FoxESS Community - Owners & Installers Forum

Hi, I was tinkering and I put together a wiring diagram of what I would like to deploy in my house. Please, qualified electricians, tell me what I'm doing wrong 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.

Additional info:
- The delay relay is to avoid re-energizing the change-over relay if the grid were to come back intermittently, so a delay between 1 minute to 10 minutes would be a neat fix for this potential toggling.
- The changeover will be fast but not immediate (sub 20ms), so not all the house circuits will be suitable for uninterrupted power. The essential electronics (HA Server - Router & ONT - Switching - Modbus - CCTV) will be connected to the EPS CU where uninterruptible power will be available.
- I'm aware that power lines in the loft are a hot topic, but the idea is to manage them in the safest possible way, even if it comes to treating them as outdoor lines (e.g. SWA).
- I've not shown the wires of the circuits coming off the RCDOs because it's beyond the scope.

Interesting little project, assuming the timer will operate in a delayed on mode I can see nothing wrong with the functionality.

You have an earth rod fitted which is correct, the one omission is significant and can be dangerous - you *must* create a Neutral to Earth bond when the grid is disconnected. This bond would be made at the DNO cutoff where the grid enters your home and you have to assume that this is no longer in place.

The 2 pole contactor is correctly disconnecting the grid connection to form an island, if you can change that for a 3 pole contactor you could make the EPS N.E bond on the 3rd pole, alternatively you would have to fit a single pole contactor to bond the EPS Neutral to Earth (a PE-N link) when the grid is lost.

The new Hx G2 series does this internally on the EPS port, but the original H has no internal bonding and if you disconnect the grid you will have a floating Neutral.

You can't appreciate how thankful I am for your answer.
I just spent some time looking into what you just explained and I have so many questions now

- Why shouldn't I permanently connect the EPS Neutral to ground at all times?

- Because of my permanent Earth Rod, have I become a secondary earthing point for my neighbourhood and, more dangerously, a primary one in case of complete PEN loss?
Is this something I should be concerned of or just a minor risk?

Peeshow wrote: ↑Sat Apr 27, 2024 7:06 pm You can't appreciate how thankful I am for your answer.
I just spent some time looking into what you just explained and I have so many questions now

- Why shouldn't I permanently connect the EPS Neutral to ground at all times?

It’s illegal (in the UK) to connect an off-grid system to any of the incoming supply connections to avoid the potential for electric shock when engineers are working on the incomer - so in effect when you disconnect from the grid you must isolate all of the connections from the incoming grid, if you have a 3 pole changeover you will do that so then you can simply connect the EPS Neutral to ground on the EPS port sure in the knowledge that it will be isolated from the grid when the grid fails.

Peeshow wrote: ↑Sat Apr 27, 2024 7:06 pm
- Because of my permanent Earth Rod, have I become a secondary earthing point for my neighbourhood and, more dangerously, a primary one in case of complete PEN loss?
Is this something I should be concerned of or just a minor risk?

It’s not really a risk, there are many earthing points between your home and the transformer and as long as you are connected to the grid it’s fine but as said ‘technically’ you should ensure when the grid is ‘off’ that you are no longer connected to the incomer not so much for your safety but the safety of engineers working outside.

Tony has done a really good diagram which shows the best way to configure it, a 3 pole changeover switch to isolate the incoming grid, a N->E bond on the EPS connector etc..
In your case you have located the EPS C/O switch in the main consumer unit (with the timed relay driving it) and the house load MCB’s are where the EPS consumer unit is, and your EPS CU would be where the EPS isolator is - note the EPS Earth is bonded to Neutral.

The problem as ever is equipment, finding a 3 pole changeover contactor - the best i've found is a Schneider 2NO+2NC 63A contactor, and so I ended up using a separate 1 pole changeover for the PE-N connection.

