summerhouse/shed supply earthing

[Shweblet]

Hi guys,

widely covered topic but still a lot a mixed opinions/interpretations. I have scoured the BS7671, on site guide and the internet is search of anything black and white. I'm not looking for somebody to tell me how to do anything, just some experienced opinions on what appears to be a disputed topic.

Quick bit of background: 12 years experience in the industry with control systems, qualified to c&g L3 but still learning and just finishing off my portfolio and have a registered electrician overseeing any work I'll be doing which he will test/inspect and then sign off.

I have a summerhouse 4m long, 3m away from the fabric of the house (please see mock up sketch) I plan to have in future proofed which is why I wouldn't just have a fused spur from an RCD protected circuit. I already have an armoured cable in the ground which will be supplied from a spare way in the house consumer unit via a 10mm T+E with an isolator between the two on the external wall of the house (not yet connected obviously) - The T+E is clipped direct at height through the garage at the side of the house. Have been advised to install a weatherproof CU at the summerhouse with an electrode to create a TT system and only have the SWA earth connected at the house CU end (apparently the NICEIC dictate that you have to stake it). This is the way I will do it, i'm not arguing against it but thought i'd try and get some further clarification for my own benefit in the future.

My question is really whether a TT system is beneficial when the additional installation is so close to the house if the CU will only be supplying radial sockets and lighting, no extraneous parts in the summerhouse, no hot tub (yet - Although I do know if there were one it needs an electrode). the slabs in the garden will be at the same potential as the house and the summerhouse is timber construction with the decking. Surely exporting the earth 7metres and having the whole summerhouse supply off an RCBO from the main CU in the house is safer than having an MCB supply out to the outhouse and then staking it?

Thanks,
Shweblet

 

[davesparks]

Have been advised to install a weatherproof CU at the summerhouse with an electrode to create a TT system and only have the SWA earth connected at the house CU end (apparently the NICEIC dictate that you have to stake it). This is the way I will do it, i'm not arguing against it but thought i'd try and get some further clarification for my own benefit in the future.

Who has advised you to do this and which regulations did they use to support their advice?
The NICEIC do not dictate that you have to 'stake it' in the same way that they do not dictate anything about the way you install things, they only state that youust comply with the regulations. The assessor will ask you why you have installed it the way you have and exoect you to be able to explain why you did or did not use the supplied earthing system.

For clarification setting up a TT system for an outbuilding is only usually needed if there are extraneous parts in the outbuilding which need bonding but the cost of running a suitable main bond is prohibitive. For example if you are running a 32A supply to an outbuilding with extraneous parts but the installation requires 50mm main bonding, it would be prohibitively expensive to run a main bond to tbe outbuilding.

Special cases or locations may also require setting up a separate TT system.

 

[buzzlightyear]

considering a new supply to a shed that will require a new circuit and part p for the building control.

 

[Shweblet]

Hi Davesparks,
thanks for the feedback, the conversation I had was only verbal with nothing to back it up but I did not ask - I started reading up to find answers, hence why I started digging because I don't fully agree from a safety standpoint, IMO protection from the main CU by RCD would trump having protection from only an MCB and converting to TT - but again, this is why im asking the questions as I know there are mixed opinions.

Hi Buzz,
I know it is notifiable work and comes under part P - it is being inspected periodically during installation and will be tested, inspected and signed off by himself, this is not what im trying to get at though. I am not a DIYer, I do this for a living but I still see myself as learning. The point of this post is not to get information to carry out work - the work is getting done how the guy giving me the EICR says.

Im asking - For educational purposes why some people would insist there is a earth electrode installed rather than just using the original earth back to the MET - to me it seems more work and not overly beneficial, obviously open to correction.

Some sources (here and the IET forum) state that the DNO will not allow you to export their earth, others for the loss of the neutral on the incoming supply (which sounds like clutching at straws as the whole house would be at risk anyway), I have another who has stated the NIC will ask for it to have an electrode.

Just trying to get experienced opinions on different theories and practices.

Thanks,
Shweblet

 

[Strima]

I think John 'monotone' Ward explains it quite well.

