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Islanding mode earthing requirements with PME supply to inverter in garage

I have the following situation, in my own house I might add:
TN-C-S single phase PME supply
50A B curve MCB in main distribution board feeds 10mm2 3C (one core used as CPC), SWA cable going to garage ~5m away from the house
In the detached garage there is a consumer unit feeding a 32A type B RCBO for an EV charger and a 32A type A rcbo for an inverter, both 30mA
EV charger has an open PEN detection relay
(I know about the selectivity issue here with the MCB but it's determined not a safety issue, short circuit unlikely. Sockets and lighting on separate 2.5mm supply)

Now the question is, if I want to enable the inverter to work in islanding mode, what earthing provisions do I need to add?
The house is detached and more than 30m away from other houses and metal street furniture
Ze from supply is 0.12 ohm, main fuse 100A
Inverter can perform N-E link and coordinate with changeover switch at incoming supply

Am I right in thinking that I should remain connected to supplies earth but add an earth rod? What impedance requirement should this earth rod have?
I think it's 200 ohm because the inverter is protected by a 30mA RCBO and in connected mode, the supply earth will be sufficient.
I also think the earth rod should be connected to the inverter directly (i.e. downsteam from the rcbo and not upstream) so that the RCD will be able to detect the leakage in islanding mode.

Thanks!

Parents
  • BS 7430 usually says 20Ω for an electrode for a generator ... although the exact reasons for that figure seem to have been lost in the mists of time.  Certainly if you had a TT system hanging off you generator-fed system somewhere you'd have to include the generator's electrode in the Zs calculations for the TT part - and 20Ω would then be convenient as it aligns with the maximum figure the DNOs use. But for a pure TN system where the electrode doesn't form part of the earth fault path, physics would suggest that higher values could still be satisfactory, and 20Ω isn't going to be the easiest value to achieve in practice.

    If you do connect the electrode at the generator end, do consider that it will effectively be an extraneous-conductive-part when running from the grid supply - and thus by BS 7671 rules the entire path of the protective conductors from the MET should be sized for PME main bonding. (Typically the electrode's resistance to Earth will limit the current that can flow, but BS 7671 doesn't take that into account).

    Legal wise (for the UK), you'd need to comply with The Electricity Safety, Quality and Continuity Regulations - which makes compliance with  British Standard Requirements (i.e. BS 7671) mandatory, even if the generator can only operate as a switched alternative to the grid. If it can operate in parallel, there are further specific requirements (e.g. automatically disconnecting when grid supplies fail) and DNO rules to comply with - G98/99 (previously G83/59).

    There are some other gotchas if the generator (inverter) can run in parallel with the grid supply - specifically because after the point where the two supplies combine normal assumptions about overload protection can be undermined - both for circuit cables and switchgear. For instance a DB fed by a 100A fuse that can also accept 32A from on-site generator may have to carry 132A internally - so the usual 100A rated equipment won't necessarily comply.

       - Andy.

  • For instance a DB fed by a 100A fuse that can also accept 32A from on-site generator may have to carry 132A internally - so the usual 100A rated equipment won't necessarily comply.

    Effectively does have to now - Regulation 551.7.2 was changed in Amendment 2:2022

Reply
  • For instance a DB fed by a 100A fuse that can also accept 32A from on-site generator may have to carry 132A internally - so the usual 100A rated equipment won't necessarily comply.

    Effectively does have to now - Regulation 551.7.2 was changed in Amendment 2:2022

Children
  • When I get home I will read up on this regulation 

    the maximum demand of the house with diversity applied is approx 60A without the inverter. The inverter is also connected to a Modbus energy meter that the entire house is connected to and can clip to ensure <=100A is present in the CU.

  • The Regulation now states that the rating is not based on demand but simply the rating of the supply protective device and the sum of connected generation. However, whilst  this approach appeared in BS 7671 in March 2022, it has been in the IET Code of Practice for Electrical Energy Storage Systems since the 2nd Edition was published (December 2020).

  • Doesn’t seem well thought out to be honest. This would mean I need to fuse down everything to 80A as the inverter is 16A rated to protect against a scenario that’s extremely unlikely and would likely be harmless anyway. Or somehow find a consumer unit rated 116A or higher…