craigc avatar image
craigc asked

Do I need RCD protection on AC-In on Multiplus-II (UK based)


I'm planning my installation of a Multiplus-II 5000/70/50 in my home in the UK along with ESS and later solar. I will be splitting my existing consumer unit into essential and non-essential loads to enable UPS mode in the event of a grid outage.

My question is whether the circuit protection device fitted inside the non-essential loads consumer unit which feeds the Multiplus-II AC-In input needs to have RCD protection or if an MCB can be used. Of course I will have a full set of RCBOs on the various other circuits on both essential and non-essential consumer units but I'm concerned about discrimination if I was to also use an RCBO to feed the Multiplus-II input. I could probably get a time-delayed RCBO but I wonder if this is really needed.

The Multiplus-II installation manual simply states that

The AC input must be protected by a fuse or magnetic circuit breaker rated at 32 A (for 3 kVA model), 50 A (for 5 kVA model) and 100 A (for 8 kVA and 10 kVA model) or less, and cable cross-section must be sized accordingly.

so my guess is that this might not be so much an inverter product question but more of a UK wiring regs question.

I have the regs and on-site guide so can read up on that but just wondering what others have done? The cable feeding Ac-In will be 10mm2 (singles ideally but T&E possibly) and will be running a short distance through 75x75mm trunking to the AC isolator and then again in the same trunking to the Multiplus-II so risk of damage to the cable is minimal. I could use SWA but that would be cumbersome and probably overkill.

Any thoughts? My feeling is that using a 50A MCB is the pragmatic choice but I'll need to check the regs to see if the short run of non RCD protected cables in trunking is a problem.

Thanks, Craig.

Update: Checked the OSG in section 7.2.5 and as far as understand it, as long as the cables are not “installed in walls or partitions”, which surface mounted trunking isn’t, there is no requirement for RCD protection. The prospective fault current in a fault condition inside the Multiplus would easily cause disconnection of the 50A MCB inside the allowed time so I think an MCB is appropriate. Anyone aware of any other regs which contradict this or am I misunderstanding anything?

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1 Answer
sharpener avatar image
sharpener answered ·

AFAIR an RCD (and a lockable 2-pole isolator) on the feed to the inverter is a specific requirement of the IET Code of Practice for EESS and it should not be shared with final circuits so it needs to be a dedicated RCD.

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craigc avatar image craigc commented ·
Hi - thanks for responding.

I have the lockable 2-pole (4-pole actually) isolator and it’s an RCBO board so no sharing with final circuits but does it need to be an RCD or is it just overload protection (eg. MCB) that is required? I’ll see if I can find the reference you mention.

Many thanks, Craig.

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craigc avatar image craigc commented ·
@sharpener regarding the code of practice for EESS, is this a publication that you have access to and if so, would you recommend it for someone like me, a graduate in electronics & electrical eng. who is planning and building his own ESS system? And if the answer to that is yes then what is the cheapest way to buy it? I think it's about £72 on the IET website but just wondering if there is another way to buy it, perhaps as part of a package or subscription. Thanks
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sharpener avatar image sharpener craigc commented ·

It's definitely an RCD it specifies, to catch earth-leakage faults inside the inverter IIRC, so an RCBO would also be acceptable. Mine is separate as I had a spare 100mA one and this gives some discrimination for faults on the final circuits. As someone (?@wkirby?) has pointed out, you don't want a fault there to trip the inverter off (perhaps more of a problem with clearing overcurrent faults so ideally the RCBO would be a Curve C).

I borrowed the CoP from the IET library, the current (2nd) edition was out on loan but they sent the 1st edition meanwhile, there is not a great deal of difference. Maybe given your quals it would be cost-effective to join the IET, there is a discount for members. Or you could try asking your local public library if they can get it.

IMHO a big limitation is that none of the examples in the installation diagrams shows the type of inverter that has separate input and output ports (even though there was a Victron rep on the list of consultees). This makes interpretation of the requirements for Victron systems non-trivial. But it is quite good at explaining the rationale for the various requirements.

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craigc avatar image craigc sharpener commented ·

Thanks - very much appreciated. I'll look into the cost to join the IET as you suggest or maybe I'll just buy the CoP. I bought the 7671 blue book and OSG although it's mostly the OSG that I find useful which I guess is no surprise.

I'll look into what protection device options I can get from FuseBox. IIRC they didn't have a compact style 30ma RCBO as large as 50A but my DB can accept the larger style RCBOs if necessary. But I definitely want some discrimination because I don't want an earth leakage somewhere down the line on say a kitchen toaster causing my Multiplus AC-In to disconnect before the RCBO on the kitchen ring final circuit. I guess I could install a 100mA RCD unit in a separate box if need be.

In general this is something I'm acutely aware of - I don't want the addition of UPS-mode on my ESS system to actually make power failure MORE likely than they would be if I just relied on the grid at all times. We live in the countryside with an 11kV transformer outside our house just for us and we probably get power cuts 3 or 4 times a year so I think on balance we can improve that reliability with an ESS in UPS mode. But if I get details such as the RCD discrimination wrong then all bets are off!

Thanks again.

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