Wooden Shed and how to proceed TT or not

[Fleeting]

Whilst the Regulations does not permit the use of PME on some types of installation and the ESQCR 2002 forbids the supplier from connecting a PME derived earth to the metalwork of a boat or caravan I cannot see where this exporting of a PME earth is mentioned however I stand to be corrected.

 

[Murdoch]

What does the electronic version say?

I don't have the electronic version

So ......... AFAIR when the BBB was issued the printed version and the electronic version were identical but then some updates were made to the electronic version of the BBB - and hence why people with the printed BBB may be reading different regs to the people with access to the electronic version of the BBB

Source JW on one of the E5 podcasts

 

[Sidney]

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.

I'd just leave it on the RCBO. You don't mention how the cable supplying the socket is installed as this has a bearing  as well.

I would just leave it connected to the existing earthing arrangement.

 

[binky]

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

I can't remember where the term was used either - probably guidance notes. 

Judging by the diagram posted above, the NICEIC no longer supports this view either, and a quick look through the BBB offers nothing. 

The argument for not exporting earths is long winded and based on worse case scenarios, which, to be frank, always seemed unlikely to happen in 99.99% of situations. Based on having TN-C-S, the concern was that a neutral fault with a neighbours system or somewhere outside of your property could mean your installation becoming the easiest route back to the nearest transformer with the potential to overload your neutral, and make your earthing live. The only case of this I've ever known about was in Truro shopping centre, where by the main gas pipe was still steel. The first anyone knew about the fault was gas appliances going 'mad'. The gas pipe was getting hot, heating the gas and increasing gas pressure. By the time Western Power got to the issue, the gas pipe was glowing red apparently (I only know of this becuase of a friend who worked for WP). Somewhere deep in this forum is a very long technical discussion about the potential issues. 

 

[steptoe]

Same old same old, 

The first issue is the continual confusion of TN-C-S and PME , 

They are NOT the same , 

They are meant to be, but they are not, unfortunately. 

For what it's worth, if you have PME I would see nothing wrong in extending the PME , if you only have TN-C-S , then you should TT the shed. 

You cannot simply change the RCBO for an MCB , by what I have understood the SWA does not go all the way back to the CU, so the internal cabling still requires RCD protection. 

 

[Fleeting]

Same old same old, 

The first issue is the continual confusion of TN-C-S and PME , 

They are NOT the same , 

They are meant to be, but they are not, unfortunately. 

For what it's worth, if you have PME I would see nothing wrong in extending the PME , if you only have TN-C-S , then you should TT the shed. 

You cannot simply change the RCBO for an MCB , by what I have understood the SWA does not go all the way back to the CU, so the internal cabling still requires RCD protection. 

But you aren't extending PME that stops at the service head, you are using a cpc derived from a PME supply. 

I can't remember where the term was used either - probably guidance notes. 

Judging by the diagram posted above, the NICEIC no longer supports this view either, and a quick look through the BBB offers nothing. 

The argument for not exporting earths is long winded and based on worse case scenarios, which, to be frank, always seemed unlikely to happen in 99.99% of situations. Based on having TN-C-S, the concern was that a neutral fault with a neighbours system or somewhere outside of your property could mean your installation becoming the easiest route back to the nearest transformer with the potential to overload your neutral, and make your earthing live. The only case of this I've ever known about was in Truro shopping centre, where by the main gas pipe was still steel. The first anyone knew about the fault was gas appliances going 'mad'. The gas pipe was getting hot, heating the gas and increasing gas pressure. By the time Western Power got to the issue, the gas pipe was glowing red apparently (I only know of this becuase of a friend who worked for WP). Somewhere deep in this forum is a very long technical discussion about the potential issues. 

I understand the open PEN scenario but this "exporting PME" business to my knowledge has never been quoted in the Regulations.

 

[steptoe]

But you aren't extending PME that stops at the service head, you are using a cpc derived from a PME supply. 

I understand the open PEN scenario but this "exporting PME" business to my knowledge has never been quoted in the Regulations.

