Yes ELVs <50Vdc.  IMHO its sheer madness to be involved with MV dc Panel strings - way too dangerous  IMHO in a domestic environment, and we dont have the skilled tradesmen to recognise this type of micro grid nor is there a recognised route to upgrade to this unfamiliar but hazardous system.  Way too many unschooled wannabees playing with fireworks.  It is utter negligence by our Political leaders to allow this to continue.  They only woke up to the hazards of amateur plumbers who didnt understand flammable atmospheres till a few houses were blown up flattened.  Then British gas had to step in a institute proper training courses with local tech colleges.  Where do our Electricians get proper upgrades?  A one man jobber cant afford an apprentice (with all the paperwork and oversight involved). He will only have a family member or friend involved.

Pandora's box has been opened here

Microgeneration Certification Scheme (MCS)

https://www.gov.uk/government/publications/register-energy-devices-in-homes-or-small-businesses-guidance-for-device-owners-and-installation-contractors/register-energy-devices-in-homes-or-small-businesses-guidance-for-device-owners-and-installation-contractors

Just when you thought that the summary dumping of Part P responsibilities on to the LABC (who had no historical involvement in electrical installations) was a complete dog's breakfast, and IMHO remains so to this day and from experiences of Landlords, widely open to brown envelopes.

Remember gentlepersons that the domestic electrician's primary skill involves "house bashing - that's a trade term and needs years of experience" - this is not to be deprecated in anyway. The industrial electrician is an elite class holding authentic credentials in order to be allowed on site or employed by a Company.  These persons wouldnt trade their coveted positions for the Solar Wild West country.

The average domestic electrician is a sole trader operating from home and he has to weigh the costs of trading legitimately against competing with rogue chancers who abound.  You can be a hairdresser today, slap some stick on vinyl flashy advert on the side of your van and carry on as "Instant Electrics".  Here today, gone tomorrow Strawman.

Given that over half the cost of a Roof Panel installation comes from labour and scaffolding (mandatory ???) you can see that installation persons dont need any electrical certification (maybe half day course on how to use a ladder) and can plug in MC4s (even crimp them up).  We have the dubious practice of a young persons finishing tails under supervision of a "Competent Person"  You get the picture?

Nowhere in any of this labyrinth of obfuscation is there any requirement for anyone to be properly trained in dc power systems. As a certified and authentic EE (not Power btw) I have spent a year looking at the problems of domestic DIY Solar systems on most of the relevant Forums covering USA/CA OZ EU and the Leisure craft/RV.

If you want a good example of the kind of hazards you can face then the widely acclaimed site https://www.youtube.com/@OffGridGarageAustralia  will surely open your eyes.  Ignore all the bells and whistles of batteries and BMS just look behind at the electrical installation behind it all  and think about kids playing with fireworks with a can of petrol nearby.  The guy is a former IT technician and it shows a superficial grasp of dc power systems  (think of MCBs for solar panel disconnect under full sunshine  an OZ scandal since corrected when resulting in numerous alarming arc flash fires)

(FYI the ABYC has a long history and wealth of experience  certifying electrical systems on boats, they have their own "Codes" and authorised inspectors to UL standards - if your craft is not signed off you dont get insurance and that limits where you can moor your boat - its quite draconian and there are numerous examples of fire aboard caused by electrical faults)

There is no credible UK Solar forum to exchange User experience.  We cant even recognise the problem of highly publicised  Escooter fires from faulty products we blithely allow to be imported from Cheap Charlie  see YT https://www.youtube.com/results?search_query=escooter+fires.  The CE mark is a treacherous joke (meaning China Export)

mapj1 said:

I will point out that there are not enough accidents prevented to justify

IMHO you need to look at the fallacy of that statement - how do you measure the number of accidents prevented when they didnt happen?

I've been looking at this recently. I'm building a house in Scotland, part of the Planning requirement is to have PV panels. (I'd have them anyway). I've had one quote, and , asking around, its a typical, maybe low price, 4kW PV, 10kW battery, supplied and fitted for £8850.

Now, money is getting tight, so looking around, the same materials, can be bought for £5500. That's some mark up to fit those for £8850, especially as the scaffold will already be up, and the Roofer is fitting the trays to mount them (in roof system). I can fit the panels/inverter etc myself, but need the MCS sign off. Except I no longer need that, as Octopus will allow PV export with just an electricians sign off, no need for the MCS requirement, hence now, I'll be DIYing.

