How to actually TT a shed/garage?

HappyHippyDad · May 29, 2019

I have never TT'd a shed/garage I always just extend the earth from the house.

In the example of a shed/garage being supplied with SWA is the armour of the SWA earthed by the house (lets say TNCS) or by the earth rod?

Also, if the SWA is earthed by the TNCS then you have both TNCS and TT present in the garage CU. If the SWA is earthed by the TT then you have TNCS and TT present in the Main CU? Is that ok with regards potential difference between earthing systems?

ps... I realise you can extend the equipotential zone but I am just looking at a TT for a shed, this is because of the chance of an electric charging point being in the shed (as per my other thread).

Gavin John Hyde · May 29, 2019

If the cable going to shed is sufficient with installation method and size for the sockets, lights and thr charger then you can supply it from the house cu. Then the shed/garage cu has seperate circuits for each of the 3 above. I would where possible have the rcd protection in the shed rather than house in this instance.

Risteard · May 29, 2019

Rpa07 said:
Wasn’t that their intention for the 18th until they redacted it because North of the border they can’t sink rods in granite. Same issue but a shed.

Most of Donegal's rock but it's a requirement there to have an Earth electrode in every installation. Can be hard work sometimes.

Nigel · May 30, 2019

telectrix said:
can't see any reason not to use the TNC-S and add a rod at the shed.
the argument for not using TNC-S is the problem caused if the supply neutral is lost. adding a rod would solve that problem. tin hat time .
View attachment 49651

Perfectly fine but the installation would need to be designed for a TT and not the PME.

ipf · May 30, 2019

I'd just use the TNCS to the shed. You could always TT the EVCP individually at the shed DB.

happysteve · May 30, 2019

Regarding the question of terminating the SWA and ensuring you can't simultaneously touch exposed-conductive parts from two different earthing systems, I use a 25mm nylon compression gland on the shed/garage DB for the SWA rather than an SWA gland. After R1+R2 tests, I cut the armour off using the standard method, and leave the serving sheath the same length. I make sure the compression gland insert is completely on the sheath when tightened, but with not much inside. The end of the armour is enclosed within the body of the compression gland, out of the reach of fingers (from inside the board), meeting IP2x requirements. It's a lot neater than using a separate insulating box, containing a "proper" SWA gland.

Deleted member 26818 · May 30, 2019

You can purchase plastic glands for SWA to isolate the earthed armour from the shed.

HappyHippyDad · May 31, 2019

spinlondon said:
You can purchase plastic glands for SWA to isolate the earthed armour from the shed.

Do you have a link? When I search for plastic swa gland I just get stuffing glands.

davesparks · May 31, 2019

I see no reason not to use a stuffing gland as long as the cable is properly secured by cleats etc.
A bit of heat shrink over the end of the cut end of the armour will keep it neat, just remember not to shrink it until after the Zs test

Deleted member 26818 · May 31, 2019

happyhippydad said:
Do you have a link? When I search for plastic swa gland I just get stuffing glands.

CMP ZEN INSULATED BRASS CABLE GLANDS, CMP ZEN B327 GLANDS, SWA SCREENED CABLE GLANDS CIEL CMP - https://www.cablejoints.co.uk/sub-product-details/cmp-zen-cable-glands-high-voltage/cmp-zen-cable-glands-b327

kris jones · Jun 1, 2019

happyhippydad said:
I'm going to extend the example now!!

If the shed/garage is going to have sockets/lights AND an electric vehicle charging point would you need to take 2 separate supplies (i.e 2 x 2 core SWA) as the charging point needs to be on it's own dedicated circuit? Or could it be on it's own dedicated circuit from the Garage CU? I would have thought the former as otherwise it's not really a dedicated circuit.

Apologies if this has been covered but If supplying a ev point I think youll need something like 40a (min depending on size of charging point) with no diversity allowed? (not done the course but im sure thats right) so in a normal house you'd probably split the tails into a new small board purely for the ev. Based on that if you were powering a shed that subsequently supplied an ev you might have to take that into account? If just a shed then its unlikely youll be using that much power at one time but might be worth considering??