I am trying to square this up with a public supply, in which the LV end has either one or two conductors at earth potential, which is ensured by earthing at the transformer.

So is the output of the inverter any different from the output from a public transformer. Granted the cables are short, but they are also short if you live adjacent to a transformer.

If the PCE has an internal impedance of 5 Ω to 6 Ω, then ignoring whether this is Ze or not, how can any Zs be satisfactory?

It’s different in many ways, the main one being its very limited fault current. The inverter manufacturer calls for over current protection between the inverter and the loads, and I’ve fitted the required B curve mcb to satisfy this requirement.

There’s no way the inverter can clear this 32a breaker though, it has such a limited overload/surge capacity.

Guidance notes for testing do cover this this, but I don’t have it in front of me to remember exactly what they say.

Chapter 82, 2.6.24 says:

Supply characteristics may change between installation operating modes;

(b) The prospective fault current is likely to be far less and the effective EFLI greater, than in connected mode. 

I won’t quote it all, but basically ‘use RCDs or the overcurrent protection built into the PCE to provide protection against electric shock’. MI’s should be consulted…

If the PCE has an internal impedance of 5 Ω to 6 Ω, then ignoring whether this is Ze or not, how can any Zs be satisfactory?

You just have to accept a high Zs and design accordingly for shock protection. If using ADS then pick an appropriate device (most likely an RCD - just like for TT), or use double/reinforced insulation (likely upstream of the 1st RCD), or even rely on the characteristics of the inverter to collapse the voltage to acceptable levels in the case of a L-PE fault of negligible impedance (section 419 style).

   - Andy.

AJJewsbury said:

You just have to accept a high Zs and design accordingly for shock protection. If using ADS then pick an appropriate device (most likely an RCD - just like for TT)

Thank you, Andy. Yes, I can see the need to protect in a similar way to TT, so is Zs relevant?

AJJewsbury said:

Ze is defined as the loop impedance external to your installation - if your system is entirely self-contained (off grid) then there is no external part - so the test and concept simply don't apply. I'd record it as "N/A".

Yes, I think that sums up the situation.

Hugh Frater said:

but this will just give the impedance of the output stage of the inverter, currently around 5-6ohms. This value being a fail because the supply from the PCE to the distribution board is T-N-S and this requires a lower value.

Which is why I mentioned Zs. Is the installation really TN-S?

Hugh Frater said:

Earth rod is connected to the MET, N-E link is made in the PCE (although code of practice states it should be in the distribution board). All circuits are on 30mA rcbo’s.

Still doesn’t help with the original issue/question though.

I think that Andy has answered it. In Section I of the EICR, put "N/A" for Ze, but you must also give details of the earthing in Section J including the resistance of the rod.

With a public TT supply, you can estimate the resistance by undertaking a Ze test, which ignores the resistance of the transformer's earth. You cannot do that in your case so the rod's resistance needs to be measured properly.

Chris Pearson said:

Thank you, Andy. Yes, I can see the need to protect in a similar way to TT, so is Zs relevant?

It is relevant in TT systems also ... even if RCDs are used ... see 411.5.3 (i).

Regardless of "very high" Zs, it's still a requirement to know that the RCD will operate in the stated disconnection time. To achieve 0.1 s disconnection times, we are talking in excess of 1 k ohm for 100 and 300 mA RCDs ... but can be as low as 250 ohms Zs for 500 mA RCDs.

I don't believe, though, that for island mode there is a common standard for protections built into the inverter, so I'm not sure that you can always guarantee the RCD will operate before the inverter current limits ?

Chris Pearson said:

Thank you, Andy. Yes, I can see the need to protect in a similar way to TT, so is Zs relevant?

It is relevant in TT systems also ... even if RCDs are used ... see 411.5.3 (i).

Regardless of "very high" Zs, it's still a requirement to know that the RCD will operate in the stated disconnection time. To achieve 0.1 s disconnection times, we are talking in excess of 1 k ohm for 100 and 300 mA RCDs ... but can be as low as 250 ohms Zs for 500 mA RCDs.

I don't believe, though, that for island mode there is a common standard for protections built into the inverter, so I'm not sure that you can always guarantee the RCD will operate before the inverter current limits ?