SWA 2 core or 3 core?

[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.