question

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 kevgermany ♦♦ commented ·
Awesome - that is great news - thanks so much.
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sharpener avatar image sharpener kevgermany ♦♦ commented ·

Yes, I have now found it, on the first tab (General) of VEConfigure. If this setting is exceeded the inverter/charger will reduce the charging current and then if necessary invert to support the load (so long as the Power Assist box is ticked on the Inverter tab)

"Use PowerAssist to prevent an external circuit breaker to trip due to excessive load. If the load exceeds the AC input current limit the Multi will start inverting in parallel with the external AC supply and will provide the extra current needed"

However this relies on there being sufficient charge in the battery to support Power Assist. You might imagine that ESS would keep a reserve for this purpose (as for the UPS function) but I don't think it does so you would have to set up ESS accordingly. Normally (i.e. in "Optimised" mode) it will discharge the battery in preference to drawing power from the grid so by definition the battery is already empty when you need Power Assist(!)

But as you say @Taddy, you will have the cooker and shower on the grid side anyway so the 30A feed to the garage is not likely to be a big limitation. With current sensing on the meter tails the Multi would also feed the cooker/shower from the battery up to its 4kW power limit even though they are wired on the AC-In side, and a 30A circuit is enough for this. OTOH as you will need a second cable run for the AC-Out why not upgrade to two 10mm2 circuits and then get the unrestricted 50A capability of the Multi?


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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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sharpener avatar image sharpener taddy commented ·

Yes. If you can get one that is lockable it will also satisfy the requirement for a two-pole lockable isolator on the AC-In. I bought an MCB lock on amazon but it did not fit. Otherwise it will have to be a separate switch.

In terms of safety I think fitting a separate earth electrode as @WKirby says is much more important !

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taddy avatar image taddy sharpener commented ·

Thanks again. I'm just looking into the separate earthing electrode . Just wondering what the rationale is behind that? When we use mains power we are relying on the network-provided earth (at the transfomer?). So if that is ok for mains power, why is it not ok when we use the inverter in island mode? And if we are adding an earthing point then we have 2 earthing electrodes and so possibility of damaging ground loops...? Apologies if I am way off the mark.

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sharpener avatar image sharpener taddy commented ·

The supply cable might be completely disconnected from the substation (e.g. when it is being repaired or replaced). The neatest solution is to convert your installation to TT, then you can't get any earth loops and your neighbours can't put their fault currents down your earth electrode.

From the link @WKirby sent (maybe it is behind the IET paywall, I can't tell as I am logged in automatically as a Fellow):


By: EUR ING Graham Kenyon CEng MIET and Dr Andrew F Crossland CEng PhD


In island mode, an installation with EESS must comply with Regulation 21 of the ESQCR, and the PCE operates as a switched alternative to the grid. All live conductors, that is line(s) and neutral, that are to be powered in island mode must be disconnected from the grid. The installation may remain connected to the distributor’s means of earthing (where this is provided).

Earthing arrangements for island mode operation

In connected mode, an installation with a TN earthing arrangement (TN-C-S or TN-S) may use the distributor’s means of earthing. In a TT system, the consumer’s earth electrode is used – but care needs to be taken to ensure that this provides an earth of sufficient quality.

However, when the installation moves to island mode, it is important to make special earthing provisions.

Regulation 551.4.3.2.1 of BS 7671 states that, in TN systems, generators operating as a switched alternative to the public supply cannot continue to rely on the distributor’s means of earthing.

551.4.3.2.1 pop out box

In systems where a low voltage supply is provided to the installation, the neutral of the supply is earthed at the distributor’s transformer.

Accordingly, in systems operating in island mode, the distributor’s neutral-earth link cannot and must not be relied upon, as this is switched out when the live conductors are disconnected.>>>

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taddy avatar image taddy sharpener commented ·
Hey thanks so much. Makes sense. Any tips on a good place to get hold of grounding rods + clamps + box etc?
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sharpener avatar image sharpener taddy commented ·

Most decent electrical wholesalers stock them e.g. TLC or CEF, I use both from time to time, not much to choose between them. Testing it does not exceed the maximum allowable impedance (200 ohms) is not straightforward.


BTW you do know that if you are self-installing all this you still need to make a G98/G99 application to your Distribution Network Operator? They may like mine (WPD) want you to limit the inverter power to 3.68kW anyway which might affect your calculations.


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taddy avatar image taddy sharpener commented ·
Thanks again. Yes, I'm about to do that. Still mapping out the project atm.
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