Who from the IET is working with the government to allow <800W inverters to be 'plugged in'?

In respect of BSI committee work (BS 7671 is developed by a committee jointly managed by the IET and BSI, but that follows the BSI rules for developing standards) specific discussions in committees cannot be shared. Similarly, content of standards and draft standards can only be discussed when they are published, or made available as drafts for public comment.

Would those rules prevent someone confirming whether a committee (or indeed which sub committee) had been asked to consider a particular request or requirement?

The comments on RCD operation seem somewhat unclear to me. Mark started off taking about a certain type of RCD that the Germans had the foresight to drop many years ago (which I take to be AC types) - but the article says "But some older RCDs were never designed for electricity flowing back into the circuit" - but bi-directionality is a completely different feature - and not one universally found even in the latest RCDs. I suppose we should add in single pole switching RCDs (mostly RCBOs) where the local generation isn't separated from Earth when the RCD trips, so the shock hazard persists until inverter shutdown - again a problem that isn't mitigated by ensuring your installation is up to current standards.

   - Andy.

AJJewsbury said:

Mark started off taking about a certain type of RCD that the Germans had the foresight to drop many years ago (which I take to be AC types)

BS 4293? ;-)

Even if you have an RCBO, what happens if somebody touches the line conductor of the same final circuit when it is exporting? If it is not bi-directional, the imbalance between L & N will not necessarily be sensed.

It will trip from an earth fault there there if the only NE bond is the one at the incoming main - I agree it won't if there is also an NE bond at the inverter.  So long as the inverter has anti-islanding that cuts off its output when the connection to the grid is lost, the bidirectional thing is moot, or at least no worse than a spinning down motor that carries on generating for a bit after the supply has gone.

Mike

what happens if somebody touches the line conductor of the same final circuit when it is exporting? If it is not bi-directional, the imbalance between L & N will not necessarily be sensed.

Why shouldn't it see the fault just the same? With a N-PE link only on the grid side a L-earth fault on the load/local-gen side there will be an imbalance just the same - only difference is that the N coil will carry the "extra" rather than the L. 

We seemed to survive for many years with fixed-wired PV systems designed where "the public supply shall be considered the source as PV installation shall be considered the load" without any noticeable piles of bodies. The main difference with bi-directional RCDs that I've found so far is that they have an extra contact to disconnect the test button - so resistor can't burn out if the button is held in after the device has tripped.

   - Andy.

AJJewsbury said:

Would those rules prevent someone confirming whether a committee (or indeed which sub committee) had been asked to consider a particular request or requirement?

From what is published in past international standards, and national standards up to and including the current BS 7671, it is easy to see that standards committees involved in national and international standardisation for electrical installations have been talking about the possibility of plug-in solar for well in excess of 10 years.

AJJewsbury said:

The comments on RCD operation seem somewhat unclear to me.

This is not easy.

The 'Type AC vs Type A' is a long-standing discussion.

The 'bidirectional vs unidirectional' is far more recent, and itself has further complexities.

Chris Pearson said:

BS 4293? ;-)

Not specifically, Type AC, but the debate might have started with international standardization on RCDs prior to the introduction of BS EN 61008 series and BS EN 61009 series.

A key thing about BS 4293 series, and earlier versions of BS 7288, is that they never included requirements for EMC testing, yet at the same time never prohibited electronic circuits from the detection and actuation paths.

AJJewsbury said:

Why shouldn't it see the fault just the same? With a N-PE link only on the grid side a L-earth fault on the load/local-gen side there will be an imbalance just the same - only difference is that the N coil will carry the "extra" rather than the L.

gkenyon said:

The 'bidirectional vs unidirectional' is far more recent, and itself has further complexities.

I don't know, but Amendment 3 was made for a reason.

If an RCD is unidirectional, it may well trip in both directions, but it is not certified to do so.

In an all RCBO board, all the other circuits will be fine irrespective of whether the source is the mains or solar, but that does not apply to the circuit to which the solar is plugged in.

I wonder how HMG will deal with this issue. Perhaps they will simply say that balcony solar is out of scope of BS 7671 because it is not part of the fixed wiring?

Chris Pearson said:

If an RCD is unidirectional, it may well trip in both directions, but it is not certified to do so.

Agreed, and because protective devices have been marked for many years (started with older British Standards prior to the ENs we use today), isn't it the installer's 'fault' if the connection is incorrect, or an inappropriate device is selected for a particular application?

However, if there's confusion in the industry, we need to address that.

Chris Pearson said:

I wonder how HMG will deal with this issue. Perhaps they will simply say that balcony solar is out of scope of BS 7671 because it is not part of the fixed wiring?

Well, that's my personal perspective on this pretty much summed up. BS 7671 doesn't tell consumers what they can plug into a socket-outlet. That's what the Electrical Equipment (Safety) Regulations is for.

