only/just the conductor from the earth (TN or local electrode) to the MET, and I presume that one installation can only have one MET (the clue being the 'M').

Whatever it gets called, it may be sized by the adiabatic equation. Call it a separate installation if you want.

I would have thought that there would still be a minimum CSA for whatever material was being used, that minimum being equivalent to 6mm² of copper - in other words, if the minimum for copper is 6mm², the minimum for steel would presumably be a lot greater than 6mm².

I don't see why if it is for mechanical protection.

You seem to again be assuming that the CPC to the outbuilding can be regarded as an 'earthing conductor' and that a bonding conductor is therefore only required to have a CPC of half the CSA of that CPC, rather than half of the required CSA of the (house's) installation's true earthing conductor (from DNO/rod earth to the {house's} MET).

Rod? I am only talking about true TN-S installations
where the large bonding conductors to cope with high current are not needed.

Would each flat in a block supplied by TN-S have to have bonding conductors sized by the incomer?

I'm not at all sure that one can make those assumptions.

I'm not at all sure they are assumptions.

 

[JohnW2]

Quite, so 6mm² steel - possibly less.

That's surely just a guess, and I wouldn't think that one should interpret regs on the basis of a guess, should one? Even if the reason for the minimum CSA is a 'non-electrical' one, steel can 'rust away' but copper doesn't, so another (different) guess would be that the minimum CSA for steel might be greater.

Whatever it gets called, it may be sized by the adiabatic equation. Call it a separate installation if you want.

On the basis of what OPD would you propose to undertake an adiabatic calculation? For the (house's) real 'earthing conductor', one would presumably use the cutout fuse?

I don't see why if it is for mechanical protection.

Perhaps not - but, as above, you seem to be making up your own rules on the basis of a guess about the reason for the minimum CSA specified in the regs.

I am only talking about true TN-S installations where the large bonding conductors to cope with high current are not needed.

The requirements for main bonding really all relate to very rare fault conditions. Don't forget the "generally Earth potential" bit of the definition of an extraneous-c-p. In very unlikely situations, the potential introduced by an extraneous-c-p could be 230V, in which case high currents could flow through bonding conductors, even in TN-S installations. If it is your view that one can ignore the very small possibility of a relatively large pd arising between a TN earth and an extraneous-c-p, then you logically would argue that main bonding is not required with TN-S ??

Would each flat in a block supplied by TN-S have to have bonding conductors sized by the incomer?

I don't know how the regulations regarding main bonding are interpreted in such situations or what the normal practice is - can you tell me? I have a horrible suspicion that it may come down to another discussion regarding the meaning/interpretation of 'an installation' in that situation.

Kind Regards, John

 

[EFLImpudence]

That's surely just a guess, and I wouldn't think that one should interpret regs on the basis of a guess, should one? Even if the reason for the minimum CSA is a 'non-electrical' one, steel can 'rust away' but copper doesn't, so another (different) guess would be that the minimum CSA for steel might be greater.

I do believe steel is stronger than copper.

On the basis of what OPD would you propose to undertake an adiabatic calculation?

The OPD protecting the supply circuit to the out-building.

For the (house's) real 'earthing conductor', one would presumably use the cutout fuse?

Yes.

Perhaps not - but, as above, you seem to be making up your own rules on the basis of a guess about the reason for the minimum CSA specified in the regs.

Well, I did ask and you suggested mechanical protection. What else can it be? It's not the smaller size just calculated with the equation, is it?
Is it not the same for earthing conductors to rods?

The requirements for main bonding really all relate to very rare fault conditions. Don't forget the "generally Earth potential" bit of the definition of an extraneous-c-p. In very unlikely situations, the potential introduced by an extraneous-c-p could be 230V, in which case high currents could flow through bonding conductors, even in TN-S installations. If it is your view that one can ignore the very small possibility of a relatively large pd arising between a TN earth and an extraneous-c-p, then you logically would argue that main bonding is not required with TN-S ??

I don't follow.

I don't know how the regulations regarding main bonding are interpreted in such situations or what the normal practice is - can you tell me?

