SWA earthing

[ricicle]

Also remember that the VD figures are in the Appendices section of BS7671 and as such are only guidance and not part of The Regulations themselves.

 

[JohnW2]

Not really looked. Tried to message you, but can't?

Everyone else seems to manage! - are you sure you typed my 'name' correctly? I'll trying sending a message to you, so you can reply to it.

Cheers, John

 

[flyingsparks]

Assuming a Ze or Zdb 0.02 (my softwares default), I get the below.

Lectrician, what is that software, if you don't mind me asking?

 

[JohnW2]

Also remember that the VD figures are in the Appendices section of BS7671 and as such are only guidance and not part of The Regulations themselves.

Indeed....

Can he not get away wiith more than 3% if the light is not susceptable to such voltage drop?

That is always an option to consider - and the same for the 5% for non-lighting loads. Don't forget that the 3% and 5% figures only exist in an 'informative' Appendix of the regs.

Kind Regards, John

 

[mfarrow]

Also remember that the VD figures are in the Appendices section of BS7671 and as such are only guidance and not part of The Regulations themselves.

All references in this text to the Wiring Regulations or the Regulation(s), where not otherwise specifically
identified, shall be taken to refer to BS 7671:2008 Requirements for Electrical Installations as amended by
Amendment No 1, 2011.

That's the title of the whole book, warts and all

RF

If you can't get the Ze to work with 70mm² 2c, you might want to consider costing up for a further option: 2x 35mm² 2c cables. This has an armour resistance of 2.6 Ω/km vice 2.0 Ω/km for 70mm² 2c; or 1.3 Ω/km as a parallel run.

2 cables will also be a lot easier to install than a single, large cable.

 

[JohnW2]

All references in this text to the Wiring Regulations or the Regulation(s), where not otherwise specifically identified, shall be taken to refer to BS 7671:2008 Requirements for Electrical Installations as amended by
Amendment No 1, 2011.

That's the title of the whole book, warts and all

. Are you trying to suggest that, because they are part of 'the whole book', the Appendices count as 'regulations'? Page 267 of that whole book (BGB) says:

Appendix 1 is normative, and therefore is a requirement.
All other appendices are informative, and are provided as guidance.

RF If you can't get the Ze to work with 70mm² 2c, you might want to consider costing up for a further option: 2x 35mm² 2c cables. This has an armour resistance of 2.6 Ω/km vice 2.0 Ω/km for 70mm² 2c; or 1.3 Ω/km as a parallel run. 2 cables will also be a lot easier to install than a single, large cable.

... but, I would suspect, considerably more expensive?

Kind Regards, John

 

[RF Lighting]

If the Ze is an issue on steel, I'll just run in a single copper earth alongside the SWA.

At the moment the client has budgeted for 3 core SWA, but if the extra earth isn't required, it seems a waste of money putting it in.

I certainly don't want to open the whole cables in parallell can of worms!

 

[securespark]

FWIW, I never relied on the SWA alone. I always ran in 3 core whether the calcs suggested it or not.

 

[mfarrow]

Are you trying to suggest that, because they are part of 'the whole book', the Appendices count as 'regulations'?

No, I'm saying that The Regulations are the whole book. Most of the Appendices may be 'guidance', but they all relate to a mandatory regulation. If we choose to ignore that guidance, and due diligence says that we should follow published guidance/best practise where possible. If we breach the guidance, we are still breaching The Regulations.

I would suspect, considerably more expensive?

Not necessarily. The price would probably be similar, yes, but a lot of the cost is in the copper, so 1x 70mm² 3c may turn out more expensive than 2x 35mm² 2c.

The 'cables in parallel can of worms' is one you'll be opening up anyway if you need a supplementary CPC or certain 3c scenarios, as any fault condition within the cable will rely on a parallel path from each side of the faulty armour.

Under other fault conditions, i.e. loss of a live conductor on one of the parallel cables, the circuit will still function and, owing to the size of the cable you're installing versus the load, there will be no risk of an overload condition on the remaining conductors.

 

[JohnW2]

No, I'm saying that The Regulations are the whole book. Most of the Appendices may be 'guidance', but they all relate to a mandatory regulation. If we choose to ignore that guidance, and due diligence says that we should follow published guidance/best practise where possible. If we breach the guidance, we are still breaching The Regulations.

I wouldn't say that. The crucial regulation is 525.100, which says:

"...the voltage at the terminals shall be such as not to impair the safe functioning of the equipment"

525.101 the goes to say that 525.100 is deemed to be satisfied if the (3% and 5%) VDs given in 6.4 of Appendix 4. However, (IMO) that clearly doesn't mean that this is the only way to satisfy 525.100, so I don't think you are right in saying that one (necessarily) breaches the regulations by 'breaching' the guidelines'. With many loads, I don't think it would be at all difficult to argue/demonstrate that their 'safe functioning would not be impaired' at VDs well in excess of the Appendix 4 guidelines.

Throughout life, and particularly throughout the Building Regs, 'deemed to satisfy' provisions are included for those who can't be bothered (or are not able) to use their own design skills to demonstrate that regulations are complied with - a classic example being the use of ('deemed to satisfy') Table 54.7 by those who can't be bothered to undertake an adiabatic calculation to determine required sizing of protective conductors. 'Deemed to satisfy' options are nearly always 'conservative' - i.e. they result in more 'pessimistic' answers than one could get/justify by proper design.

