SWA 2 core or 3 core?

[reidmaguire]

I dont do a lot of external works, however, I have been asked to install some power for general lighting and powerto an outbuilding with no extraneous conductive parts 15 meters from the main board and my thoughts were;

The supply is tt, so 10mm 2 core swa from spare way in main board to CU in outbuilding - outher sheath eathed to main board only. the outbuilding will be seperate tt and rcd protected.

I have been asked why I am not using 3 core SWA, and when I say it is not required, I am starting to question this myself!!

Any though on this appreciated

 

[nrbspark]

Your thoughts using the 2 core are exactly the advice that was given to me by the NICEIC a couple of months ago when I posed myself with the same question and was not 100% sure myself. The reason that I was given was Parallel earth paths would be present if there is a spike at either end and also a cpc connected at both ends within the SWA.

So they said 2 core SWA with armoring connected at mains distribution end and divorced at new DB, then an earth spike at the new DB with the obvious RCD protection.

Hope this helps.

 

[Guest123]

I dont do a lot of external works, however, I have been asked to install some power for general lighting and powerto an outbuilding with no extraneous conductive parts 15 meters from the main board and my thoughts were;

The supply is tt, so 10mm 2 core swa from spare way in main board to CU in outbuilding - outher sheath eathed to main board only. the outbuilding will be seperate tt and rcd protected.

I have been asked why I am not using 3 core SWA, and when I say it is not required, I am starting to question this myself!!

Any though on this appreciated

Spot on.

 

[Davey101]

I've never really understood why you cant use the earth from the main building (not saying you can't, just that I've never had it explained properly)
Can anyone give a practical explanation please?

 

[AA410]

So could you still used a 3 core in this situation what comes to mind is if the armoured was damaged in anyway some time ago i came across a job were a 2 core cable had been pulled out of the gland hence no earth

 

[Staffie]

I've never really understood why you cant use the earth from the main building (not saying you can, just that I've never had it explained properly)
Can anyone give a practical explanation please?

To my knowledge, something to do with any extraneous stuff in the out-building being at a different potential to that in the main building, hence the need for a seperate earth.....I think...

 

[AA410]

I dont get that if you used a 3 core and earthed the outer steath with the cpc it is at the same potential

 

[Davey101]

So could you still used a 3 core in this situation what comes to mind is if the armoured was damaged in anyway some time ago i came across a job were a 2 core cable had been pulled out of the gland hence no earth

I think the point is that you DONT WANT an earth at the outbuilding, the earth must be provided by a seperate earth rod, the earth from the existing building should be insulated from the outbuilding, or have I got it totaly wrong?

 

[pennywise]

Your not suppose to export the earth from a PME system to an area outside of the equipotential zone ie the house.

 

[nrbspark]

How do you determine the area of a buildings equipotential zone, does anyone have an equation?

 

[Staffie]

How do you determine the area of a buildings equipotential zone, does anyone have an equation?

I don't think there is one?

Basically, if the building to be supplied is not part of the fabric of the building where the supply is coming from, then the earth is not to be imported from the PME and a new earth should be utilised, an earth rod.

 

[nrbspark]

This suggests that if you run a supply to a detached garage from the house that a spike should be used every time and not the 3rd swa core as the cpc. Also should you use a spike if you are supplying an outside light in your garden.

I think that I may be confusing myself but I have seen the 2 items listed above many times on a TNC-S system.

 

[Toonlad]

just been looking through my course work and found a similar example to the ops question.I hope this helps.If not my lecturers are talking ****e.

bs 7671 542.1.8 -If the protective conductor forms part of a cable, the protective conductor (ie the swa) shall be earthed ONLY at the installationcontaining the associatedprotective device. This by default the HAS to be the main house end, utilising the supply companys earth,but for the supply cable only.

Additionally you would not bond both ends.For this would then connect the outbuilding earthing system to the suppliers earth , which would cause circulating eddy currents in the supply cable swa, causing nuisance tripping of the outbuilding main switch rcd.

 

[AA410]

What about on a TNS systeam what earthing arangements are for that on an out building

 

[Guest123]

No issues with extending a TN-S earthing system to an outbuilding, providing circuit parameters allow it.

