Islanding mode earthing requirements with PME supply to inverter in garage

Question

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!

gkenyon · Accepted Answer

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

gkenyon · Answer

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.

AJJewsbury · Answer

It&rsquo;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&rsquo;t easily get a cable out. 
 
 I don't see any great technical problem with connecting the electrode to the MET via the 10mm&sup2; c.p.c. in the submain - it's quite common to bond extraneous-conductive-parts to the earth bars in sub-boards, 
 If running new cables to the intake position is a non-starter, I do wonder how you're going to make the connection between the inverter control system and the disconnect-from-the-grid contactor though. I suppose a wireless link plus a UPS and the intake might be an option, but I've never heard of it being done that way. 
 - Andy.

gkenyon · Answer

Akbar Ramzan said: The inverter has the ability to send an ELV signal for coordinating islanding mode. 
 The control system needs to do more than "send a signal" - what about failure modes (island mode isolator fails to open or close, same with N-E bond relay)? This is why a G99 notification is usually required from the DNO for this type of functionality. 
 It's not trivial to do, and consequences of something going wrong are either breach of legislation, or a whole host of electrical safety issues which can be lethal.

gkenyon · Answer

Akbar Ramzan said: To be honest, I will probably end up not going with the earth rod idea and just having a single RCD protected socket at the EPS output as it's a lot easier. 
 How is the socket-outlet earth maintained if the DNO are working on the system and cut the supply cable? The guidance is not to have a "floating" socket-outlet in island mode - the reasons are stated in the Code of Practice for Electrical Energy Storage Systems.

AJJewsbury · Answer

The inverter has G98 and G99 approval, 
 
 I suspect there's more to G99 than just approval for the individual items of equipment - unless it's changed significantly from the old G59 the DNO will want to come out to inspect the overall system and witness that it behaves correctly under a number of adverse conditions. As Graham said, it's all too easy to put together items of compliant equipment in a non-compliant way. 
 - Andy.

gkenyon · Answer

Akbar Ramzan said: Manufacturer's instructions allow 1 socket for a class ii item only 
 Whilst that would be OK, the issues (as stated in the IET Code of Practice) include: 
 
 The use of a standard socket-outlet permits the connection by general users of multi-way adaptors, thus enabling more equipment to be connected. 
 in the UK we don't have ' Class II socket-outlets' to prevent users connecting Class I ... and generally users are unlikely to be aware of the safety issues. 
 
 In reality, though, aren't most people wanting backup going to want to power lighting (OK could be Class II, but more than one device), internet and TV ? The router may have a "safety voltage" power supply, but there are design features of Ethernet that require a controlled discharge to earth to prevent static charging ... and this also causes "fortuitous earthing". Similarly TV etc. - quickly, you get to the point that, sadly, it's no longer a "floating" power supply for a single device.

Islanding mode earthing requirements with PME supply to inverter in garage

Community Rules & Guidelines