Posted by: @transparent

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Posted by: @transparent

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Posted by: @transparent

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MCBs for AC circuits are normally OK to use bi-directionally, but the trip-time might not be within spec.

Is this an issue and if so when?  As I understand it MCBs protect the cable only and functionally replace fuses, so the trip time is presumably not a particular issue so long as a cable is unlikely to set fire during that time.  Does it specifically relate to plug in solar or is this a separate observation

 

 

Well it's enough of an issue that both B-curve and C-curve MCBs are readily available.

The choice of the trip-time curve isn't related to the cable, but rather to the apparatus being supplied on that circuit.

I have C-curve protective devices on circuits where there are (large) motors. I don't want the trip to operate merely due to switch-on surges and back-EMF. The well-pump and the (mechanical) workshop are my best examples. The workshop has a fixed table saw, radial-arm saw and a TIG-welder.

OK thanks thats clear now, but not an issue that particularly relates to plug in solar then and not particularly a safety issue, more an issue of functionality in relation to the equipment requirements.

The key learning from this exchange, at least for me, is that the 'bidirectional issue' applies only to RCBOs not RCDs.  This is different, so far as I read it, to what BEEMA imply (but do not expressly state), but not so different from what the IET says, albeit that I don't think either of these is entirely clear!. 

Considering its a safety issue its rather disappointing that neither the IET nor BEEMA are clear (at least to my reading) and that it needed you to clarify it.  For reference (because someone is bound otherwise to challenge it) are you able to quote your source or rationale for the conclusion?

Posted by: @transparent

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I might create some more diagrams if that would help others to better understand what we're discussing.

I think visual aids would definitely help here!

@editor would Indevolt like to comment on the shut down time of their inverter in the case of mains failure/very severe undervoltage and/or the risk to unidirectional RCBOs.  If they can tell us its <30ms then I cant personally see that there is any risk, unless someone can explain otherwise. 

Posted by: @jamespa

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For reference (because someone is bound otherwise to challenge it) are you able to quote your source or rationale for the conclusion?

Not easily.

Almost 5-years ago I had an email exchange with a couple of Chinese trip manufacturers.

That was instigated by me needing to understand how DC-approved MCBs operated. But their explanations also included details of the the thermal and electro-magnetic overload detection mechanisms, which was helpful.

Then in April/May '24 I was communicating with a senior manager at Proteus. They're a UK-based trip manufacturer who were already selling bi-directional RCBOs well before BEAMA alerted the IET of the problem with solenoid coils burning out.

Thus I have a great deal of respect for Proteus, and would certainly buy their products in future.

Although I went on to explore how RCBO's initiated the firing of the solenoid, this got complex. Each manufacturer has their own electronic circuit, and aren't wanting to openly state how it operates.

I was merely able to conclude that the electronics could stay 'live' in a uni-directional RCBO once the main contacts had opened. There's no contact which opens to remove power from that internal pcb.

The overall message is "Be prepared to talk directly with a manufacturer".

Posted by: @jamespa

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@editor would Indevolt like to comment on the shut down time of their inverter in the case of mains failure/very severe undervoltage and/or the risk to unidirectional RCBOs.  If they can tell us its <30ms then I cant personally see that there is any risk, unless someone can explain otherwise. 

At @indevolt-uk, when you get a moment, please can you clarify this?

Posted by: @transparent

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Posted by: @jamespa

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For reference (because someone is bound otherwise to challenge it) are you able to quote your source or rationale for the conclusion?

Not easily.

Almost 5-years ago I had an email exchange with a couple of Chinese trip manufacturers.

That was instigated by me needing to understand how DC-approved MCBs operated. But their explanations also included details of the the thermal and electro-magnetic overload detection mechanisms, which was helpful.

 

Then in April/May '24 I was communicating with a senior manager at Proteus. They're a UK-based trip manufacturer who were already selling bi-directional RCBOs well before BEAMA alerted the IET of the problem with solenoid coils burning out.

Thus I have a great deal of respect for Proteus, and would certainly buy their products in future.

 

Although I went on to explore how RCBO's initiated the firing of the solenoid, this got complex. Each manufacturer has their own electronic circuit, and aren't wanting to openly state how it operates.

I was merely able to conclude that the electronics could stay 'live' in a uni-directional RCBO once the main contacts had opened. There's no contact which opens to remove power from that internal pcb.

 

The overall message is "Be prepared to talk directly with a manufacturer".

That all makes sense thanks, albeit that it is not exactly a quotable reference which is no fault of yours. 

Presumably somewhere in this you also spoke with them about RCDs and concluded that the trip circuits for these are different to RCBOs and thus immune from the problem.