At present, there isn't a designated standard for the products, so we can't really develop a 'use case' in BS 7671 with any additional provisions that may be necessary.

And any designated standard would have to address the fact that the products must meet the essential requirements of the Electrical Equipment (Safety) Regulations, including items (b) and (d) on that list, i.e. not generate unsuitable temperatures and consider the prevailing conditions (i.e. existing wiring practices in the UK).

So, if we will not be able to rely upon the presence of bi-directional RCDs (or any at all) in the fixed wiring, the balcony system will need one in its output.

So, if we will not be able to rely upon the presence of bi-directional RCDs (or any at all) in the fixed wiring, the balcony system will need one in its output.

But it won't have one.

What it will do is shut off immediately if the mains supply is disconnected. Which means that a non-bidirectional RCD won't burn out.

The whole bidirectional RCD thing is trying to solve a problem that in practice doesn't actually exist.

the balcony system will need one in its output.

But without a N-PE link at the balcony system (which would be prohibited for parallel operation with the mains) - an RCD at that end would never "see" the imbalance - so never trip.

   - Andy.

Simon Barker said:

The whole bidirectional RCD thing is trying to solve a problem that in practice doesn't actually exist.

AJJewsbury said:

But without a N-PE link at the balcony system (which would be prohibited for parallel operation with the mains) - an RCD at that end would never "see" the imbalance - so never trip.

What about island mode arrangements, where the inverter is self-commutating and is re-energized once the grid has been disconnected by the island mode isolator (now termed switching device for islanding, SDFI) and a system referencing connection made by the system referencing switch (now called SRCSD or system referencing conductor switching device)? 

What about island mode arrangements

I don't think anyone's suggesting an 800W plug-in balcony system should be capable of island operation (at least not when feeding back into the main "plug" - it would need upstream isolation from the grid - which would defat the easy plug & play aspect).

   - Andy.

AJJewsbury said:

I don't think anyone's suggesting an 800W plug-in balcony system should be capable of island operation (at least not when feeding back into the main "plug" - it would need upstream isolation from the grid - which would defat the easy plug & play aspect).

If we are keeping this purely to even if there are no other inverters, and we are looking at the plug-in solar debate, the power to the final circuit might be disconnected by the operation of the RCD because of a faulty appliance on that circuit ... mains loss protection is not instantaneous, as in 'zero time'.

G98/G99 has time delays for under-voltage and under-frequency, that are at least 500 ms (longer ... nearly twice as long ... as the disconnection time of the RCD). RoCoF doesn't have a time delay, but would still need some time to detect the phenomenon (somewhere between 100 and 500 ms).


So, I believe it still leaves the unidirectional vs bidirectional to be considered; I'd be happy for testing to demonstrate to me it's not an issue.

I accept that, if the RCD disconnects the neutral, the shock risk isn't increased as the disconnection time of the RCD isn't increased.

BUT

Many RCBOs are single-pole with unswitched neutral, so the presence of plug-in PV has effectively increased the RCBO disconnection time from 40 ms to at least 140 ms.

So ... split-load boards may well be safer for plug-in PV than single-pole RCBO boards !

Below from G98 Issue 2 2025:

G98 is not an appropriate standard for plug in solar so quoting is doesn't really help (as identified at the start of this thread).  In particular, G98 is built around hard wired equipment with a degree of tolerance to voltage / frequency fluctuation, with a reasonably long delay in ceasing generation so as to avoid unfortunate events where the grid might suddenly lose a significant chunk of generation at a national level because all the solar systems shut down.

With plug in solar, the trade-offs have to be different.  Plug in solar, by its nature, can be unplugged and the risk presented by a plug remaining live (or a connected circuit remaining live after a RCBO has tripped) for more than a few milliseconds is intolerable.  Plug in solar will either need a separate standard akin to G98, or a significant update to it.  Of course, plug in solar will as a result be an inherently less stable source of generation (more accurately load reduction!) for the grid than "normal" solar and that is something that those who operate the grid will need to buy into if plug in solar is expected to achieve significant scale.

As I noted in one of the other threads on this subject, there is a class of devices (inverter / chargers often used on road vehicles / smaller boats, operating in parallel with a shore supply) which have much commonality in terms of shutdown requirements with plug in solar - I don't know what standards they adhere to (I suspect there may be none applicable with respect to loss of mains detection) but whatever approach is taken there seems to work and perhaps might work for plug in solar.  I suspect that "line interactive" computer UPSs also need to detect loss of mains in order to avoid their plugs being live on loss so there might be some standardisation or learning in that area too.

An equipment standard on the subject of fast loss of mains detection and disconnection of plug in equipment connected to a separate power source would be a sensible thing to exist if it doesn't already given the various classes of device which might benefit from such a standard.

the trade-offs have to be different.

