Exporting Pme 2016-09-21

[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 ?