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Posted By Topic: Electrical Brain teaser.

Nov 06 2019 19:59

Hi All

Need some clarity on a installation.

There is a main switchboard {MEN} on a building/shed, this is a very large building/shed.

from the main switchboard there is a submain feeding 2 switchboards in other out buildings, both of them will be MEN boards. (they are both in seperate out buildings).

But from the 1st submain (MEN)board there is another submain (PEC) running back into the building with the main switchboard, the main switchboard is about 120 meters away.

my question is: can I have a distribution board (PEC) in the same building as the main switchboard (MEN) EVEN THOUGH THE DISTRIBUTION BOARD (PEC) IN FED FROM THE SUB MAIN (MEN) THAT IS FED FROM THE MAIN SWITCHBOARD(MEN)




Nov 06 2019 21:10

No you can't do that. Because the submain back to the main building will be a parallel conductive path.

Nov 06 2019 23:07

Or maybe I've read more into that clause than I should of.

Slightly unclear wording in that clause regarding earthing or bonding conductors, then pipes or metalwork.


Nov 07 2019 07:54

I was thinking the same thing

Isn't one of the main issues of a separate MEN that any earthed parts of the individual installations could be at different potentials and that's why it's only allowed in an outbuilding and not within the same building

By running it back from an outbuilding MEN you'd have a DB in the building of the Main SB but there could be many parts earthed to each and earthed parts at different potentials?


Nov 07 2019 09:02

There's certainly a risk of parallel paths; depending (in part) on the big barn's construction.

There's also a risk of simultaneously accessible parts being connected to different earthing systems at different potentials, eg equipment supplied by / earthed to MSB and equipment supplied by / earthed to the PEC-fed DB within the barn.

The length of conductor via 2 submains to barn DB especially the 140m between the electrodes, could easily cause different potentials - worst case being when highly loaded (fault currents worse again, but shouldn't last long enough matter).

But not enough info to be sure of exactly how the rules apply.

Nov 08 2019 02:39

The wiring rules shouldn't permit this to be done, even if they do.

The supply is TNC, and at the main building, the MEN link provides a TNCS supply to that building. The TNC supply continues to the outbuildings, whereupon in each outbuilding it goes to a MEN board, which provides a TNCS supply. One outbuilding then returns a TNCS supply back to the main building.

The problem is that once you "drop down" from TNS to TNCS it's a one way street, so, if you look at it from the perspective of the TNCS in the main building, and then follow the potential path round to the TNCS that came from the outbuilding, it goes via a TNS connection, so it's "stepped up" to TNS and that isn't permitted. I can't remember where that rule or regulation comes from, but its some kind of law of earthing systems; once you drop down to TNCS, there is no going back.

Nov 08 2019 10:03

It hasn't actually 'gone back" at any stage.
What it's ended up as is that the big barn has two (separate0 parts of the installation in it.
At least separate as far as A & N goes.

The problems are that
a) there is high likelihood of a parall earth path, resulting in some N curent being carried by an earthing conductor;
b) there is a high probability of someone being in simultaneous contact with 2 earthed items connected to the 2 different earthing systems - which will be at different potential. At least slightly different; and under some conditions different enough to be hazardous touch voltage.

I agree wiring rules should not allow it - but at present they don't explicitly prohibit it; and don't even warn about the problem of touch voltage.

Bottom line is that a PEN submain should inly be used where there is
a)substantial distance between structures, and
b) no parallel path
leading to very low chance of simultaneous access to parts connected to both systems. PEN should never be used just to save cost of installing a PEC.

Nov 08 2019 11:59

> there is high likelihood of a parall earth path

That is exactly the definition of "going back" - the definitions of the TN-whatever are all about earthing and if somehow there is earth continuity, even inadvertent, accidental, temporary or incidental between the two "isolated" earth derivations, then, you've "gone back".

I wish I could remember where the fundamental rules for building TN-xx earthing systems were, I suspect they are IEC documents as the TN nomenclature is European in origin (French for ground)

Nov 08 2019 12:28

I had interpreted your term 'going back" as meaning we can't use a PEN downstream of any conversion from TNC to TNS.

Which we do all the time, as in this case; for submains to outbuildings. Fairly sure it's permitted under IEC.

Here we have TNC distribution 9norrmal for NZ); with TNCS for loads supplied directly from main switchboard.
Ongoing TNC for the first submain, then TNCS for loads supplied from DB1 (including submain "back" to main building.)
and ongoing TNC for other submain to 2nd outbuilding.

So nothing that's been stepped down to TNS steps back up to TNC.

But even if it did, there's no problem as long as we maintain adequate separation between systems. Eg if 1st submain was TNS (including PEC) then ongoing submain to second outbuilding could be TNC or TNS; no problem either way.

The problems come about not because of the submain from OB1 back to main barn being TNS - being TNC wouldn't have made any difference.
The problems come because the barn has ended up with two supplies, each connected to a different bit of ground.

At 150 m long, we'd have the same issue if all submains were PEN and there were an electrode at each end of the barn.

And only slightly better if all submains had PECs.
even within same structure, if submain 1 goes to DB1 at far end, and subamin 2 comes back to DB2 close to MSB, we'd have a possibility of potential difference between simultaneously accessible parts due to the length of PEC between MSB & DB2 - even with only one MEN in the system.

I'll have to check through IEC to see whether they deal with this possibility; but AS/NZS certainly doesn't deal with it effectively enough - yet.

the new "combined outbuilding" rules are an improvement (at least for outbuildings); but still don't prohibit a submain back from a DB to the building housing a higher-level swbd.

Nov 08 2019 16:27

Just think how as/nzs 3000:2007 or 2018 (doesn't matter which one as this fundnmental requiremennt has not changed) clause (b) touch voltage is going to be delivered in the electrical installation under consideration.

Remember when using a PEN connection, the load current voltdrop in the neutral conductor adds to the touch voltage of the outbuilding switchboard.

You can forget about the local earth electrode by the outbuilding switchboard controlling the touch voltage as it is untested and unlikely to be of a low impedance to hold the voltage rise occuring.

PS by the way, the IEC do not permit the use of the outbuilding provision as detailed in as/nzs 3000.

Further an TN-C-S csupply an NOT be made into TN-C supply again after the neutral to earth connection point (or MEN link in AS/NZS 3000 terms).

Nov 11 2019 14:13


can you provide a reference for your assertion?
I have found nothing to support it in the "fundamental principle" of IEC 60364-1