This photo shows a bi-directional RCBO by Proteus at Redditch.

The symbols are correct, and the third-contact which the solonoid switches is visible in the diagram on the side.

What I still don't like is:

• the terminals being labelled 'In' and 'Out'
• no bi-directional symbol or marking on the part of the RCBO which remains visible to the householder after installation is completed

This particular RCBO is Type A

and Curve C... which defines the time it takes to trip due to over-current.
Most household MCBs and RCBOs are curve-B, which operates faster.

I use Curve-C trips to supply circuits which are likely to have large inductances connected (motors in a workshop or an inverter).

Update 28may24:

I have a response from Proteus Switchgear to say that they are already acting on the BEAMA directive and "will remove the in/out and also add on a bidirectional arrow symbol to the front of compliant devices"

The changes will filter through once existing stocks have been exhausted.

I had a read and I still cannot understand the below. We are talking about the direction of power flows. An alternating current changes direction 100 times a second. So the issue cannot be about the direction of the current. 

But if there is power still being applied in the reverse direction by an inverter or storage battery, when the contacts open, the solenoid remains energised.

As it is about DC currents, why not describe it as such?

And it does seem thar this Beama originated confusion was related to a British electrical labelling standard? 

See matching discussion here in the topic about Plug'n'Play Balcony Solar kits.

Thanks for the clear explanation @transparent.  I think I agree with two sides of this issue:

There is a risk that when a unidirectional rcbo/rcd trips out or is flicked off with an inverter connected and running, that inverter may continue to backfeed for long enough to destroy the rcd solenoid inside the rcbo rendering it non functional as a protective device.  It also would be odd if this prevents the engagement of the wider public with solar. 

Risk management is standard practice in such circumstances - eg: we're happy to use non earthed power supplies in our home, simply by using higher levels of insulation internally - all those usb chargers, laptop supplies, so many things!  These are still sold to people for use without an RCD.  There must be some way that an inverter can pose an acceptable risk to a unidirectional rcd/rcbo, and if necessary some new standard imposed for UK legal balcony solar - whilst simultaneously phasing out unidirectional devices.  For example, a new standard mandating the power down on disconnect to be much faster than the present 0.5s.  Of course the inverter could be faulty; this feature could be 100% tested.  A fault could develop - so could a fault with the insulation I mentioned above.

I had a look for bidirectional rcbos, and they don't seem to be obbviously more expensive, screwfix have them for £17 ish.  Other distributers are available:-)

We have an EV charger, which has an expensive type B rcd which can trip in the case of dc leakage, which normal rcd/rcbo's may not.  I think that most power electronics devices (solar inverters, inverter heatpumps...) are recommended to have type B.  I think type B are £200 ish, probably this reduces adoption!

Posted by: @robl

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There is a risk that when a unidirectional rcbo/rcd trips out or is flicked off with an inverter connected and running, that inverter may continue to backfeed for long enough to destroy the rcd solenoid inside the rcbo rendering it non functional as a protective device. 

Are you saying that an RCD solenoid would burn only when a current is not flowing through it? So (and I am taking the risk of making the options up):

- if inverter power is off, it would not burn

- if inverter power is fed from the load side, and not used from grid side, it burns

- if inverter power is fed from the load side, and used from the grid side, it does not burn 

So in this latter case, how much power would need to be drawn from the grid side in order for it to not burn? 

It would be great if we could access a single test report for a product version with such a risk.  

Whether power flows in one or the reverse direction does not change the physics that an AC current changes direction 100 times a second... 

@batpred

Below is a simplified problematic RCD circuit I ‘found’.  The trigger is the difference between L and N currents, which is then amplified using a power supply from the output, which remains possible for 1/2s after switch off so could burn out the coil.  I imagine speed is crucial, so it can’t drive anywhere near continuously.  I must admit it looks like it should only cause the coil to fail if there is an actual RCD trip current AND the inverter power is on, rather than the more general case I mentioned before, apols.

Posted by: @robl

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For example, a new standard mandating the power down on disconnect to be much faster than the present 0.5s.  Of course the inverter could be faulty;

And that's the nub of the problem, isn't it?

Even if such a Balcony Solar Standard were imposed, I'm not convinced that the IET would accept it.

The Wiring Regulations are valid based on 'ignorance' of the appliances which get connected.
The assumption is "safety first", and that means the rules are there to cope with an appliance which is not complying with a 'standard' as it should.

Posted by: @batpred

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- if inverter power is fed from the load side, and used from the grid side, it does not burn 

So in this latter case, how much power would need to be drawn from the grid side in order for it to not burn? 

Please recheck the schematic of the bi-directional RCBO at the top of this page.

The issue has nothing to do with the amount of power being drawn, but what happens when the contacts have opened in response to an earth-leakage fault.

@transparent I do like the safety first precautionary approach.  We could do with it in other aspects of life (PFC's, CO2 emissions...).  I trust a balanced approach will be taken of risk versus PV adpotion.  In my experience, once an individual is engaged in green topics - be it PV, EV, a heatpump, diy loft insulation - they've started on a journey that keeps going.  I think individuals are more engaged if hands on, rather than paying someone - so I really hope this is all possible somehow, even though I myself am unlikely to do it even if available.  

I don't think expensive type B (DC capable) devices are very common*, and all the usual cheap parts can be "blinded" by a DC leakage path which is very concievable with earthed PV panels, even if bidirectional.  As mentioned before, you can use non-earthed devices without an RCD at all - it's easy to see this causing a serious problem that's only "discovered" when charging your iphone while touching a radiator.

*actually - I just googled it, and I think they are now mandated.  I'm guessing most installs avoid the whole issue by having MCB only for solar/battery/heatpump installs.  

@robl

Thanks, that makes sense. The issue would occur just after the RCD trips (probably depending on the coil material, etc), in case the inverter is still providing power. We should bear in mind that all inverters approved for domestic use have anti-islanding, so when they detect a loss of grid, they should stop providing power within a certain time. I deduct it could also depend on how quickly the inverter stops providing power. And it would also be obvious that the RCD is faulty whenever the RCD test button is pressed. 

As the real risk depends on the coil material and heat dissipation and the inverter timings involved, I can only hope that the IET made the decision after careful risk analysis of the equipment combinations (RCD and inverter) that had been historically deployed at the time. And how the risk would evolve. 

So the IET officially named these as "bi-directional" RCDs which essentially requires that all installations use them and so are able to cope with an inverter where anti-islanding is not active. This also has the effect of allowing inverters with that type of fault to continue to work undetected (most would as they are probably on MCBs). I am glad I do not have any role in advising government in these matters! 

Posted by: @robl

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*actually - I just googled it, and I think they are now mandated.  I'm guessing most installs avoid the whole issue by having MCB only for solar/battery/heatpump installs.  

Yes, for sites with multi generation sources that seems to be the case. Thankfully all MCBs are bi-directional! 

I assume heatpumps will be fine on an RCD...

Posted by: @robl

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In my experience, once an individual is engaged in green topics - [...] - they've started on a journey that keeps going.  I think individuals are more engaged if hands on, rather than paying someone

So that again comes back to the Public leading the way, rather than having Net Zero strategies imposed on them...

... in which case we're going to need

• more hands-on community energy groups
• a nationwide Energy Knowledge dissemination program

There are a number of reputable manufacturers in China who offer Type-B RCDs

Geya, Chint and Tongou are worthwhile considering.

I haven't yet found a Chinese manufacturer of bi-directional RCBO units.
But at least their uni-directional ones are properly labelled for 'load' and 'supply'.