Discuss Outbuilding supply from TT system in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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robsparx

I am wiring an outbuilding, to be used as an office at the moment. I am nearly at the stage of running the supply out to it from the main house.

The main house has a TT supply. Would the correct way to do it be to use a 63a 100ma time delayed rcd to protect the swa supply, and 30ma protection in the outbuilding CU?

Also as far as I can understand, I dont HAVE to put a seperate earth rod at the outbuilding. It isn't too far from the existing rod anyway. Am I right in saying this?

I would probably just put the rod in anyway if that would make it extra safe and give me lower Ze readings, but there isn't anywhere really to put it. Outbuilding is surrounded by tarmac
 
Do you mean 63a fuse or mcb plus 100ma S type RCD to protect the cable? If so this would be correct.
If you use 3 core 16mm SWA then IMO you will have no problem using the existing earth rod.
Earth the SWA armour from the supply end only and use the 3rd core as your earth conductor.
 
Why earth the armour at the supply end only? If exporting the earth from the main building with the other rod then why not use 2 core and armour or 3 core and use the 3rd core and armour as CPC combined. The bigger earth the better, can't see why you wouldn't use it if it's going to be there.

All else sounds fine as long as readings are nice and low, and a 100ma type S sounds like a good idea for the cable too, with everything in the outbuilding on a 30ma
 
I haven't checked but my gut feeling is that the equivalent csa of the armour will not meet the adibiatic equation requirement for csa of cpc.
I believe that if you use 2 cpc conductors you have to ensure that the one with the smaller csa will operate under fault conditions and meet the adibiatic equation by itself. However I could be wrong.
 
The armour of a 3 core 16mm swa is equivalent to 22.1mm copper so is fine.

2 core 16mm swa is equivalent to 20.3mm copper so again it's fine.
 
I stand corrected.
As I said I hadn't checked and for some reason thought that the equivalent CSA of the armour was less than that of the conductor. Doh.
 
I think i am right in saying that a TT system only needs a 2.5mm earthing conductor so most armoured cables will be ok when using the armour as the earth conductor.
 
Think they are refering to reg 543.1.1 which in it states that the CSA of a protective condutor can not be less than 2.5mm^ if protected and 4mm^ unprotected as copper conductors.

Though this reg does not only concern TT but all systems. Also very few main earthing conductors would be this small after doing the adibiatic equation in 543.1.4
 
why the RCD to protect the SWA, surely the CU has RCD protection anyway if it is TT so you should be able to take a supply from a spare way in the CU. I would also bang in another rod anyway as they cost very little and the danger of losing the earth to the origin rod (armour corroding etc) is overridden
 
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why the RCD to protect the SWA, surely the CU has RCD protection anyway if it is TT so you should be able to take a supply from a spare way in the CU. I would also bang in another rod anyway as they cost very little and the danger of losing the earth to the origin rod (armour corroding etc) is overridden

Because if he used an MCB covered by the main RCD instead of it's own type S RCD there would be no discrimination between the 2 buildings, so a fault in the outbuilding would likely trip the main house out too
 
Thanks for the replies guys, not much to worry about then.

Off out to pull the cable in now, in a foot of snow. Gonna be fun
 
What do you mean?


If the earthing conductor is buried for any length and not mechanically protected then it must be a minimum of 16mm. Table 54.1 page 127 of BS 7671 see also 542.3.1
 
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Isn't the rod classed as the electrode Lenny and the conductor the cable that links the electrode to the MET. So though the rod would be buried the actually conductor connected to the rod is terminated above ground and may run above ground, or am I miss interpreting again ;)
 
Isn't the rod classed as the electrode Lenny and the conductor the cable that links the electrode to the MET. So though the rod would be buried the actually conductor connected to the rod is terminated above ground and may run above ground, or am I miss interpreting again ;)


Thats right, as long as the actual conductor or cable etc isn't buried than this doesn't apply.

Most earthing conductors to electrodes aren't buried and are terminated in electrode boxes above ground, happy days.

I was just explaining what another poster meant when he asked what would happen if it was buried for any distance,
 

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