SWA as Main Bonding.

[EFLImpudence]

SWA as Bonding.

544.1.1 Except where PME conditions apply, a main protective bonding conductor shall have a c-s a not less than half the c-s a required for the earthing conductor of the installation and not less than 6mm². The c-s a need not exceed 25mm² if the bonding conductor is of copper or a c-s a affording equivalent conductance in other metals.

To what figure does this equivalent conductance refer as there is no mention of the actual resistance of a main bonding conductor.

Every forum thread (not necessarily here) I have read and indeed GN8 states that if s w armour is used it therefore must be 8.5 times the c-s a of a copper conductor.
Does this mean that a TNS minimum of 6mm² must be 51mm² or
with an actual required 3mm² therefore 25.5mm² or
any c-s a of armour which offers a low enough acceptable resistance?

Does this mean that a TNCS minimum of 10mm² must be 85mm².
As this only has to be 10mm² for thermal reasons with no reference to resistance and could be very short why would it have to be anything like that size?

If this 8.5x is the case then the s w armour can virtually never be used as a bonding conductor.


However, Table 54.7 states that the c-s a of the armour can be calculated using the formula k1/k2 x S
as in this table:
//www.diynot.com/wiki/9d70c45e2e8e92df325a7cb84cf1761c
which has been dismissed in some of the threads read.

Why the discrepancy? What am I missing?

 

[Spark123]

The table you refer to in the wiki is for use as a circuit protective conductor, not main protective bonding.
It isn't something you need to concern yourself about in day to day use as the DNO look after this side of things, it is important where you are running a remote supply from DNO equipment and keeping their earthing arrangements.

 

[EFLImpudence]

The table you refer to in the wiki is for use as a circuit protective conductor, not main protective bonding.

Is it?

It isn't something you need to concern yourself about in day to day use as the DNO look after this side of things,

Do you mean they will wire a customer's garage for me?

it is important where you are running a remote supply from DNO equipment and keeping their earthing arrangements.

Yes, so what is the answer?

Have you misunderstood?

 

[JohnW2]

SWA as Bonding. 544.1.1 Except where PME conditions apply, a main protective bonding conductor shall have a c-s a not less than half the c-s a required for the earthing conductor of the installation and not less than 6mm². The c-s a need not exceed 25mm² if the bonding conductor is of copper or a c-s a affording equivalent conductance in other metals.
To what figure does this equivalent conductance refer as there is no mention of the actual resistance of a main bonding conductor.
Every forum thread (not necessarily here) I have read and indeed GN8 states that if s w armour is used it therefore must be 8.5 times the c-s a of a copper conductor. .... If this 8.5x is the case then the s w armour can virtually never be used as a bonding conductor. What am I missing?

So far, I don't think you're missing anything. Although conductivities of different steels vary quite a lot, a factor of 8.5 is that frequently quoted for the 'average' relative conductances of copper and carbon steel. As you say, that means that, in practice, no SWA is going to be adequate for use as a main bonding conductor per the requirements specified in 544.1.1

However, Table 54.7 states that the c-s a of the armour can be calculated using the formula k1/k2 x S
as in this table:
//www.diynot.com/wiki/9d70c45e2e8e92df325a7cb84cf1761c
which has been dismissed in some of the threads read. ... Why the discrepancy? What am I missing?

I think what you are missing is 543.1.1. This indicates that the minimum csa of a protective conductor other than a protective bonding conductor should be determined either by 543.1.3 (adiabatic calculation) or, for the lazy, by 543.1.4 (i.e. Table 54.7). It therefore appears that none of 543.1 applies to main bonding conductors, and nothing in 544.1.1 says that any approach (e.g. adiabatic calculation) other than what it states is acceptable for determining the minimum csa of main bonding conductors.

That's how I see it, anyway.

Kind Regards, John

 

[JohnW2]

... I might also add that the IET seem pretty confusing in what they say about required copper and steel CSAs in other situations. Table 54.1 (relating to buried earthing conductors) indicates that, if the conductor is protected against both corrosion and mechanical damage, the minimum CSAs are 2.5mm² for copper and 10mm² for steel (ratio 1:4) but, for conductors not protected against either corrosion of mechanical damage, the minimum CSAs are 25mm² for copper and 50mm² for steel (ratio 1:2).

Admittedly these figures are probably based more on risks of (chemical and/or mechanical) damage than on conductivity but, even for conductors protected against both of those risks, the ratio is only 4:1, rather different from the 'usual' relative conductivity figure of 8.5:1.