I agree entirely. The Germans felt it necessary to have their own standards as installation conditions vary across nations. As I recall Germany tends to use TN throughout, whereas we have a mixture of TN and TT installations - so while compromise of the effectiveness of RCDs might "only" result in loss/reduction in additional protection there, here it might mean basic ADS is compromised. The Germans have decided on somewhat different overall parameters too (e.g. the overall rating) so there's no precedent to comply with a single common standard (at least not in the short term - longer term no doubt a common denominator will emerge).

I think we should keep our minds open to other means of achieving similar ends too - e.g. if it were to be decided that the last major stumbling block was the loss of additional protection to other outdoor equipment fed by the same final circuit (e.g. cut lawnmover lead) then consumer advice to revert to using plug-in RCDs for their garden equipment (as they would in days past if the sockets weren't 30mA RCD protected) might be an adequate compromise.

But if on the other hand we decided that the only practical way forward was for inverter shut-down time had to be so fast (on top of typical RCD actual opening times) that neither ADS nor additional protection was significantly compromised, then that's what we should stipulate. (Cue others to point out technical limitations!). We don't necessary have to be limited to what has already been produced.

From the German paper showing waveforms on unplugging:

it seems that the current drops to zero just about instantaneously (green line) - perhaps a signal such as that could be used to trigger a much more prompt shutdown (or at least disconnection from the mains side).

   - Andy.

(edited to fix diagram)

It would not be hard though to add one line about plug in units  to the annex of G98 that covers loss of mains,  and make it the same as the standard for domestic and commercial equipment with a plug on it (EN IEC 62368-1:2024)  where no more  specialized standard applies.

That would require that pins must not remain "hazardous live" above ES1 limits for longer than 2 seconds after the mains plug is withdrawn.

Something like

'units connected by a removable plug with accessible pins shall meet the residual voltage requirements of EN 6236801, and residual voltage be below ES1 limits within 2 seconds of removal'

Now I've typed than in a few seconds at a cost of pence, so realistically, a joint govt and experts committee should be able to manage something similar in a few months and at a cost of millions.;-) 

In this context, depending what voltages appeared, those ES1 limits could require the voltage to fall below 60V DC or 30V RMS AC, if the available short circuit current at that time was more than 2mA DC or 0.5mA RMS AC respectively.

This long standing requirement is one reason we have bleed resistors on filter capacitors and brakes on very large motors that otherwise generate while spinning down.

That 2 second figure compares to G98 table 2 - while a modest under voltage should  hold on for ~ 2,5 seconds note that the thing that will operate when unplugged is the 2nd level fast under frequency detection of 0.5 seconds.  I suspect that most units will be fine with little or no adjustment.

note that the reputable German balkony  units seem to do all this, and also claim to stop inversion within 200ms of mains loss, and to be below 60V residual voltage on the pins within 2 seconds.

Mike.

mapj1 said:

That 2 second figure compares to G98 table 2 - while a modest under voltage should  hold on for ~ 2,5 seconds note that the thing that will operate when unplugged is the 2nd level fast under frequency detection of 0.5 seconds.  I suspect that most units will be fine with little or no adjustment.

There is no time delay for RoCoF ... and this will operate if the power is completely removed.

However, there is a delay whilst it's detected, so when we are looking at single-pole RCBOs protecting the final circuit, the inverter can continue to generate after the 40 ms disconnection time ... perhaps up to 100 ms. So, instead of max 40 ms for RCBO to disconnect, it's now 140 ms. OK for a fault to Earth in TN and TT systems, but NOT where the RCD is required to provide additional protection because someone is in direct contact with live parts (broken case of equipment, cut lawnmower lead etc.).

I take the point, but the risk to the user is not much changed as the lawnmower itself  was always allowed to hold up the voltage by generating as it spins down - that's the less than 2 seconds to less than 30V RMS. The fixed wiring is held to  a very different standard to the appliances that connect to it, in many ways actually, not just this.

If someone is between live and true earth, having cut an extension lead plugged into the same circuit as their plug in solar, the RCD will still do its task in time. This is just yet another appliance that adds some pulse-stretching to the voltage on the disconnected side. Actually I can imagine having fully loaded washer or a tumble dryer on the same circuit as the victim may be  more problematic than a mower, as there is no deliberate braking action and stopping time maybe longer. 

In these example graphs, the power is cut at the first vertical line, after the first 2 mains cycles, and the Vertical grid-lines are 0.2 second intervals, so this whole plot shows a machine that stops generating altogether in about a second, as both voltage and frequency decay.
Scaling the starting voltage from 230V, this example passes  below ES1 limits by 0.6-0.8 seconds. This is from a paper about designing control circuits for traditional  induction motors.  Big motors on inverters and VSDs don't behave like this of course and in any case it is all horribly variable with what else is plugged in at the same time and the impedance  reactance that is presented to the line, these are very clean plots.



regards,

Mike.