If you do it correctly you can extend the PME , nothing to do with exporting it.

You don't export PME, you extend it, 

Nothing wrong with that, 

But, you should never export a TN-C-S earth outside the EZ  

Whether it's in the rega or not, remember, the regs are just guidance, not statutory, a competent person with a sound understanding of earthing arrangements will understand the inherent dangers associated with exporting a TN-C-S earth 

 

[Fleeting]

If you do it correctly you can extend the PME , nothing to do with exporting it.

You don't export PME, you extend it, 

Nothing wrong with that, 

But, you should never export a TN-C-S earth outside the EZ 

Equipotential Zone is no longer a term cited in BS7671 although I know in some literature it is quoted but where is it quoted you cannot "export" TN-C-S?

 

[steptoe]

Equipotential Zone is no longer a term cited in BS7671 although I know in some literature it is quoted but where is it quoted you cannot "export" TN-C-S?

I pressed post instead of return, 

Read the last para 

 

[Fleeting]

I understand the dangers of TN-C-S and the so called "exporting" so aside from an engineering judgement which could be discussed until the cows come home there is nothing officially prohibiting this.

 

[steptoe]

I understand the dangers of TN-C-S and the so called "exporting" so aside from an engineering judgement which could be discussed until the cows come home there is nothing officially prohibiting this.

Taking an earth from a TN-C-S supply outside the EZ is leaving the installation in a less safe manner than it was before you started,  

Lack of design, and I don't understand how you could sign off an EIC for something that was inherently dangerous. 

I understand the dangers of TN-C-S and the so called "exporting" so aside from an engineering judgement which could be discussed until the cows come home there is nothing officially prohibiting this.

The regs are not statutory anyway, so it doesn't matter whether they prohibit it or not, 

But, 

EAWR are statutory 

 

[Fleeting]

The E&WR 1989 don't apply to the OP as it is a dwelling.

I'm not signing of an EIC I am trying the gather whether this "exporting" is quoted anywhere. 

 

[steptoe]

The E&WR 1989 don't apply to the OP as it is a dwelling.

Of course they do, it's a statutory document 

Applies to any electrical work undertaken whatsoever, 

Reg 16 , applies under all circumstances  

 

[binky]

But you aren't extending PME that stops at the service head, you are using a cpc derived from a PME supply. 

I understand the open PEN scenario but this "exporting PME" business to my knowledge has never been quoted in the Regulations.

but it's still reliant on the PME, it's not separate from it and therefore under certain fault conditions can behave differently to TN-S or TT. 

Lts of things aren't in the regs, but in guidance notes and other materials that attempt to explain the regs. 

 

[Fleeting]

but it's still reliant on the PME, it's not separate from it and therefore under certain fault conditions can behave differently to TN-S or TT. 

Lts of things aren't in the regs, but in guidance notes and other materials that attempt to explain the regs. 

Yes it relies on a PME derived earth you don't export it stops at the service head.

 

[binky]

Yes it relies on a PME derived earth you don't export it stops at the service head.

by that argument it doesn't matter if it's TN-S or TN-C-S, so why is there so much written about correctly identifying the earthing arrangements if it doesn't matter as it stops at the service head? 

 

[Fleeting]

by that argument it doesn't matter if it's TN-S or TN-C-S, so why is there so much written about correctly identifying the earthing arrangements if it doesn't matter as it stops at the service head? 

Not understanding what you are trying to say.

 

[binky]

from 2009

https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=30762

 

[binky]

and one of the many discussions on here 

 

[GD16]

Great thanks to be honest the selectivity issue wont matter practically as the RCBO is in the Main CU only supports the radial circuit to the outside socket on the house currently If it triggers its not going to affect the rest of the house.

So wether the RCBO triggers in the Main CU or the RCD protected circuits in the shed go first it's not going to make any difference to the Shed.

How do you think an inspector will interpret this from the perspective of selectivity and  the requirements of 18th ed?