MCS is also changing this year, there are far too many complaints that their eligibility requirements are too tight, rumour has it that entrance to their scheme is going to be loosened a lot. It's a same scenario in the Heat Pump world, good sole traders are not bothering trying for MCS as the costs and paperwork requirements are far too stringent, so good Designers and Installers are not fitting heat pumps, as they are not allowed to apply for the £7500 grant, yet Octopus/BG et al are fitting them with an awful design (50 deg C flow temperature) that will be fitted cheaply, but will cost the customer a lot more in the long run with their poor performance and increased electric usage.

Well Alan that sums up our typical bureaucratic incompetence we've come to expect. Armchair diktats from faceless mandarins.

IMHO if you can at all then instal ground level panels.  There are numerous horror stories of roof panels - gggle and yt for info.  Tip to consider as its new build, install a cat walk tray on your roof ridge and make it accessible from a gable end by ladder. Its and unobtrusive convenience for maintenance which you will need.

Wire each panel with its own cable thru ridge into loft space and route to a marshalling cabinet and maybe use panel optomisers. You avoid the dubious MC4 connectors (use barrel but splice ferrules and heat shrink apply si grease around metal then a 50mm length of 4:1 adhesive sleeves - these make a water resistant cover

Beware of fake Solar cable (4mm2 solar as 6mm2 solar)

Are you sure your location and insolation (sunshine hours) can justify the full expense of a solar system - capex vs payback time (10yrs +).  Dont believe Solar cowboy calcs. Octopus are just a shed load of accountants check very carefully TnC.  How much energy do you expect to export? Is it really worth it, store your excess as hot water or underfloor heating.  This Solar thing in Scotland has to be one of those political panjandrums  BEWARE

Martyn said:

However, SEG and export are not worth the aggravation for the payback these days

I tried doing some rough example calculations.  Say you average 2kW export for 6 hours a day.  Sign up with a supplier who offers £0.15/kWh.  That's about £657 per year.

The panels could pay for themselves in 10 years.

(use barrel but splice ferrules and heat shrink apply si grease around metal then a 50mm length of 4:1 adhesive sleeves - these make a water resistant cover

Are you sure such an arrangement would meet BS 7671 requirements for joints (526.5)?

   - Andy.

Simon Barker said:

The panels could pay for themselves in 10 years.

Have you amortised the total cost of installation inc wiring switchgear inverter (fails in 5 years maybe) panels need replacing etc?  Batteries?

AJJewsbury said:

Are you sure such an arrangement would meet BS 7671 requirements for joints (526.5)?

I cant justify purchase of IEE regs plus add plus the user manual, not a practising designer anymore

can you quote the relevant excerpt 526.5 that outlaws crimped joints?

OBTW what I meant was "paraphrase" the relevant passage (mustnt violate copyright), but explaining/discussing an extract is ok

can you quote the relevant excerpt 526.5 that outlaws crimped joints?

It wasn't the crimp I was alluding to, but the method of enclosure...

526.5 Every termination and joint of a live conductor or a PEN conductor shall be made within one of the following or a combination thereof:

(i) A suitable accessory complying with the appropriate product standard

(ii)An equipment enclosure complying with the appropriate product standard

(iii) An enclosure partially formed or completed with building material which is non-combustible when tested to BS 476-4.

which a bit of heatshrink doesn't obviously meet, if used on its own to extend tails to reach your marshalling boxes.

   - Andy.

can you quote the relevant excerpt 526.5 that outlaws crimped joints?

It wasn't the crimp I was alluding to, but the method of enclosure...

526.5 Every termination and joint of a live conductor or a PEN conductor shall be made within one of the following or a combination thereof:

(i) A suitable accessory complying with the appropriate product standard

(ii)An equipment enclosure complying with the appropriate product standard

(iii) An enclosure partially formed or completed with building material which is non-combustible when tested to BS 476-4.

which a bit of heatshrink doesn't obviously meet, if used on its own to extend tails to reach your marshalling boxes.

   - Andy.

