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.

gkenyon said:

We know that, if you cut one corner, accidents might happen, but they'd be unlikely. However, the more corners you cut, the more risky things become.

Whichever way you look at it, we are degrading the protection currently offered by BS 7671, in the full knowledge that that doesn't protect all of the people all of the time ...

Well then this is where the organisations and institutions developing and operating with BS7671 need to come in with some cost-effective, efficient solutions to support consumers to adopt these low cost balcony / plug-in solar solutions in a safe way.

Does it need some educational guidance producing, that these plug-in systems should be connected to socket circuits protected by suitable double pole, or true SP+N RCDs/RCBOs and that the consumer should budget for getting the circuit's RCD/RCBO upgraded? Is there a technical solution, in terms of the inverter's built in protection settings and response times for these situations?

Does BS 7671 follow the approach of VDE of advising use of an dedicated, economical plug/socket for these, so that where possible they are connected to their own dedicated circuit? Maybe working with the ENA and having some clear supporting guidance to ensure new circuits for plug-in solar can be installed as economically, lowest cost possible to avoid them being plugged into the socket circuits?

There's measures which could be taken to reduce the risk from these devices, to steer consumers towards safer installation arrangements.

gkenyon said:

1. With the lawnmower lead, the lawnmower has stopped, because I cut the lead ..

Not necessarily - I've seen leads that have been "shaved" - exposing (at least) one of the conductors (and maybe thinning it a bit) but not completely severing either of them.

  - Andy.

Question - TN disconnection times are based on a touch voltage of half the line voltage - 115V-ish so 0.4s - while TT disconnection times (due to much higher impedance between c.p.c. and Earth) are based on full 230V touch voltages - 0.2s (or 200ms) - so why 40ms for additional protection where the assumption is also that the victim is exposed to 230V?

   - Andy.

mapj1 said:

I agree, but my point is more that  we already do that every time we plug any appliance in that holds up the voltage for some short time after the breaker breaks - and there are more of those than just inverters, plenty in common use. As another example we also weaken the protection against direct contact when we plug in a bedside lamp and the protection damage with every flex. 

Hi Mike, not sure you missed the following points:

• the appliance is still going to hold up the voltage after the additional disconnection time, so it is definitely worse.
• When I cut the lawnmower lead, there's an assumption there's always an appliance connected somewhere else on the circuit that holds up the voltage for a short period of time. Not necessarily. But if there is, see previous bullet.

AJJewsbury said:

so why 40ms for additional protection where the assumption is also that the victim is exposed to 230V?

RCD measures the residual current, which is, potentially, all passing through the person. It's not a touch-voltage-for-time but a touch-current-for-time paradigm.

Compare with a fault to Earth where most of the available current (hopefully) passes back down the CPC and, where applicable, supplementary local equipotential bonding.

Different paradigms.

For me, the question is what is a reasonable (supply) hold up time for an appliance.  As mapj1 has pointed out, there is a standard for this (EN IEC 62368-1:2024) which imposes some limits.  I suspect that in many cases a solar inverter should be able to do a lot better as I suspect that standard is written around things like lawnmowers and vacuum cleaners where there is no cycle by cycle control of current drawn / returned.

If we take the pure solar (no battery) case, solar is a constant power device (at least over the timescales we are talking about here) - in most cases in the event of a protective device trip I would expect the inverter output to over or under voltage and trip within half a cycle or so.  It is only in the case where the loading on the inverter side of the protective device is such that a stable equilibrium can be reached at a voltage within the permitted range that the delays associated with RoCoF or similar come into play.  Have the studies been done to understand what proportion of the time that might happen?  Is it practical to reduce the permitted voltage range to somewhat narrower than standard so as to reduce the likelihood of that happening?

The solar plus battery case is more complex.  The simple solution is to force the interface to the supply to act like constant power solar if export is to occur.  That doesn't prevent the equipment itself having UPS like sockets but there will need to be a contactor to disconnect the inverter from the supply once loss of mains is detected.  That contactor will mean that systems featuring batteries will be slower to disconnect than pure solar (unless we're willing to accept a solid state device for that function?).

In this case I'm considering the main holding-up appliance is the solar inverter - its not clear what happens when you put multiple holding-up appliances in parallel or which ones you want to disconnect. 

The situation is of course cleaner when the solar unit is on its own radial on its own special plug and socket, which is not much different to hard wiring,  and that should be the gold standard, but see my gripe in the other thread about the accidental consequences of the plug and socket legislation as to why we seem unable do the dedicated connector in the UK.

The question is how much more dangerous is it if we throw caution to the winds and plug an inverter in anywhere, compared to any other combination of appliances with more or less hold-up

Mike

PS

This non-standard plug is not as common as it used to be ;-)  but we need to remember why we once needed that sort of safety campaign and compulsory plug and socket stuff, so we we know where we came from in terms of common/acceptable behaviour.

They MAY be called Balcony solar but when the DIYer gets it home they could put it on a fence/wall in the garden or on the slopped roof of their house or even just lie it flat on the roof of the kitchen extenion probably with no mounting restraints at all.  (Just wait for a windy day and see what happens).  The mind boggles to the positioning options and how many PV units the DIY will buy in one go or over time as they percieve the savings of energy cost.  However if they don't get some kind of battery storage then they are only doing half a job.  Now when we start to talk about battery storage that opens up a whole can of worms/question about positioning and fire safety and......

It's not a touch-voltage-for-time but a touch-current-for-time paradigm.

Understood - but they two aren't entirely unrelated (I'm sure Mr Ohm had a formula we could plug body resistance into).

Compare with a fault to Earth where most of the available current (hopefully) passes back down the CPC and, where applicable, supplementary local equipotential bonding.

At that's good if the detection method relies on higher currents (e.g. ADS by 100mA RCD) but unfortunately the conventional fault-of-negligible impedance still leaves a considerable current available to pass through the victim - e.g. with 230V and a 100Ω electrode we'd be looking at 2.3A flowing through the c.p.c. with negligible drop in L voltage as a result  - but a victim with a body resistance of say 1000Ω could still have 230mA flowing through them. But still 200ms (rather than 40ms) is considered OK (ignoring the 250mA vs 150mA/5x RCD testing debate for the moment).

   - Andy.

Is such an arrangement not rather unlikely for 'no skill' plug-in solar  ?

Is such an arrangement not rather unlikely for 'no skill' plug-in solar  ?