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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
  • I think your idea of a local electrode connected in shunt with the suppliers CPC will be fine So long as the CPC is not switched then exactly where it connects only matters if some part of the path to it is too thin for the likely fault current - most unlikely for a modest electrode to be less than some tens of ohms to terra-firma so really 4mm2 will be fine, thinner if you can ensure it is well protected  against mechanical damage and corrosion.

    Do convince yourself that there is no condition where the inverter can generate but there is inadequate protection against faults and overloads. ( I presume the inverter does not generate 32A, or if it does, not for long) And let us know how you get on.
    Mike

Reply
  • I think your idea of a local electrode connected in shunt with the suppliers CPC will be fine So long as the CPC is not switched then exactly where it connects only matters if some part of the path to it is too thin for the likely fault current - most unlikely for a modest electrode to be less than some tens of ohms to terra-firma so really 4mm2 will be fine, thinner if you can ensure it is well protected  against mechanical damage and corrosion.

    Do convince yourself that there is no condition where the inverter can generate but there is inadequate protection against faults and overloads. ( I presume the inverter does not generate 32A, or if it does, not for long) And let us know how you get on.
    Mike

Children
  • Thank you for your answer. You’re right, the inverter is only 16A rated but it can passthrough up to 32A. It relies on the user to manage their load during a power cut to reasonable levels or else it would overload alarm then cutoff shortly after. Whilst connected to the grid, the full 32A is available.

    yes the CPC will remain unswitched. Im surprised that you said that where it connects does not really matter. Given the inverter does an N-E bond, I would have thought the earth rod can not be connected on load side of EPS RCD. I will need to think about this more carefully. Terminating to the grid / connected side consumer unit would be easiest and I believe it may also enhance the safety of the EV charger.

  • The location of the NE bond matters - and of course an NE bond will trip an RCD by looking like an NE fault if the bond is on whichever side of the RCD is acting as the load side, hence the need to switch off the mains neutral as well as live,  (which has its own NE bond) when running from inverter.
    But what I meant was that the location of the electrode can be more or less anywhere in the earthing  system that never gets isolated.
    Switched NE links tend to be a UK problem, in many European and further locations (Oz, Nz SA among others), to have more than one NE bond is not prohibited, just one has to think about where current may circulate, and of course it plays merry hell with RCDs in any country.
    Mike