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matt1309 asked

Obscure UK earthing question TNCS

Hi folks,


I've got an obscure earthing questions.

Background

Current setup is grid paralleled multiplus ii. Pv ac coupled on AC In, loads and grid also on ac in only.

"Off grid mode"

My plan was to add a 3 way switch between ACIN ACOUT1 and the grid/load.

In the event the power went out I would disconnect the grid manually and move the 3 way switch essentially powering loads on acout1 rather than ACIN. (I cant leave loads on a out all all the time as the multiplus is undersized for loads hence needs grid parallel, even power assist its undersized, I plan is to add another multi in parralel one day when funds allow for it and change system to all loads using ACIN)


Question

My question is in regards to earthing. I have ground rod at multiplus in the shed 100m from house (loads) and grid and therefore I dont export the TNCS earth from the house/grid.

However if grid goes down and I go into offgrid mode using the above procedure I've realised the loads at the house would be using the grid TNCS earth as we don't pass earth from shed (where multiplus is) to house or vice versa. I imagine this will cause issues with RCDs, can someone confirm this. If it doesn't cause issues is it even allowable to use the earth from TNCS system when live and neutral from grid are disconnected via the grid switch when I'm in offgrid mode?

I imagine the solution is another ground rod at house making it TT system.

Id really appreciate it if someone knows or knows where I can look to research the answer.


Thanks,

Matt



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3 Answers
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sharpener answered ·

Correct, the IET Code of Practice says "In accordance with BS 7671, the EESS cannot rely on the distributor's earth connection during island mode operation". And the thinking is also that you don't want to import other customers' fault/earth potentials when you are in island mode.

I can see another problem: when set for UK grid the MultiPlus will in island mode forcibly connect the neutral to the ground rod at the shed, whereas the loads in the house should be earthed to the new ground rod there. The solution is to connect both ground rods together (assuming you have an earth core in your 100m cable) so there is a equipotential zone encompassing both, I think that complies with both the letter and the spirit of the rules.

I was fortunate to have had a TT connection already, even though the DNO provides a PME terminal. Presumably this was because there are two outbuildings with concrete floors; with a TN-C-S supply they would each have required their own ground rod, but a single one at the house is sufficient for a TT setup.

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wkirby answered ·
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sharpener avatar image
sharpener answered ·

Yes, you would need another ground electrode at the house. I think exporting the earth from the shed back to the house although perhaps theoretically allowable would be frowned upon. The ground rod needs to be less than 200 ohms, I have read that IF you are sufficiently technically skilled you can check this by injecting a test current via a 60W light bulb and measuring the voltage wrt a remote ground spike and/or the DNO's PME terminal, you should not see more than 50V (mine gives 15V so about 60 ohms), take adequate precautions while doing this test!

These 125A changeover switches here are robust and good value, IME their action is quick enough to change from grid to inverter without any of the house loads or computers dropping out.

HTH

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matt1309 avatar image matt1309 commented ·
Thanks for the help guys.


So in summary I cant use the PME earth as a make shift earth rod at the house.

The DNO earth is at the house but I assume when I was in "off grid mode" I wouldn't be able to use that earth given live and neutral from DNO would be disconnected even if the resistance to earth is low?

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