It no longer applies. Blanket 10mm² MPB now.

Do your questions not relate to all situations as well as what I have said?

TN-S ECs are calculated by the adiabatic equation and the MPBs are half that, subject to the minimum.
It's surprising how small they used to be.

 

[ban-all-sheds]

This chart shows which (90°C XPLE) SWA cables comply with Table 54.7 when using only the armour as the CPC:

Green = compliant, Red = Non-Compliant

 

[JohnW2]

I do believe steel is stronger than copper.

Of course it is, size for size, when both materials are intact. However, once the stell has corroded significantly, it could be less strong that copper (again, size for size).

The OPD protecting the supply circuit to the out-building.

There's surely something fundamentally wrong with the reasoning that's being implied here ...

... a main bonding conductor exists for one, and only one, purpose - namely to carry whatever current is necessary to minimise any pd which arises (for whatever reason) between an extraneous-c-p and the installations 'earth' (i.e. its MET), thereby attempting to create an equipotential zone. Any such current does not even flow through any OPD within the installation, so it makes no real sense that the required CSA of that boding conductor should be in any way dependent on, say, "the OPD protecting the supply to the outbuilding", does it?

Well, I did ask and you suggested mechanical protection. What else can it be?

Yes, you asked, and I offered a guess - but, as I said, I don't think we should interpret regulations on the basis of guesses!

Is it not the same for earthing conductors to rods?

Indeed it is - that's why I mentioned rods (and you asked me why I'd mentioned them!).

It no longer applies. Blanket 10mm² MPB now.

Are you talking about current practice or regulations - and, if the latter, what regulation? In any event, 10mm from where to where?

TN-S ECs are calculated by the adiabatic equation and the MPBs are half that, subject to the minimum.

Indeed, but you seem to be using the term "Earthing Conductor" to refer to something which is not what I understand to be the (one and only) 'real Earthing Conductor' in an installation. As far as I can see, the required CSA of that ('one and only') 'real Earthing Conductor' can be determined by an adiabatic calculation based on the characteristics of the cutout fuse, and then any/all main bonding conductors within the installation have to have a CSA at least half of that (subject to the minimum). Do I take it that you disagree?

Kind Regards, John

 

[JohnW2]

This chart shows which (90°C XPLE) SWA cables comply with Table 54.7 when using only the armour as the CPC:

Yes, we often discuss that Table. However, (a) it relates to Table 54.7 figures (per 533.1.4), which are ones designed for those too lazy to undertake adiabatic calculations (per 533.1.3) and, much more important to this discussion, is merely talking about the required CSAs of CPCs in general. The requirements for minimum CSA of a protective conductor which is a main bonding conductor, as being discussed in this thread, are defined in 544.1.1 - and those minimum requirements may be greater than those for CPCs in general per 543 (adiabatic calc or Table 54.7).

Kind Regards, John

 

[EFLImpudence]

This chart shows which (90°C XPLE) SWA cables comply with Table 54.7 when using only the armour as the CPC:

That table - using s x K1/K2 is sort of contradicted by the note below Table 54.8 which says that the conductor must be of the equivalent conductance which for steel is eight or nine times less.

Do you think this is a mistake?

 

[JohnW2]

That table - using s x K1/K2 is sort of contradicted by the note below Table 54.8 which says that the conductor must be of the equivalent conductance which for steel is eight or nine times less. Do you think this is a mistake?

I'm not sure that one can necessarily describe it as a 'mistake', since I suppose they are free to impose requirements specifically for bonding conductors in PME installations which are different from the requirements for CPCs in general, if they so wish . For the former they seem to have decided that 'conductance equivalence' is what ('all') is required for non-copper conductors. For the latter, they use 'k values' which, as we have discussed before, appear to take on board factors (e.g. behaviour with temp rises) other than just relative conductance.

Kind Regards, John

 

[EFLImpudence]

between an extraneous-c-p and the installations 'earth' (i.e. its MET), thereby attempting to create an equipotential zone.