View: https://youtu.be/NvENsVUhDUs

 

[westward10]

I fail to see what this has to do with the DNO, connecting a cpc from a consumer unit is nothing to do with them.

 

[telectrix]

an old myth from back in the 80's - all outbuildings must be TT. .... you are not exporting the earth. you are extending the equipotential zone. davesparks sums it up in post #2.

 

[pc1966]

I think the issue of "exporting the earth" with TN-C-S is the case of a PME fault creating the risk that external metalwork rises to a dangerous voltage relative to the true Earth around it (as in regs on caravans & car chargers).

But for most cases of outside buildings I doubt that is a real issue.

 

[westward10]

Nearly all readings on this refer to exporting PME but you are not. PME stops at the service head you are merely using a cpc which happens to be derived from PME.

 

[davesparks]

Im asking - For educational purposes why some people would insist there is a earth electrode installed rather than just using the original earth back to the MET - to me it seems more work and not overly beneficial, obviously open to correction.

Some sources (here and the IET forum) state that the DNO will not allow you to export their earth, others for the loss of the neutral on the incoming supply (which sounds like clutching at straws as the whole house would be at risk anyway), I have another who has stated the NIC will ask for it to have an electrode.

Some people insist on setting up a seperate TT system regardless because they don't understand the regulations, good installation design or some of the fundamental principles of electrical safety. Personally I wouldn't trust someone who takes this approach.

You've mentioned an EICR being issued, but that is a completely seperate thing and doesn't relate to this propped work does it? An EICR cannot be used to certify new work, only to report on the condition of an existing installation.

No you absolutely cannot export PME to an outbuilding, that would mean running a PEN (neutral and earth in the same conductor) conductor from the intake to the outbuilding, this is illegal.
What you are asking about is installing a distribution circuit and using the earth which is supplied by a PME supply.

 

[Shweblet]

Some people insist on setting up a seperate TT system regardless because they don't understand the regulations, good installation design or some of the fundamental principles of electrical safety. Personally I wouldn't trust someone who takes this approach.

You've mentioned an EICR being issued, but that is a completely seperate thing and doesn't relate to this propped work does it? An EICR cannot be used to certify new work, only to report on the condition of an existing installation.

No you absolutely cannot export PME to an outbuilding, that would mean running a PEN (neutral and earth in the same conductor) conductor from the intake to the outbuilding, this is illegal.
What you are asking about is installing a distribution circuit and using the earth which is supplied by a PME supply.

Apologies Dave, I meant EIC.

 

[westward10]

Bit of entertaining light reading as this is an old debate.

Exporting the earth?

Im not 100% on earthing for submains. If the supply is tnc-s you are not ment to export the earthfrom db to say a detached garage?If i did export the earth could it not be classed as TNS instead?? Or to keep it TNCS run a seperate earth form cutout.Or just have the garage as a TT. Are you...

www.electriciansforums.net

 

[Ian1981]

Bit of entertaining light reading as this is an old debate.

Exporting the earth?

Im not 100% on earthing for submains. If the supply is tnc-s you are not ment to export the earthfrom db to say a detached garage?If i did export the earth could it not be classed as TNS instead?? Or to keep it TNCS run a seperate earth form cutout.Or just have the garage as a TT. Are you...

www.electriciansforums.net

Jesus that was painful to read?

 

[DPG]

Bit of entertaining light reading as this is an old debate.

Exporting the earth?

Im not 100% on earthing for submains. If the supply is tnc-s you are not ment to export the earthfrom db to say a detached garage?If i did export the earth could it not be classed as TNS instead?? Or to keep it TNCS run a seperate earth form cutout.Or just have the garage as a TT. Are you...

www.electriciansforums.net

That's ten minutes of my life I'll never get back.

 

[Shweblet]

Thanks for the detailed information guys.

I found a book I purchased from the IET when I bought the 18th edition and it sums it up in there and couldn't have made it any clearer really - Wish Id found that before the post but hey ho.

I've attached the front cover and relevant pages.

Thanks,
Shweblet