How to actually TT a shed/garage?

[HappyHippyDad]

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]

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

Connect shed CU to Rod.

 

[HappyHippyDad]

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]

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 .

 

[mattg4321]

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]

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]

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]

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

 

[FatAlan]

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]

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]

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]

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

 

[HappyHippyDad]

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

 

[HappyHippyDad]

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]

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.