Exporting Pme 2016-09-21

[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 .