Yes but in true TN-S the installation's earth is in the ground.

Are you talking about current practice or regulations - and, if the latter, what regulation? In any event, 10mm from where to where?

As it will be PME, from extraneous-c-p flat entry point to the flat's MET (which isn't really a MET).

Indeed, but you seem to be using the term "Earthing Conductor" to refer to something which is not what I understand to be the (one and only) 'real Earthing Conductor' in an installation. As far as I can see, the required CSA of that ('one and only') 'real Earthing Conductor' can be determined by an adiabatic calculation based on the characteristics of the cutout fuse, and then any/all main bonding conductors within the installation have to have a CSA at least half of that (subject to the minimum). Do I take it that you disagree?

No I don't necessarily disagree but as it's TN-S the installation earth will be at true earth potential so the size of the bonding conductor can be very small and in response to the original question it is likely that the swa cable cpc (or armour) can be used.
Obviously different installations may work out differently which may require a larger cable than the 2.5mm² you mentioned but it will not be anything like 10mm². I do not think the 6mm² minimum would apply to the cpc being used as MPB.

There will not be a lost neutral taking the earth with it and so relying on the bonding.

 

[flameport]

The chart about the SWA cables also makes the assumption that they will have an initial temperature of 80C, rather unlikely in many installations.

The deal with bonding conductors is that in TN-C-S installations they will often carry current in normal operation and are sized based on that, rather than the intermittent current in the event of a fault. Hence the actual conductivity of the material is relevant, rather than some other calculated value based on temperature rise over a very short period.

 

[JohnW2]

Yes but in true TN-S the installation's earth is in the ground. ..... No I don't necessarily disagree but as it's TN-S the installation earth will be at true earth potential....

If I understand you correctly, is that necessarily true? TN-S merely means that the earth conductor is separate from the neutral. You seem to be assuming that the 'earth' in TN-S supply cables always consists of a bare sheath which is in contact with the ground. Is there never an insulating layer around that sheath? ... and (I must say I've never seen or heard of it!) is there such a thing as an overhead TN-S supply?

In any event ....

... but as it's TN-S the installation earth will be at true earth potential so the size of the bonding conductor can be very small ....

Firstly, as above. However, even if the TN-S earth is at true earth potential, then, as I wrote before, that does not guarantee that high pds cannot exist (or try to exist) between that earth and extraneous-c-ps - as I said, they are defined as conductive parts liable to introduce a potential, generally earth potential. However, that potential could be 230V (or higher). As an example, if structural metal entered the premises from an adjacent adjoined building, a combination of a fault and a 'faulty installation' in that adjoining building could result in the potential of that extraneous-c-p becoming very high relative to true earth. Under those circumstances, the (required) main bonding conductor could find itself having to carry high currents (to 'true earth').

Just to let you know, I'm probably not going to be around for much longer tonight.

Kind Regards, John

 

[JohnW2]

The deal with bonding conductors is that in TN-C-S installations they will often carry current in normal operation and are sized based on that, rather than the intermittent current in the event of a fault. Hence the actual conductivity of the material is relevant, rather than some other calculated value based on temperature rise over a very short period.

I'm not sure I fully understand that. It's obviously fair enough to size the main bonding conductor so as to be at least adequate to cope with the currents it may carry in normal operation, but it surely must also be adequate to cope with brief fault currents - and, for that, factors other than just conductivity are relevant??

After all, minimum required bonding conductor CSAs with TN-C-S are generally appreciably greater than would be acceptable with other earthing systems, the general feeling seeming to be that this is because of the possibility of the much-discussed 'lost neutral' situation.

Kind Regards, John

 

[Benway]

Heh, that escalated quickly! Voting was 3 to 1 going straight into the Cu, so that's my plan.

Cheers dudes.

 

[JohnW2]

Heh, that escalated quickly! Voting was 3 to 1 going straight into the Cu, so that's my plan.

I don't think that I 'voted'. For what it's worth, if I had voted that would become "3 to 2" if the CU were plastic, particularly fairly flimsy plastic.

Kind Regards, John