Exporting Pme 2016-09-21

[DPG]

Thanks for providing this resource. I have been researching this subject which seems to be a poorly documented & poorly understood topic in the industry & with people working from home in the pandemic, we are seeing lots of new outbuilding installs for garden offices.
My understanding is a major reason for not exporting the earth is because of the dangers it would pose when you have a broken PEN conductor & the earth becomes live..
This feeds into selection & erection: It's not just about the bonding a water pipe scenario typically quoted. If you have an outbuilding with say steel walls and/or a concrete floor - (dampness can be a factor) the risk is much higher from the earth becoming live than say if you have a wooden outbuilding with a wooden floor & plastic accessories. In the first scenario you would need a TT system and to isolate the armoured cable earthing from the outbuilding.
P.S. I have just found this resource on electricalapprentice.co.uk which is helpful & even has a picture. It refers to the broken PEN conductor scenario:
Armoured cable supplies to outbuildings

I'd suggest reading GN8.

 

[i=p/u]

Just a circuit like any other, as for bonding if required, use armour of swa, as long as greater than the 10mm required .. that's what I know

 

[Msitekkie]

Thanks. I had a look at the GN8 table of contents (downloadable) and while it covers general principles, I didn't see any specific sections on outbuildings, so thought I would note that here for other's benefit. Hopefully the resource I linked to, combined with the original submission will answer most questions.

 

[Msitekkie]

Which regulation states this? When i asked my NICEIC assessor about exporting earth on a PME TNC-S (in this case to metal lamp post) , he told me your not exporting anything , he also said what’s the difference between an outbuilding and a part of the main building ? , in a metal building can not the adiabetic equation be used or just run an extra earth .

I am not aware of any specific regulation, perhaps it's left more to general principles. "thou shalt avoid people getting electrocuted as far as is reasonably possible". I think because it is not specifically covered in the regs is one of the reasons this is poorly understood & applied. The resource I linked appears to have come from ELECSA originally & I see they say "ELECSA consistently receives enquiries relating to supplies to detached outbuildings of domestic premises".
not "exporting" - semantics. I'm not sure I could come up with a better word to describe it. It's certainly not affected by Brexit if that's the type of export he has in mind... ?
The size of the earth is irrelevant for protection if you have a broken PEN conductor, because it's the earth that's the source of danger - carrying the live current.
Yes, if you have a broken PEN conductor there is also danger in the main building, but It's about the likelihood of you being in contact with earth in some outbuildings (e.g touching metal wall or concrete floor, especially in the presence of damp) and say a metal accessory or the case of a Class 1 appliance (earthed metal case). As with so many things in our industry it's a case of balancing risk and reasonable protective measures.

 

[DPG]

I am not aware of any specific regulation, perhaps it's left more to general principles. "thou shalt avoid people getting electrocuted as far as is reasonably possible". I think because it is not specifically covered in the regs is one of the reasons this is poorly understood & applied. The resource I linked appears to have come from ELECSA originally & I see they say "ELECSA consistently receives enquiries relating to supplies to detached outbuildings of domestic premises".
not "exporting" - semantics. I'm not sure I could come up with a better word to describe it. It's certainly not affected by Brexit if that's the type of export he has in mind... ?
The size of the earth is irrelevant for protection if you have a broken PEN conductor, because it's the earth that's the source of danger - carrying the live current.
Yes, if you have a broken PEN conductor there is also danger in the main building, but It's about the likelihood of you being in contact with earth in some outbuildings (e.g touching metal wall or concrete floor, especially in the presence of damp) and say a metal accessory or the case of a Class 1 appliance (earthed metal case). As with so many things in our industry it's a case of balancing risk and reasonable protective measures.

But you haven't read GN8. Surely this would be a good starting point?

 

[LastManOnline]

I'm confused by this statement. Daz

Your confusion is justifiable. The explanation given by "Daz" does,nt add up. But the point in question about "not having 2 earthing systems simultaneously accessible" is valid. For the reason that if earthing system number 1 (TNC-S in main house) was to have an open PEN, then the armouring in the SWA would be at 230 volt, while the exposed metal in the shed (TT) would be at 0 volts. Hence the importance of ensuring the SWA armouring is glanded off into a plastic box

 

[DPG]

Just realised this thread was about 5 years old.

 

[LastManOnline]

Just realised this thread was about 5 years old.

And you my friend are the link to the past. The good news is... you are still making sense!

 

[DPG]

And you my friend are the link to the past. The good news is... you are still making sense!

I'm glad someone thinks so ?

 

[marcuswareham]

Your confusion is justifiable. The explanation given by "Daz" does,nt add up. But the point in question about "not having 2 earthing systems simultaneously accessible" is valid. For the reason that if earthing system number 1 (TNC-S in main house) was to have an open PEN, then the armouring in the SWA would be at 230 volt, while the exposed metal in the shed (TT) would be at 0 volts. Hence the importance of ensuring the SWA armouring is glanded off into a plastic box

It might not be 230v, could be more or less depending on phase balance, Perhaps we need to contact all our neighbours on our local transformer and get everyone to use there oven, tv and electric shower, etc all at the same time of day so the phases stay balanced, then no voltage on CPC in a PEN fault , hahahaha ?

 

[marcuswareham]

Instead of making a metal outbuilding TT and having a high impedance electrode and then relying on RCDs and having the possibility of 2 earthing systems in close proximity, could you use an earth electrode (for measurement only) and a voltage monitoring relay that opens a contactor (L, N & PE) if the voltage between PME earth and the electrode is >50v , although I guess there is a risk of N current and fault current going down said electrode if things went wrong so would probably need 10mm bonding, doh ?


