I agree with the importance of good training however I am not sure I see the level of danger that you are suggesting.

Most countries other than the UK allow power sockets in bathrooms. There doesn't appear to be a higher accident level in these lands.

Many countries have far lower electrical installation standards. On one Chinese building site I complained about a twisted and taped joint in a 400V 3phase cable being out in the rain. They considered this to be resolved by putting an upturned plastic bucket over it.

Solar panels on roofs have two intrinsic risks, working at height, and they can't be switched off, you just have to wait for it to get dark. I would suggest that injurys due to falls from height are much more common than electrical accidents.

What are you considering to be dangerous and are there any accident statistics?

I will repeat my question, what does the Fireman's switch isolate and what does it leave live? They were originally used for high voltage neon signs on shop fronts and then I believe for electric petrol pumps (may be wrong). If you break the circuit from the solar panels to the inverter the panels and the wiring down to the switch are alive. You also need a switch capable of breaking a few hundred volts DC. A shorting switch might be better?

Similar problems exist with EVs. The HV cables are required to be in defined places that are in the emergency services documentation. They must also be orange and have a grounded screen. These carry similar voltages and higher currents than PV systems.

More house fires are caused by other things than solar panels, probably smoking and cooking are the main causes. Do some Pareto deal with the 20% of the causes that result in 80% of the problems.

BTW its not a finger poke,

I meant the finger that has to make contact with the top of a neon screwdriver for it to illuminate - making the user part of the circuit (if, hopefully, at an extremely low current). Unusually dry skin, or contamination (or gloves) can mean the screwdriver doesn't illuminate even when the tip is live.

knuckle test which apprentices (if we still had any) would be well tutored

You are joking? Not this century (or even the latter part of the previous one) - reg 14 of Electricity at Work regs put paid to people being taught to do such things long ago. Use the proper safety equipment or go home is the mantra these days.

Can I safely uncouple an MC4? 

Ah, OK I see where you're coming from now. Probably more conventionally described as being 'on load' rather than 'live'. There are similar considerations with high current connectors on AC system. Generally operating procedures are used to reduce the risk (typically shut down the inverter and then open the (downstream) DC isolator, at that point there should be no current flowing ... unless there's a short in the DC wiring somewhere. I agree a clamp meter would be useful in mitigating the remaining risk.

   - Andy.

AJJewsbury said:

Generally operating procedures are used to reduce the risk (typically shut down the inverter and then open the (downstream) DC isolator, at that point there should be no current flowing ... unless there's a short in the DC wiring somewhere.

Really?  is this what you would stand up and say in court after the owners house burnt down?

Try sketching out and studying an SLD of a Solar energy system, 10kW panels, installation kit, inverters, batteries, Victron stuff, standby genny, metering to grid etc and start with Andy's https://www.youtube.com/@OffGridGarageAustralia/videos - over 500 Vids packed with solid personal experience.

Really?

Of course - the electrical industry uses similar procedures all the time - prove instruments, switch off, lock off, prove dead, re-probe instruments, before exposing live parts. The ones above are actually on a sign supplied with my own PV system and match the instructions from the inverter manufacturer for safe isolation.  Why would I even end up in court if some DIYer caused themselves damage by not following sensible precautions? Would the original installation electrician be called to account if a DIYer decided to unscrew a socket from the wall without turning off the supply first or tried to wire a 10kW shower in 1.0mm²?

   - Andy.

The problem of course that while we can detect alternating current and voltages with clamp meters and capacitive probes  respectively that are non contact, and large DC currents by their magnetic fields using a hall effect probe (in a DC clamp meter) with reasonable confidence, there is no equivalent  reliable non-contact method of detecting the presence or absence of a  DC voltage before some metal is exposed..

Breaking of high current DC is going to have to be taught with extra emphasis and maybe a  few old techniques  re-learnt - things that get overlooked include the fact that magnetic arc traps used for DC breaking are polarity sensitive - the arc being pushed one way or the other, and there are certain kinds of fault (parallel sources such as strings of panels)  where quite conceivably a fault current could be flowing either way.