Surely this doesnt apply to ELV <50V systems?  You refer to heatshrink protection.  Some of the quality products seem to be very well suited to outdoors UV resistant, adhesive forms a sealed clamp on a round sheath.  I would put additional coat of Si grease on the metal parts of the barrel and around wire strands after crimping and use a 50mm length sleeve - seems belt and braces to me.  But then electrolytic corrosion can only occur with a positive and negative splice being close together in the event of a moisture path - seems remote risk.  You dont see auto wiring harnesses suffer unless in extreme circumstances.

Rob Tes said:

Surely this doesnt apply to ELV <50V systems? 

Just to clarify, most solar PV systems are not ELV. Typical operating voltages for small domestic systems are ~600-800Vdc, up to utility installations with open circuit voltages of 1500Vdc. Operating currents are of the order 10-20A, depending on configuration, obviously more if anything is in parallel (or if the modules are in reverse current due to mismatch or fault)

It is also worth noting that solar PV installations rely on the measure of double or reinforced insulation, with class II equipment and insulation monitoring devices.

The conductors, at least for sections on the array itself, should be fine wire flexible (cl5) stranded tinned conductors under that insulation, in the most part because anything on the array will be subject to frequent thermal cycling and wind movement.

The connections will also be exposed to a wide range of temperatures (in the UK behind a panel could expect an ambient temperature from -15°C to +70°C with daily swings), are likely to see high degrees of moisture and are likely to be difficult to inspect. And yes noting the parallel thread on this topic I would expect similar in someone's loft.

Hence the use of connectors, and connections generally, made to a standard.

For domestic installations, anything >50vdc is way outside my comfort zone, but thats my personal prejudice .  Aside from shock hazard,  my concern is that switchgear/fusible links etc are notoriously suspect for circuit protection under load/fault interruption.  Properly authenticated circuit breakers (as per Siemens NF) are very expensive. The Main circuit breaker with AIC 5kA can be ca £1000 for the levels you are referring to.  I was astonished to see a well known US brand which supplies a complete solar kit down to the inverter mains outlet use a rotary cam switch (typically seen on electrical panels) as a circuit breaker between the panels and the Inverter  (no internal arc quench in that design).

At least with ELV systems you can keep the methods of last resort to hand on a shelf by the distribution board - namely a proper set of cable cutters, leather gloves, welder goggles and a plant mister bottle to spray water on an arc flash fire.  But I am old school as were fire buckets with sand.

I have the same fear of 3 phase power and wouldnt trust any old tradesman installing that system (at least it has full industrial design system credentials behind it so I would need a professional design practice to provide  detailed installation drgs and BOM. The tradesman installer has to follow the drg to the letter and not be left to fumble his own interpretations on the back of an envelope (Ive seen this approach)

IMHO the approach to Solar systems is very slap dash and careless, no site installation drgs, BOM, cable run iso

Maybe the sorry tales of fast track installations reported on various Forums has given me a jaundiced view (I hate Roofers)

Rob Tes said:

anything >50vdc is way outside my comfort zone

From experience, I'm more worried about DC current than DC voltage. Anything more than a few (<5) amperes at 12 V or more is enough to draw an arc, and 1-2 amperes is enough to sustain it. At 5 V DC, more current is needed ... but not too much to be honest.

Shock isn't the issue, fire, burns and arc flash can be problematic.

I can't disagree with some of the other points that you make.

gkenyon said:

Anything more than a few (<5) amperes at 12 V or more is enough to draw an arc, and 1-2 amperes is enough to sustain it

More homework needed here old chap. FYI you cant sustain an arc flash at 12 V. You will get a spark but you cant develop a sustained arc (at STP) below a certain voltage.  This is ca 20Vdc.  A welder will tell you that you can only weld with 2 car batteries 24Vdc or more  YT vids are there.

and anyone who has tried it will tell you that 3 car batteries strike up far better but to go easy anyway, as it does them no good at all. A few minutes non stop welding can heat the battery acid to a temperature that does irreversible damage. On the very rare occasion it is needed, I prefer to see the batteries put in a ditch or on the other side of something solid like  a vehicle, so if one does boil and vent, the scope for injury is limited, and not all on-line videos show sensible behaviour - but that is not just about welding.
It is quite a good thing to know how to do however.

Mike.

