Exporting Pme

Exporting Pme 2016-09-21

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Exporting Pme

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

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