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taddy avatar image
taddy asked

AC on the inverter charger - post RCD

Hi everyone,

Writing from the UK. I have a multiplus II 5000/70 inveter charger. Is it ok to have the AC IN feed coming after an RCD (specifically an RCCB) as I was going to have the AC IN coming from my exisiting consumer unit - so MAIN SWITCH, RCD, 30 or 50A fuse then AC IN. I saw a video where someone was saying that this wouldn't work although he didn't explain why! The guy said you need to wire a separate line from the meter of the house. I have read wiring unlimited and the manual for the multiplus but can't seem to get a specific answer to this.

My plan is to have the lower power circuits in the house running off the solar system but keep the cooker, shower etc running off the mains feed and use the mains feed to charge the solar batteries when needed. I have a circuit diagram/plan if it will help.

Thanks in anticipation,

Tom, UK


MultiPlus Quattro Inverter Charger
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3 Answers
wkirby avatar image
wkirby answered ·

The trouble with an RCD ahead of the MultiPlus is that when it is in passthrough or connected mode, then you have two RCDs in series for a given circuit downstream of the MultiPlus. This poses a selectivity problem, the upstrean RCB may respond before the downstream one.
If the upstream one responds first, the AC input to the inverter will be disconnected, but then the inverter will run and the NE relay will need to close also, afterwhich the downstram RCD or RCBO will respond. This delay may exceed the allowable time for an RCD to disconnect. In addition there is the disruption to upstream circuits.
What does your electrician say about the arrangment?

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taddy avatar image taddy commented ·
So the scenario is you have the upstream RCD cutting off. But the inverter kicks in closing the N-E link and then downstream RCD kicks in but by this time the power has hypothetically been on longer than it should...? Have I got that right?
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taddy avatar image taddy commented ·
Thanks so much for the response and explanation BTW
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sharpener avatar image
sharpener answered ·

AFAIR it is a requirement in the IET Code of Practice for EESS for there to be an RCD upstream of the inverter to protect against an earth leakage fault in it.

As it happens I originally had a 100mA RCD in my house, so I have re-used that as the upstream device and fitted a new 30mA one to my distribution board. This will give some discrimination for earth leakage.

For overload protection you will need a 50A (not 30A) device upstream of the inverter, as this will allow the maximum passthrough current to er.. pass through. Ideally a Type C as this will give discrimination in overload and cope with the surge capability of the Multi.

Of course these devices could be combined in one RCBO but I don't think they are available in the required ratings.

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taddy avatar image taddy commented ·
Hi, Thanks so much. WRT the upstream mcb, isn't it possible to change the settings on the 5000/70 to limit the input AC current to to 30A. The reason I am asking is that I haven't quite decided on the location for my equipment and might initially put in the garage which only has a 30A MCB and related cable supply.
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taddy avatar image
taddy answered ·

Would the attached wiring work? P1 is the wiring the system, p2 is a possible consumer unit wiring but without a RCD upstream of the inverter charger.

..Wiring plus poss consumer unit layout.pdf


wiring-plus-poss-consumer-unit-layout.pdf (139.6 KiB)
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wkirby avatar image wkirby ♦♦ commented ·

Your diagram does not highlight any earth electrodes, you'll need these too.

In island mode you cannot rely on the earth provided by the network.
Check here for more details: https://electrical.theiet.org/wiring-matters/years/2021/84-march-2021/island-mode-earthing-arrangements-new-guidance-in-the-second-edition-of-the-iet-code-of-practice-on-electrical-energy-storage-systems/

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taddy avatar image taddy wkirby ♦♦ commented ·
Thank you - that is very helpful
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sharpener avatar image sharpener commented ·

Well it will work but it is not compliant with the IET CoP.


To answer your question about current limits, I don't think so. I have looked in the ESS settings and cannot find anything appropriate. You can limit inverter power and charge/discharge current but that will not do what you want. It may be there somewhere or in another Assistant but IIUC the inverter circuitry cannot control the passthrough power.



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kevgermany avatar image kevgermany ♦♦ sharpener commented ·
There is in input current limit setting. It's not part of ESS, but the basic settings.
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taddy avatar image taddy sharpener commented ·
Thanks Sharpener. So if I get a 100mA RCD for upstream like yourself that should be compliant?
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