However far more important is how to relate available energy, or rather rate of delivery, into a safe arc radius to avoid instant sun-tan or worse being basted with molten metal, and then to either limit access for body parts, or the energy available during fault, so that the two never coincide. By the time you are talking about needing buckets of sand, if you are in earnest, you are already on the wrong side of a how to design things safely.

M.

(Who is very much not afraid of 3 phases either 400 or 690, when properly fused and in suitable containment, or indeed higher voltages too, but with rather more caveats. )

mapj1 said:

when properly fused and in suitable containment, or indeed higher voltages too, but with rather more caveats. )

Anyone understand this?

Gone OT here

I wasnt discussing using car batteries for regular welding, simply giving an example to show that you need a minimum voltage to sustain a dc arc, 12Vdc is not enough, 24Vdc will do it.

The real danger to health from an arc blast is the molten droplets of copper becoming instantly vapourised and expanding with explosive force ca 70,000 times expansion.  You dont want that in your face, nor the huge blast of UV and eye damage.  Its why copper fuse wire is encased in a ceramic tube and HRC fuses have sand added as well to quench the arc.  Look up YT and see some examples of fuse rupturing fault currents.

Fuses are given  an AIC or Amperage Interrupt Capability is the maximum fault current that the protective device is able to clear safely without causing damage to equipment or personnel or welding closed.

This can be 5-20kA 20,000 Amps! typically (or much larger for HV).  Plenty of room to study here to really understand the risks as I have this past year and is the reason for my OP.  Electricians are not traditionally trained in this specialised area of LV dc high current systems. 

Even something as elementary as making ring terminal bolted joints can go badly wrong (as per busbars in battery banks).  If you use a Flir IR camera it will be surprising to see where significant hotspots are developing within those bulky cables under full load.  Those hot spots over time can develop tiny arcs that grow seriously.  viz YT

Off gridders know about this.

Indeed - hence the point about enclosure/ containment. There is a tendency among those who have not seen it for real to think that ELV quite is safe to have exposed terminals - and it may well be safe from a shock perspective, and in more traditional cases where the fault current is limited such as bell transformers and dry batteries.

With all but the smallest lead-acid or Lithium-ion and similar the impedance can be very low, and there is no such protection, and the effect of a spanner or strap of a wristwatch causing a fault path to the wrong place  however can be devastating, forming that accidental welder without the current control.The burns on a finger from a briefly red hot wedding ring are pretty unpalatable as well. I've not done it but I have seen the pictures.

Even in a pocket 'vape' the current in the heater coil can exceed 20A from a lithium cell that is only a few CM3 in volume . Scale  to something the volume of a small fridge and the potential energy is pretty impressive very quickly.

The headline has to be - with battery banks, shock is not the only risk, and 'just' ELV wiring needs increasing respect the more current is available .

Mike.

Well said Mike, but out governing Powers and indeed the technical training organisation and their learned bodies have largely ignored this emergent electrical area (even though the hazards are well understood for a century in such industries as electric traction, electroplating, telephone exchanges etc).  No young engineers or electricians receive any specific instruction in the nature of ELV dc distribution.  Its left to old timers like myself who came up through industry and formal education to recognise this drastic gap in training.  When I was tasked with the recruitment "milk round" , I was very disappointed at the poor quality of graduate training for Electrical Engineers who were expected to go on to become system designers.  IMHO they would need a full 5 years alongside an experienced site engineer just to get a proper grasp of the subject.  We don't have such mentors available any more and plant electricians don't have time or patience to carry students along (neither generally are electricians funded and trained to give such assistance BTW).

So that leaves the naive public at the mercy of "Chancers" and "Cowboys" I am sorry to say  (bit like the Roofer trade)

IMHO we will fall victim to Politico whims and grandstanding as has happened with " promotion of Diesel vs Petrol cars", Electric vehicles (2nd values plummeted ,  Heat pumps, HS2 rail (never once was the concept of modern VIP style coaches with exclusive use of the motorway outer lane considered), Solar farms occupying arable farmland),  pursuit of Nuclear power stations 20years to build 10x original budget using very old outdated technology still producing environmental nightmare waste (see https://www.rolls-royce.com/innovation/novel-nuclear/micro-reactor.aspx

but what do  kno