I don't see it as a big deal but we do need to make sure the information is available and well disseminated so folk know what to look for.

Mike

Great stuff - it supports my OP - we are not teaching this emerging speciality anywhere AFAIK.  Practising Leccies dont have time to self study this area, and there are no accredited text books you can pick up on. Worse still is the amateur with a superficial grasp of electrics who thinks he knows dc systems from meddling with cars/trucks. When you get into Lithium storage there is a great deal to learn and your path is twisted and confused by YT and Ggle rubbish highly influenced by suppliers of kit or bogus courses.  Its a nightmare fighting through it all - then you have to deal with Cheap Charlie.

FWIW I recently discovered fake solar cable floating around on the market. - 6mm2 turned out to be neared 4mm2 - so be careful to check a sample before laying out for a drum.  Fake MC4 connectors are everywhere and a major cause of roof fires. Panel installers may well have no credentials as Installation is ca 50% of total cost.  Solar energy is wild west country.

What actually is your point here? There is not enough training? People buy cheap substandard components?

I can accept that substandard connectors could start fires especially if non flame retardant plastics are used. The question is what can the fire spread to. The cables again should be flame retardant. I agree if PVC cables had been used rather than the correct solar cables there could be a problem.

The current rating of solar cables is a different problem.  The current rating of a cable is based on a temperature rise in the conductor to give a temperature in the insulation that will give a defined service life. This may be 40 years for building and railway cables or 3000 hours for automotive cables. The ambient temperature obviously plays an important part. If I use a 4mm2 cable in place of a 6mm2 cable what will happen? At full load the temperature rise may be doubled (roughly). How long will it be at full load? For a typically solar installation probably at most 25% of the time. Will this reduce the service life? Probably not. The cable temperature will be higher with the addition of a higher ambient temperature due to the sunshine. This will not be a problem if the correct crosslinked polymers are used for the insulation, it will not melt. Will the temperature be enough to ignite anything nearby?

Go back and read my text and do some homework

a) There most certainly is not enough quality training in Electrical Eng of the type that existed with the tech College HNC courses.  The lecturers were great, knowledgeable guys.  The pseudo Uni factories dont offer the same level as the day release system.  Certainly you wont get instruction on the special conditions for dc ELV systems. Given the parlous state of employment conditions for lecturers, I doubt whether any quality staff can be retained (get more as a warehouse shelf filler). I have great sympathy for all those students who were cheated out of "face time" lecturing hours substituting on line instruction and tutoring which is of inferior quality yet still have to pay for huge tuition fees.

b) Try buying the Lucas pattern push fit spade connectors common in older cars.  On EB you are totally flooded with Cheap Charlie copies with substandard materials and much thinner brass sheet ( bad news, cant carry current or maintain spring contact pressure).  You unlikely to find the original item but are presented with hundreds of identical listing utterly wasting your time.  Even drilling into GGL wholesalers often offer them same gear. Eventually you find a real EU manufacturer, but the MOQ is 1000's

c) My point about Solar cables (which have EU TUV markings on the sheath - you specify 6mm2 but are supplied with nearer 4mm2 is that acceptable.  Your other questions relating to 4mm" instead of 6mm2  are for the professional designer to decide and justify

You need to look at your view of the professional responsibilities here  as what you do in your own shed is your business.  A designer must weigh up the info and credentials from the supplier and prepare a PO with tech spec and this is used the an inspector to approve delivery of a shipment before payment is approved.  Look at the utter fiasco of the recent COVID PPE masks that were found to be CHEAP CHARLIE fakes of the genuine International Std product and totally in effective as a filter for micron virus particles. 

The designer makes drawings and docs which  specify the wiring installation and components which must all be compatible and fit together. This is given to unknown buyers and electrician installers somewhere to implement.  No place for the trivial minded lazy "catalog engineer".