Much of this is discussed in here https://electrical.theiet.org/wiring-ma ... ge-systems, i'll also leave this link here as it is very helpful https://engx.theiet.org/f/wiring-and-re ... ry-storage

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:

1. 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).
2. LN Contactor Fails when Grid ON -> Potential Danger (Power is provided from Inverter without N-E Bond connection).
3. 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.
4. Both LN and N-E Bond Contactors Fail or the Master Relay Fails when Grid ON -> No Danger (This is basically Island Mode again).
5. 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 think you have summarised the failure modes very well (what do you do for a living, are you HAZOP trained by chance ?), and there are methods for you to alert yourself to floating neutral/earth, interlocks or to detect the earth fault and force a bond directly to the earth rod, but with complexity comes increasing failure rates.

You’re right the documentation shows the secondary earth connected, I’ve always thought the interesting conflict is how can you do that and fulfil the IET requirement to be fully isolated from the grid when working off-grid?… this does seem to be a grey area. I think ‘by the book’ would have to be v2-A, although I have to add my install sits within the grey area.

One last precisation:

Timing of the operation of the island mode isolator and N-E bond relay should comply with Regulations 431.3 and 537.1.5 of BS 7671. This requires:

• The N-E bond relay to be interlocked, or mechanically linked, with the island mode isolator, [...]

This means that the potential electrically related failures discussed above are already "covered" by the Regulation, simply specifying that the connection of the 2 contactors shall be physical (interlocked mechanically). Which makes sense but forces me to find (ideally) a 3NO-2NC high current contactor or finding a series that supports mechanical interlocking.

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My background is in automotive software and system testing

Peeshow wrote: ↑Sun Apr 28, 2024 1:59 pm One last precisation:

Timing of the operation of the island mode isolator and N-E bond relay should comply with Regulations 431.3 and 537.1.5 of BS 7671. This requires:

• The N-E bond relay to be interlocked, or mechanically linked, with the island mode isolator, [...]

This means that the potential electrically related failures discussed above are already "covered" by the Regulation, simply specifying that the connection of the 2 contactors shall be physical (interlocked mechanically). Which makes sense but forces me to find (ideally) a 3NO-2NC high current contactor.

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My background is in automotive software and system testing

Yes there’s something to be said for a manual changeover over automatic switch - with such high currents you might struggle to find a 3NO+2NC, instead having to work with multiple 2NO+2NC devices to achieve an interlock.

I might have found a solution.
The HS1 series from Heschen appears to support interlocking.
Then it's just matter of finding your preferred combination.

Great, the aux contacts should be exactly what you need - let us know how you get on.

Hi Guys - I found this post very helpful in pinning down the issues for my plans to implement EPS. I may be being dumb but is there a flaw to using an existing low cost solution in the form of 3pole ATSs being offered from China etc e.g. https://www.ebay.co.uk/itm/315276963374
Can't 2 poles be used to switch L& N and the 3rd to make the N-E bond on the inverter side ? I know for the price reliability may not be that good but given the 3 poles are firmly mechanically linked and the switching is in a fraction of a second, this ATS appears to meet the spec in the standards quoted.
Comments welcome.
Regards

asyrop wrote: ↑Thu May 02, 2024 10:43 am Hi Guys - I found this post very helpful in pinning down the issues for my plans to implement EPS. I may be being dumb but is there a flaw to using an existing low cost solution in the form of 3pole ATSs being offered from China etc e.g. https://www.ebay.co.uk/itm/315276963374
Can't 2 poles be used to switch L& N and the 3rd to make the N-E bond on the inverter side ? I know for the price reliability may not be that good but given the 3 poles are firmly mechanically linked and the switching is in a fraction of a second, this ATS appears to meet the spec in the standards quoted.
Comments welcome.
Regards

The problem with a TN-C-S system the Earth is bonded to the Neutral where it comes into the meter cupboard, if you disconnect only the grid L&N, the Earth would still be connected to (grid) Neutral at the incomer and so you are not fully isolated from the grid when running on EPS - hence you would need to use all 3 poles to isolate L,N & Earth from the grid side.
If you follow the schematic Tony did for EPS circuits which isolates the incoming, you can use a 3 pole changeover

Thank you for your patience, Dave. I had failed to look closely at Peeshow's diagram and spot it is TT rather than TN-C-S ! I quite get your point and for me (who has older TN-S) I will need a 4pole version of the ATS, which as I look online is also available in similar units.
I will continue my planning - suitably the wiser...
Regards