If Zs is acceptable using the armour, can the CSA be unacceptable?
I.e. does the copper equivalence apply to a CPC?
I.e. does the copper equivalence apply to a CPC?
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- Thread starter round01
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I would think that in most circumstances the armour in a 2 x 10mm^2 SWA is likely to comply as a cpc.
Of course it can, as it must be able to withstand the fault current until disconnection occurs. A satisfactory Zs does not prove this.
Perhaps my wording was not ideal.
Does the copper equivalence apply to CPCs?
That is - does steel not withstand fault current as well as copper of the same CSA?
It is not the same as the rule for bonding.
Does the copper equivalence apply to CPCs?
That is - does steel not withstand fault current as well as copper of the same CSA?
It is not the same as the rule for bonding.
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As Risteard has said, an acceptable Zs does not necessarily guarantee that the material of the conductor will 'withstand the fault current'. Zs is just about whether the fault current will be high enough to operate the OPD within the required time - the CPC's capability of 'withstanding' the current is down to an adiabatic calculation, which will depend upon the material.
The point is that there are "copper equivalences" and "copper equivalences". Some relate to equivalence just in terms of resistivity (hence relevant for Zs), others take the 'withstanding' properties of the material into account. In terms of the latter (e.g. for adiabatic calculations), the usual value of "k" taken from Table 54.3 for a copper CPC is 115 (at 70°C), whereas the corrersponding value of k for steel armour (Table 54.4) is 51. That means that, per adiabatic calculations, you would need about 2.25 times the CSA for steel armour as for a copper CPC.
Kind Regards, John
Yes but if the Zs is acceptable then the csa of the armour WILL BE 8.5 times that of copper with the same resistance.
My point is that the copper equivalence does not apply to CPCs as it does for bonding where the resistance of the conductor is not the relevant factor.
My point is that the copper equivalence does not apply to CPCs as it does for bonding where the resistance of the conductor is not the relevant factor.
In fact, "the CSA of the armour will be (about) 8.5 times greater than that of copper with the same resistance" is a general truth, regardless of whether the Zs is acceptable or not!
However, if (as will often be the case, particularly for short runs), the resistance of a copper CPC is much lower than would be necessary to provide an 'acceptable Zs', then steel armour which was (just) adequate to provide an 'acceptable Zs' could have a CSA much less than 8.5 times that of the copper one (i.e. a resistance much less than 8.5 times greater than the copper one) - and, potentially, less than the ~2.25 times necessary for it to be adiabatically acceptable.
I'm not sure I understand your point - as above, my point is that resistance is not necessarily the limiting factor for CPCs, any more than it is bonding conductors.
Kind Regards, John
My point is that 'copper equivalence' does not apply to CPCs which you said and which prompted my original post.
Re: using armour as CPC.
As I said, there is "copper equivalence" and "copper equivalence", but one way of looking at it is that both have to be satisfied for a CPC. One can say that the "copper CSA equivalent of SWA in terms of resistivity" has to be adequate to satisfy Zs requirements, and that the "copper CSA equivalent of SWA in terms of adiabatic performance" has to be adequate to satisfy the adiabatic requirements.
See above.
However, as I've said, you folks are making this discussion more complicated than I intended, by trying to get me to discuss situations in which armour alone might possibly be inadequate as a CPC. I have personally never dealt with a situation in which armour alone would not be adequate to satisfy both Zs and adiabatic requirements (and such situations may arise rarely, if ever) - but, nevertheless, I think I've always ('unnecessarily') also used an earth core as well as the armour - but that's just "me".
Kind Regards, John
The idea of using a copper core as the CPC in SWA seems to be a fairly new idea and I've no idea why people have started doing it. Maybe another knock on effect of DIs who don't know / understand any better?
Our wholesaler does not stock 2 core SWA as apparently no one buys it, but they don't stock 5 core for TP&N supplies.