Can 722.411.41 (iv) be applied to a metal outbuilding using a PME earth also, so using a voltage sensing relay and a contactor to break L, N & PE if the voltage is outside of 207v - 253v, to lower the risks (although doesn't make them go away), I know 722.411.41 is for EV but but could the same idea be used in this case

 

[LastManOnline]

It might not be 230v, could be more or less depending on phase balance, Perhaps we need to contact all our neighbours on our local transformer and get everyone to use there oven, tv and electric shower, etc all at the same time of day so the phases stay balanced, then no voltage on CPC in a PEN fault , hahahaha ?

Correct. There are so many variables involved in determining what voltage will eventually be present under open PEN that I usually just assume worst case scenario (for purposes of discussion)

 

[LastManOnline]

Can 722.411.41 (iv) be applied to a metal outbuilding using a PME earth also, so using a voltage sensing relay and a contactor to break L, N & PE if the voltage is outside of 207v - 253v, to lower the risks (although doesn't make them go away), I know 722.411.41 is for EV but but could the same idea be used in this case

Can't see any reason why not. You still feel though that the sulutions we are currently endeavouring to supply for TNC-S issues are not ideal. Reality is TNC-S as a "supply system" is not ideal.

 

[marcuswareham]

Can't see any reason why not. You still feel though that the sulutions we are currently endeavouring to supply for TNC-S issues are not ideal. Reality is TNC-S as a "supply system" is not ideal.

Indeed yes I do feel like it is an incomplete solution, but better than nothing especially if the item in question is unlikely to be touched etc (unlike an EV)

Shame we dont have access to all 3 phases in houses, its quite easy to detect a PEN fault with 3 phase

 

[Msitekkie]

Just realised this thread was about 5 years old.

Mmm, but quite relevant at the moment, with so many home offices going in

But you haven't read GN8. Surely this would be a good starting point?

Maybe.I probably will get a copy eventually. Unfortunately I can't afford a full set of Guidance

But you haven't read GN8. Surely this would be a good starting point?

Maybe. Unfortunately I cannot afford a full set of Guidance Notes at the shocking price they are available & I am still working through Sparky Ninja's online webinars, which are more accessible & certainly cheaper... Some good stuff on earthing.
I probably will get GN8 eventually, but probably by the time I have finished Sparky Ninja's resources a new edition will be out.
What I am really interested in regarding earthing & I suspect not covered by GN8, is how to go about fitting an earthing mat when I have my drive done that will help me in future to use my Solar power off-grid ("Islanding"). I gather from Sparky that proposed standards talk about earthing mats for new builds, e.g adding them to foundations, but no guidance on retro-fitting.

 

[old man sparky]

TT and PME joined happily together.
Broken PEN upstream of yer supply with Mrs Jones at number 42 having a shower and Mr Smith cooking the Sunday roast at 48. Every buggers supply load see's yer poor old Rod as a brightly shining light is seen by moths. Best keep em separate, Mind you if its a nice wooden floored shed with no bonding needs even the swa glanded properly will often be a good enough cpc for extending yer TN-C-S arrangement.

 

[LastManOnline]

Mmm, but quite relevant at the moment, with so many home offices going in

Maybe.I probably will get a copy eventually. Unfortunately I can't afford a full set of Guidance

Maybe. Unfortunately I cannot afford a full set of Guidance Notes at the shocking price they are available & I am still working through Sparky Ninja's online webinars, which are more accessible & certainly cheaper... Some good stuff on earthing.
I probably will get GN8 eventually, but probably by the time I have finished Sparky Ninja's resources a new edition will be out.
What I am really interested in regarding earthing & I suspect not covered by GN8, is how to go about fitting an earthing mat when I have my drive done that will help me in future to use my Solar power off-grid ("Islanding"). I gather from Sparky that proposed standards talk about earthing mats for new builds, e.g adding them to foundations, but no guidance on retro-fitting.

This whole area of installing (or retro fitting) earth mats/rods etc is surprisingly not as well discussed as it could be. There are options out there that could be explored like the kilometers of disconnected metallic services in urban areas already in situ that could be "reactivated" for earthing purposes

 

[LastManOnline]

TT and PME joined happily together.
Broken PEN upstream of yer supply with Mrs Jones at number 42 having a shower and Mr Smith cooking the Sunday roast at 48.

Your point highlights nicely the variables that come in to play during an open PEN FAULT.

Every buggers supply load see's yer poor old Rod as a brightly shining light is seen by moths.

Are you referring here to the standard earth rod?

Best keep em separate, Mind you if its a nice wooden floored shed with no bonding needs even the swa glanded properly will often be a good enough cpc for extending yer TN-C-S arrangement.

Agreed. The construction of the shed is a major fact in the degree
of potential danger under open PEN fault

 

[old man sparky]

Never used a twig for TT if thats to be known as a standard rod. Hate the bloomin things. So always a substantial rod which of course adds to the reasoning regarding keeping the two earthing arrangements separate. The brighter the light the more moths you attract. Thing is there may well be parallel earth paths upstream but with water and gas arrangements turning to pvc I still reckon the rods going to be a favourite. As said use one or The other but not both. Each system will do the job if installed properly.

 

[LastManOnline]

Never used a twig for TT if thats to be known as a standard rod. Hate the bloomin things.

So do I. Am obliged over here (ROI) to install them with every new install and rewire (our supply system is TNC-S). Always terrified of hitting underground pipes/services

So always a substantial rod which of course adds to the reasoning regarding keeping the two earthing arrangements separate. The brighter the light the more moths you attract.

Do I conclude from this that you feel your earth rod can be a potential danger under open PEN?

Thing is there may well be parallel earth paths upstream but with water and gas arrangements turning to pvc I still reckon the rods going to be a favourite. As said use one or The other but not both. Each system will do the job if installed properly.

Same question as above?