Many a leccie wannabee designer (who has extensive - and valuable field experience) fail utterly in the design office when it comes to putting it all down on paper as a set of instructions for another unknown installer or a buying spec.

I'm still confused as to why you think this is any different to any other electrical installation work.  For any genuine component, there is likely to be a cheap Chinese copy available from the more dubious outlets.

You describe yourself as a certified and authentic EE (not Power btw). What do you mean by certified?

a) What are the special conditions for DC ELV systems? Current ratings are the same as AC and only as you get above 24V you need to think about arcing when switching. When you get above ELV arcing is more of a problem. The largest DC power supply I work with is 15kV 15A which does require thought and care when testing as well as in design.

b) What is the relevance to Solar PV?

c) Yes, my employer manufactures Crosslinked Solar cable to meet the various national specifications so I am quite aware of the requirements and I am quite conversant with electrical design and current ratings. I have been previously involved in various aspects of cable testing, short circuit performance where the conductor can reach 250°C is quite spectacular, not something I would do in my shed.

You describe yourself as a certified and authentic EE (not Power btw). What do you mean by certified?

a) What are the special conditions for DC ELV systems? Current ratings are the same as AC and only as you get above 24V you need to think about arcing when switching. When you get above ELV arcing is more of a problem. The largest DC power supply I work with is 15kV 15A which does require thought and care when testing as well as in design.

b) What is the relevance to Solar PV?

c) Yes, my employer manufactures Crosslinked Solar cable to meet the various national specifications so I am quite aware of the requirements and I am quite conversant with electrical design and current ratings. I have been previously involved in various aspects of cable testing, short circuit performance where the conductor can reach 250°C is quite spectacular, not something I would do in my shed.

Roger B said:

You describe yourself as a certified and authentic EE (not Power btw). What do you mean by certified?

a) What are the special conditions for DC ELV systems? Current ratings are the same as AC and only as you get above 24V you need to think about arcing when switching. When you get above ELV arcing is more of a problem. The largest DC power supply I work with is 15kV 15A which does require thought and care when testing as well as in design.

b) What is the relevance to Solar PV?

Apologies about the language in the commentary of this short video but it does illustrate some of the points that have been discussed here: www.facebook.com/.../3507234699527923

I think the fact this kind of thing goes on is an issue.

And if you have a fault L+-L- before a combiner box or inverter, say because of vandalism or (possibly unforeseen) mechanical damage, it's the only way of stopping current flow with the way PV is designed ... and yes, there's perhaps a discussion on whether that is in line with "general parts" Part 4 and 5 of BS 7671 ...

Hello Graham,

I think that the replies have got a little confused. I was referring to Rob Tes’s post where he was talking about the quality of Lucar connectors. Thank you for the video, it would be useful to know what voltage and current they were interrupting. I have previously wondered that as we are dealing with an almost constant current supply whether shorting switches would be a better option that isolators as  the first action on small systems.

Roger B said:

a) What are the special conditions for DC ELV systems? Current ratings are the same as AC and only as you get above 24V you need to think about arcing when switching.

www.youtube.com/watch

gkenyon said:

short video but it does illustrate some of the points that have been discussed here: www.facebook.com/.../3507234699527923

Yes seen that one, notice the crappy electrical pliers, barely capable of cutting 6mm solar cable.  A proper pair of cable cutters should be essential safety kit, long handle snip thru in a heatbeat.  Its the method of last resort and amateurs dont learn these things (plus PPE of course), gloves, goggles and makes sure your overalls arent flammable with spilt liquids)

Roger B said:

shorting switches would be a better option that isolators a

good way to start a roof fire?

Rob Tes said:

good way to start a roof fire?

Not really.  Solar panels are inherently current limited by the way they work.  In normal operation, they will be run at a current close to their short-circuit current.  So in the case of a short, nothing much happens to the panel, and the wiring needs to be able to handle that current anyway.

Rob Tes said:

Its the method of last resort

Surely this is why BS 7671 requires a means of isolation from every source.

