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.

AJJewsbury said:

even into a dead short the maximum expected is 125% of normal output current

That's more than sufficient current to draw an arc, and sustain it for some considerable period of time (for example as an MC4 burns and disintegrates). WIth currents as low as 1-2 amperes !

In fact, if there is a fault, because they do happen, what is built into a solar PV system to enable someone to work on the system safely, or remove the arc etc.? Usually nothing!

For safe working on Solar PV, isolating at the inverter may stop current flow ... but what are the consequences if it didn't (say because there's a fault L+ to L- between the panels and in the inverter)? The only way to check for this, is to check for DC current flow with a DC current clamp meter at the point of disconnection, prior to disconnecting (because there's nowhere to "prove dead").

If not, these are the potential consequences: youtu.be/rkq1zwG9vLc

gkenyon said:

It's possible to cause an arc at 12 V DC

WRONG more homework needed GK

https://www.youtube.com/watch?v=5FXRuT6TU2U

you cannot draw an arc flash at 12 Vdc - nor at 15Vdc from a battery charger - its the Physics of the subject (very esoteric)

Here is an example with 2 batteries 24Vdc

https://www.youtube.com/watch?v=PV5oLPLUzrM

An arc flash can be sustained (out in the bush people have used coat hangers as welding rods)

Note the size of the Arc Flash and imagine that sustained in your faulty breaker or fuse holder

What will you do next ?

Are emergency procedures stuck to the wall for Joe Public and his Family?

gkenyon said:

For safe working on Solar PV, isolating at the inverter may stop current flow ... but what are the consequences if it didn't (say because there's a fault L+ to L- between the panels and in the inverter)? The only way to check for this, is to check for DC current flow with a DC current clamp meter at the point of disconnection, prior to disconnecting (because there's nowhere to "prove dead").

Indeed, a good point raised here - how to tell if a dc cable is dead.  Analogy the ubiquitous neon screw driver owned by every Leccie, automatically check for a live wire before intervention (also the bare knuckle test some believe is more reliable as your neon could fail)

So what is proposed here is that compulsory use of a Hall Effect clamp meter on solar cables before intervention.

BUT  the sad thing here which my OP highlighted is that we dont have any formal training worth a damn to promote theis level of Safety Training

I agree it isn't but then where is the public information on what to look for ?

That absent  public profile again - Rather like how to check your tyres or oil, how to see if your electrics look dicky... It would not be hard. Perhaps it is a pity the old scary safety film unit is no longer in operation...

(I did wonder if British Public information films of kids being hit by trains and tractors or being accidentally locked in fridges and other such stuff were a side effect of the film classification system being quite strict, so frustrated British horror film makers ended up doing things like The Apaches or The Finishing line instead where they could let rip in the name of education. What they could have done with an unprotected  DC supply I wonder ? )

Mike

Rob Tes said:

WRONG more homework needed GK

I think we are at odds only in terminology, and I have tried to explain that in a previous post. To sustain the arc is difficult with 12 V due to the very short distance necessary. Easier with carbon or carbonised metal ... I never said you can weld with 12 V using arc welding sticks ... but arcs can be drawn for short periods with 12 V if you know how

youtu.be/Frk3M38mfl4

Perhaps rudeness is not necessary.

Rob Tes said:

you cannot draw an arc flash at 12 Vdc - nor at 15Vdc from a battery charger - its the Physics of the subject (very esoteric)

Yes, arc flash energy at 12 V may well be low, but an "arc" is not "arc flash" - they are two different things.

An arc can lead to arc flash (or not).

Rob Tes said:

BUT  the sad thing here which my OP highlighted is that we dont have any formal training worth a damn to promote theis level of Safety Training

Agreed ... not everyone has the relevant training and experience.

That's more than sufficient current to draw an arc

Indeed, but nothing a d.c. circuit breaker towards the inverter end would do anything to help. I don't think we have DC AFDDs available yet (and no obvious place to position one in a PV string either). I guess it's back to the very old school methods - insulation, sheathing, maybe put +ve and -ve in separate conduits etc and keep some physical space between them(*) to manage the risks of parallel arcs.

