Islanding mode earthing requirements with PME supply to inverter in garage

Realised this post was long winded but it’s missing some detail on the islanding circuit.

The islanding / EPS output of the inverter goes to a 32A RCBO feeding socketed devices where short circuits are highly unlikely on 6mm2 cable.

Overall, the proposed setup is:

50A MCB -> 32A RCBO -> inverter (<200 ohm earth rod connects here) with contactor for N-E link-> 32A RCBO -> EPS/ islanding loads.

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

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.

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.

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

AJJewsbury said:

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

Akbar Ramzan said:

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.

IET Code of Practice for Electrical Energy Storage Systems says connect the electrode to the MET (BS 7671 recommends this anyway). The N-E switched link can still be carried out at the inverter.

Akbar Ramzan said:

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.

Unless there is any part of the installation that has a TT earthing arrangement in island mode, the IET Code of Practice for Electrical Energy Storage Systems says 200 ohm is an absolute maximum.

However, you will be forming a TN-S system in island mode ... but regardless, you will still need to determine the earth fault loop impedance of all circuits in island mode, to verify that they are less than the values in Table 41.5 of BS 7671 (applies to TN or TT systems where RCDs are used for ADS). Because loop impedance tests on circuits supplied by inverters are likely to give spurious results, you can only do that if you know the effective source impedance of the inverter up to its current limit, which the manufacturer should be able to provide.

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.

It’s going to be incredibly difficult to wire in a new cable to the MET. The house has been extended so the meter cupboard is right in the middle, can’t easily get a cable out. Do you think an information label would suffice in the meter cupboard?

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).