Battery installations use short string interlinks and often have overcurrent protection within, and at the top of, each string ... this, and long runs to combiner boxes, makes the solar PV system very different, and assumptions used to draw analogies between PV arrays and a battery are, in my opinion, flawed.

When we talk about "method of last resort", I'm very mindful of EAWR Reg 14. I'm not sure we have the PV array design right yet ... the risk is not reduced "SFARP" and not enough research has been carried out into isolator fires, "oh well we'll not fit them then" isn't really a good excuse for EAWR Regulation 14 in my opinion.

I should not have to see this method under any circumstances in my opinion ... especially now array voltages are at 1500 V DC ! We can, and MUST have a safer design approach!

Simon Barker said:

So in the case of a short, nothing much happens to the panel, and the wiring needs to be able to handle that current anyway.

Agreed, but it's not just about protection against overcurrent. Arc can still form, or the DC cabling must be made safe to work on.. Detecting it at the inverter end will alert someone to the fault, but if all they can do is cut cables, that's no good !

We must do better and provide a means of disconnecting the source, in line with the requirements of the General Rules of BS 7671, so it's safe to work on cables and connectors which can't be disconnected on-load either .

gkenyon said:

"method of last resort", I'm very mindful of EAWR Reg 14. I'm not sure we have the PV array design right yet ... the risk is not reduced "SFARP"

gkenyon said:

When we talk about "method of last resort", I'm very mindful of EAWR Reg 14. I'm not sure we have the PV array design right yet ... the risk is not reduced "SFARP"

Yes the convoluted EAWR written by lawyers?  I dont see any mention of the "Crowbar" technique.  Isnt this a std method in electrical traction systems for worker protection?

My OP was only concerned with Solar systems for domestic applications and the dangers posed.  We can SFARP if we limit Solar arrays <50Vdc.  IMHO its foolish to allow higher array volts of ca 100V or even 480Vdc (Ive seen on YT probably that Vid posted here)

Facebook

https://www.facebook.com/reel/3507234699527923

I suspect this was several 100Vdc commercial site judging by the length of arc flash

My proposition is to use large knife switches with a gap of ca 50mm for Solar dc<50V.  This system was used in the London underground prewar (600 Vdc).  Its simple and it works. The convenient design of MCBs did away with all the clutter of cast iron re-wireable switch fuse boxes - with their knife switches..

https://vi.aliexpress.com/item/1005003453152419.html?spm=a2g0o.productlist.main.3.5ca41bd96PlNph&algo_pvid=fb3c70a7-cb05-4e87-b7af-a6fa00ac3907&algo_exp_id=fb3c70a7-cb05-4e87-b7af-a6fa00ac3907-1&pdp_npi=4%40dis%21GBP%216.38%216.06%21%21%217.77%21%21%402103853617006671710863788e3ba4%2112000025856398788%21sea%21UK%21929926363%21&curPageLogUid=QOlMH3g17WF6

That humble domestic master switch is a clever robust design - with built-in rewireable strip fuse, (Siemens prewar pattern) and is in use for millions of dwellings in the far east  all for the princely sum of £6  63A rating (could be doubled).  I would trust that..

But roll out the tumbril for I shall be bound for Tyburn Gate and a Public hanging for my heresy.

We can SFARP if we limit Solar arrays <50Vdc. 

Not necessarily - in some ways you're just trading one hazard (higher voltage) for another (higher current).. We're actually quite good at handling several hundred volts around the home (230V a.c. is a bit over 300V pk; and for 3-phase supplies - increasingly common domestically these days - it's over 560V pk) as we generally have the materials and techniques to handle them with adequate safety), especially at modest currents.

My proposition is to use large knife switches

Humm - those simple types need some skill to use safely - fail to open or close them smartly enough and you can cause a lot of damage - from overheating, if not arcing. The less well shielded types risk allowing shorts between exposed parts too.

Detecting it at the inverter end will alert someone to the fault, but if all they can do is cut cables, that's no good !

There's always the option of employing time - wait a few hours and the sun will set

   - Andy.