Exporting Pme 2016-09-21

[GMES]

GMES submitted a new resource:

Exporting Pme - About exporting pme

A quick guide to exporting pme to an outbuilding.

Read more about this resource...

 

[GLENNSPARK]

EXPORTING...or EXTENDING.?

Two different things Glenn...

 

[andyb]

It won't be so easy to export PME when we leave the single market.

 

[Silly Sausage]

It won't be so easy to export PME when we leave the single market.

I believe we're a net importer anyway.
Can't see the Frenchies stopping selling their electric to us after we part company.

 

[HappyHippyDad]

Good resource Glenn!

I have always extended the equipotential zone from a PME and run a 10mm bonding cable if required so have not looked into the option of a TT for an out building in any great depth.

Your resource has got me thinking about the TT option:

It states :

'Where the installation in
the garage is supplied by an armoured cable, the armour
or any protective conductor in the cable must not be
connected to and must not be simultaneously-accessible
with any exposed-conductive-parts in the outbuilding.'

1. I'm assuming the armour of the SWA cable is earthed at the supply end (ie main CU in house), it will then be simultaneously accessible with the earthing bar in the outbuilding CU (if the SWA has been glanded in the outbuilding CU).

2. Similar to the above, if the outbuilding CU is metal then again if the SWA cable is glanded in that CU it will connected to the TT earthing system?

3. The neutral of TNCS or TNS is connected to earth at the transformer or cut out, so is it not a problem that the neutral bar in the outbuilding CU is simultaneously accessible to the earth bar in the outbuilding CU?

Cheers guys

 

[Richard Burns]

Good resource Glenn!
I have always extended the equipotential zone from a PME and run a 10mm bonding cable if required so have not looked into the option of a TT for an out building in any great depth.
Your resource has got me thinking about the TT option:

It states :
'Where the installation in the garage is supplied by an armoured cable, the armour or any protective conductor in the cable must not be connected to and must not be simultaneously-accessible with any exposed-conductive-parts in the outbuilding.'
1. I'm assuming the armour of the SWA cable is earthed at the supply end (ie main CU in house), it will then be simultaneously accessible with the earthing bar in the outbuilding CU (if the SWA has been glanded in the outbuilding CU).
2. Similar to the above, if the outbuilding CU is metal then again if the SWA cable is glanded in that CU it will connected to the TT earthing system?
3. The neutral of TNCS or TNS is connected to earth at the transformer or cut out, so is it not a problem that the neutral bar in the outbuilding CU is simultaneously accessible to the earth bar in the outbuilding CU?
Cheers guys

At the supply end you would usually gland the cable as normal and ensure that the gland, and so the armour, is connected to the supply earthing system. At the remote end the cable could be glanded into a plastic box before the CU and the gland not earthed to the CU and the shroud covering the gland firmly, or the SWA could be connected into the CU with a stuffing gland. Using a normal SWA gland on a plastic CU would mean that the gland end inside the CU would be accessible to the locally earthed parts of the remote installation.
The neutral of the remote supply would only come into contact with the earth of the remote supply if there were a fault and this would be the same as any neutral earth fault.

 

[HappyHippyDad]

Thanks Richard, that all makes perfect sense.

Am I right in thinking the reason behind not allowing the 2 earthing systems to be simultaneously accessible is because there could be a potential difference under fault conditions?

 

[Deleted member 26818]

No, it's because bonding conductors would be undersized.

 

[Richard Burns]

I think the simultaneously accessible prevention is because there is little likelihood that the earth from the supply will actually be at the same potential as the TT earth, even though they are both classed as zero and so it is possible that they could present a sensation of shock in normal operation. Similarly to the differences between true earth and a supply earth. Under fault conditions this would be exacerbated.
Direct connection of the two earthing systems would cause problems with bonding conductor sizes.

 

[DPG]

No, it's because bonding conductors would be undersized.

I'm confused by this statement. Daz

 

[Deleted member 26818]

I'm confused by this statement. Daz

The reason TT is used in these cases, is because the CSA of the armour does not have an equivalent conductance to the minimum CSA of copper required for bonding.
Running an extra conductor for bonding may be prohibitively expensive.

The armour is isolated at the load end to prevent it from being connected to the earthing and bonding arrangements in the TT part of the installation.

This can be achieved by connecting the armour using a standard gland to either a plastic enclosure or to a firmly fixed bracket close to the CU.
Another method would be to use an isolating gland.
I would not advise the use of a plastic stuffing gland as it would not retain the armour adequately.

 

[DPG]

Yes I get that. But the question was about why you can't have 2 different earth systems simultaneously accessible. That's nothing to do with cable size. Daz

 

[Deleted member 26818]

Who says that you cannot?
Remember any remote part of an installation is likely to have a difference in earth potential between the installation earth and the surrounding general mass of the earth.

 

[DPG]

I was referring to hhd's post above where he quotes from the document. Daz

 

[Deleted member 26818]

Ahh yes.
Simultaneously accessible is usually taken to mean accessible without the use of a tool, or not within an enclosure.
It applies to live conductors as well as earthing/bonding conductors, so if it were applied within enclosures, then we would not be allowed bus bar chambers

 

[hightower]

Simultaneously accessible is usually taken to mean accessible without the use of a tool, or not within an enclosure.

Not by my definition. Simultaneously accessible means you can touch one while touching the other, for example, you can touch a bonding pipe while shoving your fingers in a socket.

Or perhaps as another example you can touch the PME earth while touching a TT earth, one being possibly higher voltage than the other.