* but not too far apart, we don't want a big loop, for EMI reasons.

   - Andy.

Your reply and vid are out of context with my assertion  ie you are referring to a carbon arc system.  My topic concerns fault currents in wiring/ protective devices etc using metal interfaces.  I am frustrated at deliberate attempts to dilute the issue of dc power system safety.  There is so much absurdly naive rubbish promoted on YT and giving untutored  Joe Public the idea of playing around with setting up his own solar system inside his house.   

There are numerous vids with ca 100,000 followers showing how to meddle with Lion 18650 cells and rather trivialising the safety issue.

https://www.youtube.com/watch?v=CYBnEGYWgz4

How many  people just throw lion cylindrical cells in a drawer together (just like you might with spare zinc carbon cells)

Have you seen vids of refuse lorries discharging their loads in the street because a lithium battery started a fire and produced smoke.

This is a regular occurrence in recycling centers

Would you let your kids experiment with lithium batteries?

When I am developing a project using Lions on my bench, I have the following kit to hand

A large steel cooking pot with lid containing a couple inches of sand at my feet

Wire/cable cutters to hand

Leather gloves and large tongs

Wear goggles over 12Vdc

Keep a plan water mister to hand to quench arc flash

Keep room exit clear in case of catastrophic event (room rapidly fills with highly toxic HF smoke)

How many of you understand the need for these precautions indoors?

AJJewsbury said:

A simple rotary isolator between PV panels and inverter is a pretty well accepted way of doing things. It's only intended as an off-load isolator to allow work on the inverter.

A lot of people dont understand the difference between an isolator switch and a circuit breaker.  Joe Public equates it with the light switch on a wall.

Rob Tes said:

Your reply and vid are out of context with my assertion  ie you are referring to a carbon arc system. 

I didn't specify that, and nor did you.

Once carbonisation takes place on the surface of other conductors, though, more prolong "sparking" and quasi-arcs can occur, but still perhaps not "arc flash" - I wouldn't, however, like to say it was never possible under any circumstances with 12 V DC systems.

Rob Tes said:

There are numerous vids with ca 100,000 followers showing how to meddle with Lion 18650 cells and rather trivialising the safety issue.

https://www.youtube.com/watch?v=CYBnEGYWgz4

How many  people just throw lion cylindrical cells in a drawer together (just like you might with spare zinc carbon cells)

I fully support you in this, and the other things you have said ... except perhaps I would recommend wearing eye protection even at 12 V DC.

In real installations, I have seen welded and vaporized components resulting from short-circuit faults in 12 V DC and 24 V DC systems (without carbon rods ) and for that reason, I would make the recommendation for at least some eye protection, especially where the prospective fault current is going to be more than a (very) few amperes..

Rob Tes said:

Your reply and vid are out of context with my assertion  ie you are referring to a carbon arc system. 

I didn't specify that, and nor did you.

Once carbonisation takes place on the surface of other conductors, though, more prolong "sparking" and quasi-arcs can occur, but still perhaps not "arc flash" - I wouldn't, however, like to say it was never possible under any circumstances with 12 V DC systems.

Rob Tes said:

There are numerous vids with ca 100,000 followers showing how to meddle with Lion 18650 cells and rather trivialising the safety issue.

https://www.youtube.com/watch?v=CYBnEGYWgz4

How many  people just throw lion cylindrical cells in a drawer together (just like you might with spare zinc carbon cells)

I fully support you in this, and the other things you have said ... except perhaps I would recommend wearing eye protection even at 12 V DC.

In real installations, I have seen welded and vaporized components resulting from short-circuit faults in 12 V DC and 24 V DC systems (without carbon rods ) and for that reason, I would make the recommendation for at least some eye protection, especially where the prospective fault current is going to be more than a (very) few amperes..