Wooden Shed and how to proceed TT or not

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GD16

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Hi Guys

I have a CU in the house with a 20A RCBO there are no spare ways in the consumer unit the RCBO currently feeds a correctly IP rated outside socket. So all good so far however someone has placed a box alongside and incorrectly terminated a SWA cable looks to be 2.5 three core and they have placed it above ground some of the way to a shed. The rest of the installation well don't even go there it's unbelievable.

Anyway my question is it acceptable to extend the radial circuit leaving the outdoor socket in place making sure everything is terminated properly and extend into the shed where another socket would be placed with a switched fused connection unit used to protect the 1.5 lighting circuit. What about the CPC size as this is 2.5mm so too small, I know I need to bond the SWA sheath should this be both ends. There are no extraneous conductive parts.

I think I should I TT the shed SWA bonded at house end only. if so I will have to remove the outside socket from the house and remove the RCBO from the house and replace it with non RCD protected 20A MCB so that if I put a small CU in the shed it will be RCD protected there and there will be no selectivity issues. Obviously an earth electrode for TT.

Any advice appreciated otherwise I was going to follow the NICIEC guidance in which case I will need to TT or change the SWA cable so that the CPC is larger to meet the requirements.

 
Nothing wrong with extending a cct to the shed from an easily available spot. As long as Zs values are met for that cct, cable sizes are correct etc etc.

Whether or not you need to TT the shed depends on the earthing arrangements at the house. TN-S not a problem, TN-C-S, you shouldn't be exporting the earth outside of the equipotential zone of the house, so TT the shed. 

 
Nothing to stop you using a cpc derived from a TN-C-S system to an outbuilding. The term "equipotential bonding zone" is no longer used in BS7671.

 
Exporting TN-C-S earth is a bit of a misnomer as you are not exporting TN-C-S you are using an earth derived from it. What you do after the DNO equipment is nothing to do with them. I don't understand this "exporting earths" scenario.

 
Oh many thanks for all the replies yes the House is TN-C-S therefore I was thinking TT the shed, its does seem a strange idea but I know they are worried about lost neutral (PME) and as its outdoors they perceive the risk as being unacceptable even though the impedance of the earth electrode could be quite high in dry soil on a TT system as opposed to quite a low impedance when derived from the house.. I suppose its lesser of two evils.

 
So my only other question is the current house CU has a 20A RCBO can I remove this and replace with MCB only going out to the SWA as all other protection will be done at the CU in the shed.

If there is an outdoor socket on the outside of the house would this be ok connected to the MCB if the socket had a built in RCD ? The SWA would then come from this back of socket to the shed CU.

It's a shame there were not spare ways in the house CU. 

 
I asked this because that NIC document clearly shows them

just extend the zone and get on with it

I’d rather use the PME than a very unreliable TT twig

this is all a huge can of worms, AKA EV chargers ….. 

tin hat firmly on
Extend what zone?

So my only other question is the current house CU has a 20A RCBO can I remove this and replace with MCB only going out to the SWA as all other protection will be done at the CU in the shed.

If there is an outdoor socket on the outside of the house would this be ok connected to the MCB if the socket had a built in RCD ? The SWA would then come from this back of socket to the shed CU.

It's a shame there were not spare ways in the house CU. 
No. Built in rcds for socket outlets are not considered suitable for additional protection by BS7671.

 
Thanks guys I think I will just remove the socket from the house and place it at the shed location on the TT system Its not being used near the house much anyway.

Where can I get an electronic version of the 18th Edition the BSi website only seems to do paper copies, I got my paper version from IET.

 
I don't understand this "exporting earths" scenario.


the phrase comes from older versions of earthing guidelines / regs - I forget which book it was in. Likewis the bit about the DNO and applying for permission to export the earthing to an outbuilding, although I doubt that anyone has ever talked to them about a shed supply. BUt it certainly does/did exist. About 4/5 years ago I was installing a solar ground array in the garden of a large house. Another contractor was doing electrical work in the house and as part of their work modifying a detached shed (small barn size) to convert to an office. I pointed out that they should not be 'exporting the earthing,' which they had no knowledge of, so I suggested they go home and read up on it. They returned the next day and said I was right and TT'd the shed. Regs may have been changed on this. If it has been then I am not aware of it, but that doesn't necessarily mean much!  :slap

@steptoe if he's still around, is the resident expert on this.

 
Has BS7671 ever used the term "exporting earth" it is not something I can recall. It isn't in the 15th Ed.

 
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