Deleted

[Peeshow]

Deleted

[Dave Foster]

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.

[Peeshow]

Deleted

[Dave Foster]

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.

- 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

[Peeshow]

Deleted

[Dave Foster]

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.

[Peeshow]

deleted

[Dave Foster]

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.

-------------------------------------------------------

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.

[Peeshow]

deleted

[Dave Foster]

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

[asyrop]

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

[Dave Foster]

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

[asyrop]

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