Mixed earthing arrangements

[Richard Burns]

Mixed Earthing

Hi just wanted to ask some advice on (Sorry) earthing and bonding where there are mixed earthing arrangements.

Situation:
There is a TNCS house with an newish extension which has a TT CU fed from the supply by 6mm 3c SWA on which the third core has been cut short.

The armour is earthed at the main supply by the gland to the metal box.

At the remote CU it is glanded (but armour not quite in gland) into a metal box and on into insulated CU.

Supply to sheds 10mm 3c SWA (third core to earth bar) comes from CU through the same metal box and armour is glanded.

Somewhere not found this splits to two 2.5mm 3c SWA and goes to metal CUs in two sheds where the armour is glanded to the CUs but the third core is left loose (but not cut off), the supply from one shed is looped to the third shed, again glanded armour and loose third core.

There are no extraneous parts in the sheds but there are radiators in the extension not bonded to extension CU ( I did not check if they were bonded to the main supply MET).
No fly leads have been used to earth the armour.

Questions:

I think the idea of having an extension TT and the house TNCS is not a good idea, although there are no simultaneously accessible conductive parts at the changeover of the two earthing systems, the radiators would be at TNCS potential not TT potential and could cause a shock risk in case of a fault.
Is my understanding correct?

I would like to lower the metal box at the insulated TT CU down to reach the short armour of the supply , apply fly leads everywhere and connect the third cores of all SWA to the local earth bars (EXCEPT the third core of main supply as it is too short to reach at both ends).

This would make the whole installation TNCS.
The radiators would be bonded from the main MET and I would not need bonding size cables to the insulated CU or the sheds. (The 6mm 3c SWA armour is only 7.39mm2 Cu equivalent)


Alternatively I could connect together all armour and third cores of the TT part of the installation and ensure the supply armour is insulated from the TT (but earthed at the TNCS end).

This would leave the extension and sheds TT with TNCS radiators, or I could bond the radiators to the TT which would then mix the earthing systems through the bonding.

What do you think would be the best method of approach and am I wildly off track in my assumptions?
Sorry for the long post.

 

[Rockingit]

I can't see why anyone would pay for a 3c SWA to only use 2 cores of it! If it was me, I think I'd try to find a way to get that third core back/and/or gland all the SWA's properly if the CC of the armour is sufficient as an earth, then wire them all back as a PME. You could keep the TT rod in place, will just act to lower the overall Zs values. If you can't use the SWA, maybe you could run a separate 10mm from the MET to wherever you need to get to?

 

[Knobhead]

Over time no doubt the two systems will merge as things get altered, it should be one system from the outset. What the original installer was thinking of by running three core and then only using two I don’t know! This myth of exporting earths is getting ridiculous.
As for the mix of cables, someone must have been using up leftovers.

 

[Richard Burns]

I think the installers were not at all confident of what they were doing (I certainly wasn't once I had looked round it!).
I believe the first supply was put in at the start and then the sheds done at the end and I was told they were worrying about volt drop.
The earth rod was a last minute addition, but why?
Cutting off the third core is just dumb. at least they left them on most cables, I would not be able to restore that core as the cables are set in concrete there and the core is cut off just as it enters the enclosures! However I am happy using the armour so long as I do not need bonding.

I will recommend the consolidation of the systems, which I am much happier with.

Thanks for your help.