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?