Kind Regards, John

 

[EFLImpudence]

I think what you are missing is 543.1.1. This indicates that the minimum csa of a protective conductor other than a protective bonding conductor

It does apply because the c-s a of the bonding conductor shall be half that of the earthing conductor except PME.
So indirectly it can be calculated except PME.
It just doesn't have to be except divide by two.

should be determined either by 543.1.3 (adiabatic calculation) or, for the lazy, by 543.1.4 (i.e. Table 54.7).

It is therefore similar to the earthing conductor - 10mm² or calculate and halve.

It therefore appears that none of 543.1 applies to main bonding conductors, and nothing in 544.1.1 says that any approach (e.g. adiabatic calculation) other than what it states is acceptable for determining the minimum csa of main bonding conductors.

No, I don't think that.
543 applies equally to bonding by its relative size to the earthing conductor.



I don't have a problem with that but my query is the relative size of steel armour equivalent.

Either 8.5 times or k1/k2xS

As with a lot of similar problems; if 8.5x then when is k1/k2xS ever going to be used so what is it for.

Also, the relationship between the resistance, which is never mentioned for main bonding, and thermal capacity.
Surely the thermal property of steel isn't 8.5 times poorer than copper. (I know it isn't)
So couldn't a steel main bond be smaller than a copper one - actual resistance allowing?

 

[JohnW2]

It does apply because the c-s a of the bonding conductor shall be half that of the earthing conductor except PME. So indirectly it can be calculated except PME.

Ah yes, you're right. I must have subconsciously been thinking of only PME. So, yes, for non-PME, you can determine the minimum earthing conductor (for whatever material) by adiabatic calculation and then halve i to get the minimum csa for main bonding.

I don't have a problem with that but my query is the relative size of steel armour equivalent.
Either 8.5 times or k1/k2xS ... As with a lot of similar problems; if 8.5x then when is k1/k2xS ever going to be used so what is it for.
Also, the relationship between the resistance, which is never mentioned for main bonding, and thermal capacity. Surely the thermal property of steel isn't 8.5 times poorer than copper. (I know it isn't). ... So couldn't a steel main bond be smaller than a copper one - actual resistance allowing?

Ah yes, I now seem to recall that we've been over this ground before. As you say, we should be interested (and the adiabatic calculations are interested) as much in thermal properties as resistivity - and the 'k values' we use for adiabatic calculations (as in tables 52.4 - 52.6) clearly reflect this. The difference between k-values for, say, copper and steel is far less than one would expect from consideration of resistivity alone, presumably reflecting the superior thermal properties of steel.

So, with non-PME, there is no problem. Use the adiabatic calculation, using appropriate K, and then halve it to get the minimum bonding conductor CSA. Since the steel/copper ratio for k-values is much less than 8.5, this might well result in SWA armour at least sometimes being adequate for bonding.

The problem is with PME, and I think we've discussed this before (in relation to 'export' to outbuildings with extraneous-c-ps). The problem, as you have highlighted, is that 544.1.1 speaks explicitly in terms of "equivalent conductance" (aka x~8.5), not k-values. Whilst this makes little sense (at least to me), I think you're probably stuck with it if you want to work to the word of the regs!

Kind Regards, John

 

[EFLImpudence]

The problem is with PME, and I think we've discussed this before (in relation to 'export' to outbuildings with extraneous-c-ps). The problem, as you have highlighted, is that 544.1.1 speaks explicitly in terms of "equivalent conductance" (aka x~8.5), not k-values. Whilst this makes little sense (at least to me), I think you're probably stuck with it if you want to work to the word of the regs!

Accepting that,

are you thinking that a steel main bond for PME has to be 85mm² regardless even though the resistance (conductance) of a 10mm² copper main bond is not a consideration as far as the regulations are concerned? OR

Can, for example, a steel main bond of, say, 25mm² be acceptable if it is 2.5/8.5 times shorter?
I.e. as a 27m length of 10mm² copper with a resistance of 0.05Ω is allowed (although it is debatable if this is actually a maximum), can this be replaced with steel of 27m x 2.5/8.5 = 7.94m were that long enough?

I realise these figures may have no practical application but is the principle correct?

 

[JohnW2]

... are you thinking that a steel main bond for PME has to be 85mm² regardless even though the resistance (conductance) of a 10mm² copper main bond is not a consideration as far as the regulations are concerned? OR
Can, for example, a steel main bond of, say, 25mm² be acceptable if it is 2.5/8.5 times shorter? I.e. as a 27m length of 10mm² copper with a resistance of 0.05Ω is allowed (although it is debatable if this is actually a maximum), can this be replaced with steel of 27m x 2.5/8.5 = 7.94m were that long enough?
I realise these figures may have no practical application but is the principle correct?