The 'cables in parallel can of worms' is one you'll be opening up anyway if you need a supplementary CPC or certain 3c scenarios, as any fault condition within the cable will rely on a parallel path from each side of the faulty armour. ... Under other fault conditions, i.e. loss of a live conductor on one of the parallel cables, the circuit will still function and, owing to the size of the cable you're installing versus the load, there will be no risk of an overload condition on the remaining conductors.

It was RF, not me, who mentioned the can of worms but I largely agree with what you say above. In fact, in some senses one can say that it is 'safer' to have parallel conductors. The main potential worry is probably that, whilst loss of one of a pair of parallel CPCs would leave one 'safer' than losing an entire CPC, one would normally not be aware (until the circuit was tested) that this non-ideal situation (probably not satisfying disconnection time requirements etc.) has arisen.

Kind Regards, John

 

[ericmark]

John Peckham did an article on using the armour of SWA cable in the IET mag and for 70mm he said it needed 79.0mm² to meet 54.7 and it was 84mm² on two core. So one would need to check it is still that size. However once you go to three core it is 119mm² so well within it. His article shows at 120mm² one start to have problems.

Sure you will find the article on IET web site.

 

[ban-all-sheds]

If the Ze is an issue on steel, I'll just run in a single copper earth alongside the SWA.

At the moment the client has budgeted for 3 core SWA, but if the extra earth isn't required, it seems a waste of money putting it in.

What size earth would you need (apologies if that is somewhere in the previous pages)? Would 5-core 35mm² be an option?

 

[JohnW2]

John Peckham did an article on using the armour of SWA cable in the IET mag and for 70mm he said it needed 79.0mm² to meet 54.7 and it was 84mm² on two core. ... However once you go to three core it is 119mm² so well within it. His article shows at 120mm² one start to have problems.

That is all true. Table 54.7 gives 78.92mm² as the minimum steel armour CSA for 70mm² cable.

However, as I've recently written, Table 54.7 is a 'deemed to satisfy' provision for those who cannot be bothered to undertake an adiabatic calculation per 543.1.3. If one does undertake such a calculation, I would be extremely surprised if the answer (required armour CSA) one gets is not appreciably less than the figure obtained by ('lazily') using Table 54.7.

Kind Regards, John

 

[mfarrow]

The crucial regulation is 525.100, which says:

"...the voltage at the terminals shall be such as not to impair the safe functioning of the equipment"

525.101 the goes to say that 525.100 is deemed to be satisfied if the (3% and 5%) VDs given in 6.4 of Appendix 4. However, (IMO) that clearly doesn't mean that this is the only way to satisfy 525.100, so I don't think you are right in saying that one (necessarily) breaches the regulations by 'breaching' the guidelines'. With many loads, I don't think it would be at all difficult to argue/demonstrate that their 'safe functioning would not be impaired' at VDs well in excess of the Appendix 4 guidelines.

That may be so, but you're still departing from 525.100.

Throughout life, and particularly throughout the Building Regs, 'deemed to satisfy' provisions are included for those who can't be bothered (or are not able) to use their own design skills to demonstrate that regulations are complied with.

Do you mean "that the installation is safe"? Otherwise I agree with you, but it's still a departure in my book, otherwise the IET wouldn't have numbered the 'deemed to satisfy's, it would just have put notes or NB as they do with other things.

EDIT: As I think I've said before, when we leave our installations to someone else, for whatever reason, we leave them to the uninformed in most cases, except the B[coloured]B they have in their hands, leading to 'discussions' or possibly even 'rectification' either for us or someone else in the future. Following best practise as published in that BCB isn't likely, in most cases, to end up in long drawn-out discussions, nor cost a great deal more.

 

[JohnW2]

The crucial regulation is 525.100, which says:

"...the voltage at the terminals shall be such as not to impair the safe functioning of the equipment"

525.101 the goes to say that 525.100 is deemed to be satisfied if the (3% and 5%) VDs given in 6.4 of Appendix 4. However, (IMO) that clearly doesn't mean that this is the only way to satisfy 525.100, so I don't think you are right in saying that one (necessarily) breaches the regulations by 'breaching' the guidelines'. With many loads, I don't think it would be at all difficult to argue/demonstrate that their 'safe functioning would not be impaired' at VDs well in excess of the Appendix 4 guidelines.

That may be so, but you're still departing from 525.100.

I think we'll just have to disagree, then. As far as I am concerned, the fact that following the guidance in Appendix 4 is deemed to be one way of satisfying 525.100 does not in any way mean that there aren't any other ways of satisfying it, and nor does it mean that failure to follow the guidance automatically represents a 'departure' from 525.100.

Had they wanted to insist that complying with the 3% and 5% guidance etc. was the only way of complying with 525.100 (anything else being 'a deviation', why did they not simply say that (in 525.100 itself - or, at least somewhere in 525), rather than merely saying that following the ('informative') guidance was a way of satisfying it?

Kind Regards, John