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).

ferg · May 29, 2019

Earth the armour at supply then gland in to a plastic box to isolate the earth.

Connect shed CU to Rod.

HappyHippyDad · May 29, 2019

ferg said:
Earth the armour at supply then gland in to a plastic box to isolate the earth.

Connect shed CU to Rod.

In your example there will be a plastic adaptable box near the shed. From this box there will be (for example) 10mm Twin and earth in to the garage CU. Will this earth be connected to the TNCS or TT? You will have the same issue with the TNCS and TT being in the same box/CU, is this a problem?

telectrix · May 29, 2019

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 .

[ElectriciansForums.net] How to actually TT a shed/garage?

mattg4321 · May 29, 2019

Your rod and cpc to the shed could then end up taking the load of a number of houses upstream though in the event of losing the PEN conductor

Rpa07 · May 29, 2019

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.

mattg4321 · May 29, 2019

At the end of the day you'll never eradicate all risk. PME has some potentially severe flaws but these are very rare.

The biggest of these being in contact with true earth and losing the supply neutral.

IMO whatever you do will always have a potential downside.

Dustydazzler · May 29, 2019

Don’t they (other countries) get round this issue by grounding the neutral

FatAlan · May 29, 2019

ferg said:
Earth the armour at supply then gland in to a plastic box to isolate the earth.

Connect shed CU to Rod.

Presumably as no earth is exported you only need a two core SWA?

westward10 · May 29, 2019

happyhippydad said:
In your example there will be a plastic adaptable box near the shed. From this box there will be (for example) 10mm Twin and earth in to the garage CU. Will this earth be connected to the TNCS or TT? You will have the same issue with the TNCS and TT being in the same box/CU, is this a problem?

Gland the supply cable off within an all insulated enclosure. Take no other cpc into it, earth the consumer unit to the electrode.

HappyHippyDad · May 29, 2019

westward10 said:
Gland the supply cable off within an all insulated enclosure. Take no other cpc into it, earth the consumer unit to the electrode.

Do you mean use a 2 core cable from insulated enclosure to garage CU? Maybe just have the insulated enclosure very close to the garage CU and just continue the 2 core of the SWA into it, perhaps directly behind it. That way you only have the 2 cores (without an earth) for a very short distance?

westward10 · May 29, 2019

Yes you can continue the inner part of the swa into the consumer unit thus eliminating another joint in the supply.

HappyHippyDad · May 29, 2019

Thanks for all the reply's guys, very helpful as always!

HappyHippyDad · May 29, 2019

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.

Strima · May 29, 2019

I wouldn't dick around with two supplies, split the tails at source, switched fuse then supply to the shed/garage in a suitable installation method and suitable DB. TT if required for the EVCP or other services or PME if acceptable.

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??

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