I'm not in the habit of running an extra earth with every twin and earth cable I install, nor am I with SWA.
Our wholesaler does not stock 2 core SWA as apparently no one buys it, but they don't stock 5 core for TP&N supplies.
I'm not in the habit of running an extra earth with every twin and earth cable I install, nor am I with SWA.
Maybe, but ISTR that some people, including some electricians, who have good understanding still favour not relying on the armour.
Fair, enough - but, playing devil's advocate, you are presumably not in the habit of installing T+E which has a CPC made of a relatively high resistance material, which is potentially subject to corrosion and often installed in a hostile environment or achieving electrical continuity with it by means of "plumbing fittings"!
Kind Regards, John
It's been industry standard since SWA was introduced to use the armour as the earth and still is on all commercial and industrial sites unless the adiabatic requires an additional earth. I've worked on, serviced and inspected hundreds of sites employing this method and never found a problem with it.
The anomaly seems to be on modern domestic installs carried out in the last ten years or so where an unnecessary copper earth is also used.
The same goes for steel containment and singles installations. The containment provides a perfectly good earth and is fully complaint with regs but more often than not these days a copper earth is run in with the supply conductors for some reason.
The anomaly seems to be on modern domestic installs carried out in the last ten years or so where an unnecessary copper earth is also used.
The same goes for steel containment and singles installations. The containment provides a perfectly good earth and is fully complaint with regs but more often than not these days a copper earth is run in with the supply conductors for some reason.
I thought it started about 20 years ago well before the term DI, i seem to recall something happened questioning the armouring, this was followed by testing which took some time and eventually proved the armour was ok in nearly all situations.
But by then people had adopted either a seperate earth looking messy cable tied to the side then evolved to using an extra core, not all in the trade are clued up on Adiobatic equations and im quessing just opted for an easy option
Regarding the singles containment I thought a special test was needed for it, which is out of scope for most people
But by then people had adopted either a seperate earth looking messy cable tied to the side then evolved to using an extra core, not all in the trade are clued up on Adiobatic equations and im quessing just opted for an easy option
Regarding the singles containment I thought a special test was needed for it, which is out of scope for most people
If you mean the high-current test, I thought that had been dropped.
Fair enough.
In terms of CPCs, it's not necessarily just an adiabatic calculation which might require an additional earth, so might Zs considerations. As discussed with EFLI, the armour of PVC SWA (at 70°C) will ('just') satisfy adiabatic calculations if the CSA of the armour is at least 2.25 times that of a copper conductor which would (just) satisfy adiabatic calculations. In that 'just adiabatically complying' situation, the SWA armour would have a resistance/impedance about 3.8 (8.5/2.25) greater than a 'just adiabatically complying' copper conductor - so that, for long runs, Zs might be too high using the armour, but low enough if using one of the cores (or armour plus a core).
As discussed with EFLI in the other thread, the use of SWA armour as a main bonding conductor is a very different kettle of fish, and is rarely going to be compliant with regs, There seems to be no doubt that (surprising though it may be) what the regs "actually say" is that a steel main bonding conductor in a PME installation would have to have a minimum CSA of about 85mm² - and the smallest SWA which achieves that is 3-core 70mm². Nor, as I've said in other posts, do I know how one could undertake an adiabatic calculation, firstly because I don't think it can be regarded as an adiabatic process and, secondly, even if one did regard it as adiabatic, I don't know what I and t figures one could/would use for the calculation. If one assumed it to be non-adiabatic (high current might flow 'continuously' - or, at least, for appreciably more than 5 seconds), then one is down to somehow finding a cable whose armour was capable of carrying the perceived maximum possible current (goodness knows how one would determine that) 'continuously' - and, again, I suspect that would probably be an enormous cable.
In such a situation I could, and probably would, apply 'common sense'. However, for those of you required to strictly comply with what BS7671 "actually says", do I take it that (in view of the above) you would not use SWA armour as a main bonding conductor?
Kind Regards, John
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