Amazing info, thanks!
I think my system can be adapted in the way you're suggesting and be approximated by something like this new diagram (main differences highlighted in red):
So I've updated my schematic (v2-A), including the additional contactor to handle the N-E Bond connection in Island Mode as well as unbonding the Earths.
(v2-A)
> There is an intrinsic danger to this dual-contactor approach, though. If one of the 2 coils fails, you might end up in a bad place.
Looking at the potential scenarios:
- One/Two failure(s) when grid OFF -> No Dangers (Coils are energized by the Grid, hence no difference if there are failures when Grid is OFF).
- LN Contactor Fails when Grid ON -> Potential Danger (Power is provided from Inverter without N-E Bond connection).
- N-E Bond Contactor Fails when Grid ON -> DANGER! The Earthing is lost for all Maintained Loads and it now goes through the N of the Inverter in the EPS line.
- Both LN and N-E Bond Contactors Fail or the Master Relay Fails when Grid ON -> No Danger (This is basically Island Mode again).
- Am I missing something?
I think point 2 is the most dangerous. I believe, RCD will may still work but the path is very convoluted (haven't put too much thought into this yet but seems bad at first glance)
BUT
I had a look at the documentation in the link you shared, it seems like you can leave the "Secondary" Earthing System connected to the TN-C-S earth at all times, even in Island Mode (as far as you create the N-E Bond while in it).
Hence, I'm quite liking that solution and I can't find anything wrong with it (except potentially become the "neighbourhood grounding point"), it's more streamlined and less overall dangerous. I so created a new version with "permanent earth" (v2-B).
I'd rather have some current flowing into my rods from some local faults, in the extremely unlikely scenario of complete loss of earth in the TN-C-S service, than the complete loss of earthing for my loads in case of a relay failure.
I hope I have been accurately calculating these odds... Please, correct me if I'm wrong.
(v2-B)
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Appendix 1: I am also thinking that, because now I have a 1NO 1NC contactor in my system anyway and a master relay with both NO and NC "outputs", I can select the best failure mode of the N-E Bond contactor.
I'd rather have it fail opening the N-E Bond connection when in Island mode than fail closing the N-E Bond connection when the Grid is ON. So I just need to think about how to connect it.
Appendix 2: If I ended up using solution v2-B ("permanent earth"), I won't be needing a 63A contactor for the N-E Bond and I think a 25A will do just fine (advantage: 1 slot instead of 2).
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On a separate note, my earth rods are connected to the Earthing Systems in a Henley Block at the Meter Box, bugger... I'll have to route it in the MAIN CU somehow one of these days.
PS: Clearly I'm not a qualified electrician, so for everyone out there, take what I say with a barrel of salt. I will do my best to rectify mistakes and remove my content if it needs to.
I would really love to constructively find a solution to this all-in-one concept because I've got space for a 20-way CU but not enough space for an additional change-over box like illustrated in the "Level 4" example.
- let us know how you get on.