Earths at outbuildings...

[ackbarthestar]

I just re-read your post reply, maybe this was an addition written after your initial post?? But this statement is totally Wrong! I'm not sure where your information is coming from, but the armouring of either 70C PVC or 90C XLPE 2 core SWA cables DO COMPLY for use as the CPC. 3 core 10mm SWA cables and above will comply with both equipotential bonding and CPC provision for the cable (Please check the sticky section on this subject (above) particually the tables in the link within post #37 by spinlondon)

I notice that you and some others, often use the term ''CPC'' when referring to equipotential bonding conductors. They are substantially different animals in fact, and perform totally different roles within an electrical installation and should never be confused with each other.

umm.....
I have looked at this 'sticky' post and it confirms what I have said. and is substantiated by the DNOs recommendations regarding the use of SWA's supplying external installations ( albeit back in the 80's).
10, 16, 25 mm2 2-core XLPE SWAs are insufficient to be be used as both a cpc and a MPB
I don't ever remember referring to a MPB as a cpc although they may have a duel function in regard to exportation of a PME supply.

 

[Engineer54]

umm.....
I have looked at this 'sticky' post and it confirms what I have said. and is substantiated by the DNOs recommendations regarding the use of SWA's supplying external installations ( albeit back in the 80's).
10, 16, 25 mm2 2-core XLPE SWAs are insufficient to be be used as both a cpc and a MPB
I don't ever remember referring to a MPB as a cpc although they may have a duel function in regard to exportation of a PME supply.

We must be looking at different tables then, as 2 core SWA cables will comply with CPC provision upto 95mm and possibly by calculation above!
3 core SWA armour cables from 10mm will comply with bonding and CPC requirements, ...bonding via the 3rd core, and CPC via the SWA. I have certainly not inferred that the Steel wire armour was of sufficient size to be used as a means of a combined bonding and CPC conductor!!!

 

[ackbarthestar]

We must be looking at different tables then, as 2 core SWA cables will comply with CPC provision upto 95mm and possibly by calculation above!
3 core SWA armour cables from 10mm will comply with bonding and CPC requirements, ...bonding via the 3rd core, and CPC via the SWA. I have certainly not inferred that the Steel wire armour was of sufficient size to be used as a means of a combined bonding and CPC conductor!!!

The RHS XLPE 2-core .... marked in pink
[h=3]Can we use the armour of an SWA as the CPC???[/h]It would help if I made a reference so that we can be clear about things

 

[Engineer54]

The RHS XLPE 2-core .... marked in pink
Can we use the armour of an SWA as the CPC???

It would help if I made a reference so that we can be clear about things

OK, Now i understand, the table you have been looking at can be a little confusing and often requires adiabatic calculation to confirm compliance. If you check this link out, as posted in ''Post 37 on this thread, ( steel wire armour as the earth conductor ) These tables are far more convienient to use, and are what i have been using for some years now, for a quick reference point to check SWA for CPC compliance...

It is well worth reading the whole of this article, as it can explain many things relating to the subject matter. The author of this article John Peckham, is a well respected Chartered Engineer.

 

[ackbarthestar]

OK, Now i understand, the table you have been looking at can be a little confusing and often requires adiabatic calculation to confirm compliance. If you check this link out, as posted in ''Post 37 on this thread, ( steel wire armour as the earth conductor ) These tables are far more convienient to use, and are what i have been using for some years now, for a quick reference point to check SWA for CPC compliance...

It is well worth reading the whole of this article, as it can explain many things relating to the subject matter. The author of this article John Peckham, is a well respected Chartered Engineer.

Thank you for this information. I can see there would be an element of confusion between the two tables.
However, I still find it interesting that the MEB would not allow the armour of certain sizes of 2-core SWA,s above 6mm2 to be used as cpcs.

 

[Chr!s]

The tables are a little misleading, all swa is now xple 90 C, so adjustments need to made to the tables. The restraints are more prevalent where conductor tempretures are upto the 90C.

Just work out the size of the protective conductor, bonding conductor using the tables or adiabatic, then apply the equation with the correct k values and cross reference with the manufacturers details.

Regards Chris

 

[Engineer54]

The tables are a little misleading, all swa is now xple 90 C, so adjustments need to made to the tables. The restraints are more prevalent where conductor tempretures are upto the 90C.

Just work out the size of the protective conductor, bonding conductor using the tables or adiabatic, then apply the equation with the correct k values and cross reference with the manufacturers details.

Regards Chris

Spin did send me an updated version by John Peckham, but couldn't find it. However what i remember of the tables, there was very little difference in them...

 

[Chr!s]

Spin did send me an updated version by John Peckham, but couldn't find it. However what i remember of the tables, there was very little difference in them...

There's possibly quite a big difference, just looking into it at the moment.

Regards Chris

 

[Engineer54]

There's possibly quite a big difference, just looking into it at the moment.

Regards Chris

Then i await with interest...

 

[nickblake]

I always run a supply earth to out buildings un less we are specifically advised not to by the regs , i must admit i also never rely on the SWA as an earth i always use a cable incorperating a core that can be used as an earth and soley use the SWA as a meants of protection for the cable you cant beat a proper earth , when you do a caravan install using TT earthing the readings vary so much and i have had them going way over the 200 ohm limit in dry weather when it was wet it was less than 40 ohms so the earthing on a TT can be unstable and we all know how RCD can play up even the expensive ones ,an as i have said if TNS TNCS are permitted then that what ill install

 

[Deleted member 26818]

This should be the updated version.
As far as I'm aware, it is only the Reg. Nos. that have changed.
View attachment Microsoft_Word_-_SWA_PAPER_UPDATED_TO_17TH_EDITION.doc.pdf
This is the old one for comparison.
View attachment Using the armour of SWA.pdf

 

[Chr!s]

This should be the updated version.
As far as I'm aware, it is only the Reg. Nos. that have changed.
View attachment 8067
This is the old one for comparison.
View attachment 8066

The issue with this table along with the ones in GN1 and GN8 is the incorrect K value seem to of been used.

The k value for the line conductors is given as 115(PVC) the problem is the standard now require XLPE for the line conductors. The K value being 154(XLPE)

So an example 2 core 120mm 115/51 x 60 = 135.29 but the insulation isn't PVC, its XLPE

It should be 154/51 x 60 = 181.2 mm, so you can see many sizes would not be adequate using the tables, the adiabatic may well prove otherwise.

The issue seems to of come about as the XLPE is rated to 90C, the problem is most of the time we work to 70C, they haven't included a K value for this scenario, so they used the PVC values, and they don't air on the side of caution.

Ill let you know the outcome, though i don't see how it can be incorrect, several cable manufacturers agree.

Regards Chris

 

[Deleted member 26818]

Yes but no but.
Unless the equipment the SWA is connected to is rated at 90ºC, then the 70ºC tables would suffice.

 

[Engineer54]

I don't know why we keep harping on this 90C aspect for cables, it's highly unlikely that you'll ever see a 70C cable working at anywhere near that tempreture, let alone a 90C cable.

 

[Chr!s]

Yes but no but.
Unless the equipment the SWA is connected to is rated at 90ºC, then the 70ºC tables would suffice.

The tables are using a k value for PVC, initial temp 70C and final 160C.

SWA no longer since 2008 utilises PVC for conductor insulation, so when calculating for short circuit we should be using the specific K value for that material.

XLPE, initial temp 70 and final temp 250 would give a K value of 154, so quite a difference. So as it stands i disagree with the tables.

Regards Chris