IET (unlike BEMA) seem to be fairly sure that two module RCDs are immune, but less clear about single module RCDs.  Its all a bit messy really and has an air of manufacturers hiding the facts possibly even to sell more kit!

Posted by: @jamespa

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As I understand it the part that would fail is a solenoid coil and the failure mechanism is getting too hot because its energised for too long.

If the inverter cuts out in 30ms or less then I cant see it can possibly fail, because that's the disconnect time in a fault condition and the coil must obviously to survive that otherwise its not fit for its original purpose.  I would guess that it must survive this by a fair margin.

So, it seems to me, that an inverter that doesn't support islanding and disconnects on grid failure in a time comparable to 30ms poses (probably) zero risk.

I dont actually know what the required or actual disconnect time is but funding that out might be the next step.

 

PS.  It looks like the required disconnection time depends on the grid state, up to 20s for under-frequency, much less for other faults.  There are also specs for minimum time before disconnect in the case of underfrequency (presumably to accommodate grid frequency fluctuations).  

Obviously it takes time to detect a small under (or over)-frequency (you need several cycles), so this makes sense.  However one could detect serious undervoltage/underfrequency (ie a power cut) outside normal grid limits much more quickly (certainly within 40ms).   I cant immediately think of a reason why an inverter couldn't do this, whether any particular inverter does is another matter.

OK so thinking about this further, a compliant plug in inverter (as distinct from one that is permanently wired) must shut down on loss of mains within a very short period of time like 30ms because, if it didn't, someone pulling the plug could get a shock from the exposed pins.

This being the case I would venture to suggest that the potential failure mode of a unidirectional RCBO which is being discussed in this thread does not apply to plug in inverters!  Obviously if the shut down mechanism fails then it could, but in that case anyone pulling the plug is at immediate risk of electrocution, so there are bigger fish to fry in safety terms if that is at all likely.

I invite @transparent, @mars, @indevolt-uk to comment.

@editor Thank you for raising the question.

Our inverters are all designed and developed in strict compliance with local grid-connection regulations, and are equipped as standard with compliant anti-islanding protection.

Regarding the conditions you mentioned, such as a mains power outage or severe undervoltage, these will not cause impact, nuisance tripping, or any long-term safety hazard to the upstream single-direction RCBO. Under normal and compliant installation and use, there is no related safety risk in these scenarios.

In addition, our equipment is fitted with an EPS emergency power supply switching function, which enables an ultra-fast switching response of 10 ms.

Thanks @indevolt-uk… that’s a really useful clarification, and the 10ms EPS switching figure is the number that I think @jamespa was looking for.

To pick up on James’ logic, which I think is sound, any plug-in inverter that shuts down in 10ms is well inside the window that probably matters most here from a safety perspective.

The concern around unidirectional RCBOs appears to be sustained reverse current energising a solenoid coil for long enough to cause thermal damage. At 10ms, that scenario shouldn’t arise because the inverter is off before the RCBO is even aware there’s a problem.

What this exchange has actually confirmed, and it’s worth flagging for anyone reading this thread later, is that the RCBO bidirectionality question is more nuanced than BEAMA’s guidance makes it appear. The risk is real in some configurations (particularly with permanently wired, non-compliant or islanding-capable inverters), but for a plug-in device with fast anti-islanding shutdown, the practical risk appears to be negligible. It’s a remarkably complex subject.

That said, I’d still encourage anyone installing any battery or solar kit to specify bidirectional RCBOs as standard. The cost difference is marginal and it removes the question entirely.

@transparent’s point about going direct to manufacturers when the guidance is unclear is also well taken.

Thanks @editor  , @indevolt-uk Subject to any comments from @transparent I think we have made some great progress in understanding this issue.

Regarding functionality, I note that there is no current transformer/energy meter included in the standard pack but there is reference to an Indevolt energy meter and a Shelley EM.  Obviously such a device would be needed if you want to try to balance house load for (eg) zero import/export for a period during the day.  I have read the manual to try to understand what it can do and my question is

With the addition of either a Shelly EM or the indevolt meter, is it possible to set it up to

1. discharge at a sufficient rate just to balance house demand (or net demand if there is a totally separate PV system AC coupled in via the consumer unit) - obviously only within the max discharge capability of the unit and whilst there is charge left
2. charge at a sufficient rate just to balance excess generation (if there is a totally separate PV system AC coupled in via the consumer unit) - obviously only within the max charge capability of the unit and whilst there is battery capacity left
3. do either (1) or (2) as appropriate, with the unit deciding real time whether to charge or discharge and at what rate
4. Force charge/discharge at a specific time of day whilst doing 1/2/3 at other times of day

I think that all of this functionality is described in the app manual but cant be sure without a device in front of me because its all in the app which presumably doesnt work unless there is a battery and energy meter connected