No, I don't think so. I don't think that length (or resistance) really comes into it, unless the resistance gets so high that it appreciably alters the current flowing through the bonding conductor (which it very rarely would). Whether or not a conductor melts depends roughly on the amount of energy dissipated per unit length - hence dependent upon resistivity (which determines the resistance per unit length, hence energy dissipated per unit length for a given current), regardless of the total resistance of the whole length of the conductor.

More generally ....

On thinking a bit more, I think it now does make complete sense to me that one is not allowed to use the adiabatic approach to determining the required CSA for a main bonding conductor (and earthing conductor) with PME. The thing about PME is that it presents the possibility that very high currents could (in the case of a supply neutral fault) flow through the earthing conductor and main bonding cable continuously and not interrupted by any protective device. This is a situation of continuous dissipation of energy in the conductor and is most certainly not an adiabatic process (one whose duration is so short that there is negligible movement of heat from the point of generation). As 543.1.3 indicates, the adiabatic approach (and equation therein) is only applicable in the case of fault currents that are disconnected within a maximum of 5 seconds - but the very high current through earthing and bonding conductors in a PME installation could theoretically go on 'for hours'. It is therefore apparent that PME poses potential risks to the conductors greatly in excess of those that can resulted from the very brief (adiabatic, <5sec) high fault currents that one considers when undertaking CSA calculations per 'the adiabatic equation' of 543.1.3.

I therefore think that this aspect of the regs is now starting to make sense to me. What do you think?

Kind Regards, John

 

[EFLImpudence]

I therefore think that this aspect of the regs is now starting to make sense to me. What do you think?

Ah, per unit length makes a difference and more sense.

With the proviso that a 10mm² copper conductor satisfies the regulation (Table 54.8 ) with up to 35mm² Neutral conductors.
So, perhaps it is just the usual overkill especially for an installation with 60A and 16mm² tails (even 10mm² tails would do).

 

[JohnW2]

I therefore think that this aspect of the regs is now starting to make sense to me. What do you think?

Ah, per unit length makes a difference and more sense.

Exactly. It's no different from 'current carrying capacity' in any other situation - the CCC of a cable of particular CSA installed by a particular method is the same whether the cable is 1 metre or 100 metres long. The 'guidance maximum' of 0.05&#937; for a main bonding conductor presumably (given the whole purpose of MPB/MEB) relates to limiting the PD which can exist between extraneous-c-ps and MET/CPCs, rather thsn having anything to do with the current carrying capacity of the conductor.

With the proviso that a 10mm² copper conductor satisfies the regulation (Table 54.8 ) with up to 35mm² Neutral conductors. ... So, perhaps it is just the usual overkill especially for an installation with 60A and 16mm² tails (even 10mm² tails would do).

In this situation, I suspect that it is not 'the usual overkill' but, on the contrary, a 'practical compromise' (which one might even call potential 'underkill'). The problem is that, unlike most other things, one can't really calculate what is required in the event of the infamous 'lost neutral' situation with PME, because one does not know what current might flow between the installation's neutral/MET and extraneous-c-ps. I would imagine that, in the very worst-case theoretical scenarios, that current could be enough to melt a 10mm² MPB conductor, but they presumably have decided that such a scenario is so unlikely that they are prepared to accept 10mm². If they really wanted to 'cover all bases', I suppose they would have to require bonding conductors to have the same CSA as the neutral supply ones - that would at least mean that the bonding conductor would only melt if the neutral supply one did as well!

Returning to the more general point I made last night, I was rather slow to catch on. The fact that what can go on in a main bonding conductor is not an adiabatic process, hence not appropriate for 'adiabatic calculations', would become immediately apparent if you tried to undertake an adiabatic calculation per the equation in 543.1.3. Since the duration of excessive current through the conductor is unknown, and potentially very long (in comparison with the 4 seconds you would normally use for the calculation for, say, a final circuit), what would you use as a value for 't'? If you put very large values of t into the equation, you would, of course, end up concluding that you required a bonding conductor with a CSA which approached infinity as 't' got bigger!

That is, of course, exactly what you would expect of an adiabatic process (one with no heat loss) - as current continued to flow and dissipate heat in the conductor, in the absence of any heat losses (i.e. 'adiabatic'), the temperature of the conductor would just keep rising until it eventually melted, regardless of its CSA - such is an adiabatic process!

Are we getting there?

Kind Regards, John

 

[EFLImpudence]

Yes, that's all good thank you, John.

I was just concentrating on the overall resistance which, for obvious reasons now, is not considered in this respect instead of the conductance (CCC) which is the relevant criterion.