 

[Deleted member 26818]

Would you be needing a tool to touch the bonding pipe while sticking your fingers in a socket?
The BS7671 definition refers to Conductors or conductive parts.
It then lists: live parts, exposed-conductive parts, extraneous-conductive parts, protective conductors or earth electrodes.
Bus bar chambers contain uninsulated (usually copper) bars for connecting the 3 phases and usually the neutral, some also have an earth bar.
Many have the bars so close together, that you would not be able to pass your hand between them.
Complies with BS7671, simply because they are not accessible without the use of a tool.

 

[Dan]

Just testing a discussion thread about this electricians resource

 

[Msitekkie]

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

 

[newfutile]

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 .

 

[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?

 

[old man sparky]

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

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

Same question as above?

I feel if you dont combine TNC-S with TT you dont need to consider the scenarios of what can potentially happen if an open PEN situation ever developed. A floating voltage suddenly has a return path with unknown currents introduced and limited by the resistance of maybe just the rod maybe not. Why would you allow for that scenario to happen. I prefer not to, its just me. ?

 

[mattg4321]

You could have the same problem though with a metallic service pipe that’s bonded back to the MET.

 

[old man sparky]

You could have the same problem though with a metallic service pipe that’s bonded back to the MET.

If compliant the bonding would be suitably sized for TNC-S and back to the MET, this would also be within the equipotential bonding zone, so not the same issues.
With an out building using a TT system where a TN-C-S earthing has been extended beyond the bonding zone and effectively connected as a combined earth with the Rod. Your into a different scenario, if the PEN is lost downstream.
(Referring to the comments that have gone before on this thread).
So if your extending the PME then you install to that. If your going for TT then that's ok.
But choose one or the other not both. There's really no point.
When we did the installations for petrol stations back in my younger days. PME was not allowed.
TT or TN-S yes but not TN-C-S.
That's it !! off to bed, stay safe guys. ?

 

[Msitekkie]

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.

Thanks for sharing that thought provoking response.

 

[newfutile]

Would it not be better and easier to install a pen fault device as they do for EV?
like a Garo G6EV40PME Consumer Unit, MCU Type A RCBO & PME Fault, EV Charger Compatible

 

[LastManOnline]

Would it not be better and easier to install a pen fault device as they do for EV?
like a Garo G6EV40PME Consumer Unit, MCU Type A RCBO & PME Fault, EV Charger Compatible

I think so, especially if its the type that does, nt require installing a rod

 

[i=p/u]

I believe its suppliers problem

 

[Simon47]

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 ...

Except, and CBA looking up the numbers, the regs prohibit switching the earth. Or they did until the latest edition which ...

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)

allows switching of the earth, and in fact requires it, for EVs

As already said, you can extend the PME to the outbuilding if you BOND any extraneous conductive parts adequately. That mean min 10mm2 of copper, or more if calculations say so. If the bonding is inadequate in cross section, then under fault conditions it'll melt.

Yes, teh same argument apply within the main dwelling - which is why we have main, and if required supplementary, bonding to create an equipotential zone.

 

[old man sparky]

Except, and CBA looking up the numbers, the regs prohibit switching the earth. Or they did until the latest edition which ...

allows switching of the earth, and in fact requires it, for EVs

As already said, you can extend the PME to the outbuilding if you BOND any extraneous conductive parts adequately. That mean min 10mm2 of copper, or more if calculations say so. If the bonding is inadequate in cross section, then under fault conditions it'll melt.

Yes, teh same argument apply within the main dwelling - which is why we have main, and if required supplementary, bonding to create an equipotential zone.

Its a very poor situation IMHO when we are now looking at the 18th edition of BS7671 with all the regulations contained there in. And we still have a possible situation where the whole lot can become live to the touch, bypass every protective device in the install, and the only way its addressed is to fit oversized bonding and hope the potential victim is wearing thick soled shoes. Its about time PME issues like this were addressed with safety monitoring devices that cut the supply at the origin should a loss of neutral occur.
In the meantime I will stick with my thoughts that extending TNC-S to a wooden outbuilding is fine, TT my preferred option, but I will never use both methods together. Just my preference. Wonder if the 19th edition will come up with something else to fit into your TNC-S compliant, Non combustible, SPD and RCBO filled C/U. its gonna get crowded in there. ?

 

[marcuswareham]

Except, and CBA looking up the numbers, the regs prohibit switching the earth. Or they did until the latest edition which ...

allows switching of the earth, and in fact requires it, for EVs

As already said, you can extend the PME to the outbuilding if you BOND any extraneous conductive parts adequately. That mean min 10mm2 of copper, or more if calculations say so. If the bonding is inadequate in cross section, then under fault conditions it'll melt.

Yes, teh same argument apply within the main dwelling - which is why we have main, and if required supplementary, bonding to create an equipotential zone.

So is switching of the earth prohibited, but only allowed for EV charging?, and therefore if you used a device (matt-e, garo etc etc) to protect against a PEN fault by switching the earth but not for an EV but instead for an outbuilding or hot tub or similar would that be prohibited as the only mention of switching the earth is in EV charging section?, seem a bit of a gray area

I understand the reasons why the bonding of a metal shed needs to be a min of 10mm.

Although in bonding a metal constructed shed this would bring the whole shed up in potential in a PEN fault condition so you might be protected inside the shed as if everything extraneous is bonded then everything should be at the same potential inside the shed, But what about the guy leaning against said shed standing on some wet grass with no shoes on ?, or maybe more realistic: opening the metal shed door with a metal handle whilst outside and in contact with true earth