 

[nrbspark]

So please can someone explain why you can't supply an outbuilding from a TNCS system please

 

[AA410]

The reasion i am cocerned with the 2-3 core is i have an ongoing job were i had to put an earth rod down because the TNS systeam had a very poor earth but this instalation has a lamp post feed by a 6mm 3 core cable that i still have to conect up do i use the 6mm core or do iuse the outer sheath and put an other earth rod down for the lamp post ??????? can any one help with this one

 

[Guest123]

http://www.electriciansforums.net/r...matters_electrical_installations_outdoors.pdf

 

[empress]

you can export the tncs earthing system to out buildings if the armoured is big enough.
enquired about this a few months ago with regards 20meter 60amp sub main to 1 bed log cabin.
25mm 3 core was suggested along with a Ze at final CU of 0.35ohm or less.
would of thought 2 core TT would be cheaper!

elfyn

 

[AA410]

Just like to say a big thanks to Lenny for the help with the TNS answer i have found on chapter 54 in BS7671

 

[mechelec]

One question i have had puzzling me for a while is if you use 2 core swa for a supply to a remote building with extraneous conductive parts and the armour is earthed at the supply end only (remote end made tt) and the cable is damaged towards the remote end ie fork through armour to phase conductor do we need to check the Zs phase to armour to ensure the protection will operate in the required time? long winded I know but do we need to ensure only volt drop and current carrying capacity are adequate for the cable and ignore R2 armour values????

 

[electricAl]

You`d always take an R1+R2 at the extremity of a cct anyway - this would be no different...

Reading taken at end of armoured sub-main.

All beyond that is the TT subsystem.

& as stated, you can export the exponential zone, even for a TN-C-S if there are no extraneous-conductive parts at the location (a conventional shed wouldn`t `normally` have any) so perfectly acceptable in my view.

If there are Extraneous parts, bonding back to MET must be in minimum of 10mm, but as you`re `supposed` to aquire the DNOs permission anyway makes it suddenly less attractive.

TT it then...

 

[andyb]

You`d always take an R1+R2 at the extremity of a cct anyway - this would be no different...

Reading taken at end of armoured sub-main.

All beyond that is the TT subsystem.

& as stated, you can export the exponential zone, even for a TN-C-S if there are no extraneous-conductive parts at the location (a conventional shed wouldn`t `normally` have any) so perfectly acceptable in my view.

If there are Extraneous parts, bonding back to MET must be in minimum of 10mm, but as you`re `supposed` to aquire the DNOs permission anyway makes it suddenly less attractive.

TT it then...

I agree with all you say electricAl, however where do you get the DNOs permission bit.

 

[Spudnik]

The way i see it is if you export a PME earth, if the neutral becomes disconnected or damaged in the supply network, you have lost your earth to the outbuilding, which, if effect, is a higher risk area as it is outside the equipotential zone of the house.

 

[andyb]

That's true for a pme supply in general Jason. This is why the main protective conductor is sized in relation to the size of the neutral conductor with a minimum csa of 10mm.

Just re read your post Jason, the main problem is that if you lose your neutral, any equipotential bonding will carry the full load of the installation, this is why the mimimum csa for main protective bonding conductor is 10mm.

 

[MikeB]

"an exposed conductive part connected to one means of earthing must not be simultaneously accessible with an exposed conductive part connected to another means of earthing" quote from wiring matters 2005 article referred to by Lenny- however, can't find this phrase in the 17th- where is it?

I can imagine, but haven't confirmed by testing, that a TT earth electrode installed at a garden shed may be at a different potential than a supply earth TNS at the house, so this sounds like a good idea, but 542.1.8 in the 17th and that section makes no mention of it, just stating that the protective conductor is to be only connected to one earth (and insulated from the other) if it is incapable of carrying the maximum fault current likely to flow.

Is this saying, as it seems to be, that it's ok to connect a TNS supplied earth to a TT earth electrode, so long as you have considered the current that may flow and rated accordingly?

 

[mechelec]

That means that a 2 core 10mm swa whos armour is not sufficient sized for use as cpc would need to be 3 core and the third core connected to the insulated armours at the remote end and the MET at the supply end but not the remote installation EMB???? or am i missing something.