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.

Rob Tes said:

More homework needed here old chap. FYI you cant sustain an arc flash at 12 V.

Indeed ... although I think we know enough already. It's possible to cause an arc at 12 V DC (not AC) from two conductors in direct contact (we know this from car batteries) ... although it does "blow itself out" as the gap increases as the conductors melt.

Things can get interesting, though, with "constant current source" behaviour of solar panels and power converters.

We are talking "arc" and not "arc flash" here, and I would agree 100 % that we don't start to get too many problems with arc flash at 12 V DC, perhaps up to 24 V DC.

Rob Tes said:

No young engineers or electricians receive any specific instruction in the nature of ELV dc distribution

More accurately, I believe it depends where you've been employed in industry. Power supply to controls and telecoms (and 'telecontrol') never went away from ELV DC ! However, I agree in general, when we talk about "electricians" the industry moved away from DC in the 1960s, but it's coming back now !

Rob Tes said:

Agenda!!!!!!!!!!!!  I am crying out for amateurs to get themselves properly educated instead of the painful "suck it and see" attempts shown on youtube by technically illiterate bluffers pumping out dangerous practices for the credulous public to play with.  Its as bad as showing kids how to make fireworks.   Perhaps you havent browsed enough of these crass vids to see how misleading and dangerous they are.

Maybe just sit back and let the inevitable tragic accidents occur 

I wouldn't just limit to amateurs.

Simon Barker said:

Solar power is now mainstream.  Houses all over the country are covered in solar panels.

Yes, and DC battery storage is happening quickly too.

Simon Barker said:

They work, and it's very rare that anything bad happens.

I would be a little more cautious here, especially given some insurance companies' views aired in the past 12-18 months. But I would accept that's not "DC power cable fire risk" alone ...

Once again I find myself at odds... as a kid who actually did make fireworks, and indeed in the 6th form we made gun-cotton, sufficiently well to damage the greenhouse of the parents of one of my friends, there is a lot to be said for learning by doing.

We also got up to a lot of stuff that nowadays would be more than just frowned upon, but we also learnt a lot very fast and remained largely intact, and the one who did not was due to a moped accident.

To my mind the error  of modern society is to infantalise the 'consumer' and assume they are incapable of understanding and making any technical decision as if consumers are some blobby mass incapable of doing  anything more complex than signing a cheque,.

Some may be , but the rest really should have access the resources,  to find out how to do things well, and then allowed to either  get on with it themselves or to instruct an installed to do what they want. (I'm the sort of customer who comes at the tradesman when he arrives every morning to make sure that what he intends to do today matches my plan, not the other way round . )

The alternative is a sort  of societal brain rot. This I see it in many places,  folk in their 20s who cannot change the wheel on their car - fine if you live inside the M25 so you are within 20 mins of an AA truck and in an area of good phone coverage, terrible if you are on hols in the Scottish highlands,.. teenagers who do not know how to catch a bus, adults who cannot read a map.

I do not care if folk learn in a formal setting in a classroom or by the more traditional watch one do one teach one. but they do need to learn

Mike

mapj1 said:

To my mind the error  of modern society is to infantalise the 'consumer' and assume they are incapable of understanding and making any technical decision as if consumers are some blobby mass incapable of doing  anything more complex than signing a cheque,.

Not sure what that has to do with what we're talking about?

For example, is it OK to put gg type fuseholders in accessible places on battery storage systems in homes that can be simply "pulled on load" by anyone ... including children? Similarly, sitting series/parallel monoblocs without internal overcurrent protection on the floor (for example in the garage or even on boards in the loft)??

Because that's what's happening in some ("not amateur") installs?

And an educated customer or a customer who's friends felt able to offer an opinion would pick up on this and say 'that's not right' much as we all know helpful folk who will tell you that your car has a problem - even if as in  my case you already know.

Thus the wider educated collective can correct - being a far better policeman than some trade body that looks at a few jobs a year that are within a short drive and easy to look at, where a large part of their aim is to keep the subscription payment flowing.

Mike.

mapj1 said:

And an educated customer or a customer who's friends felt able to offer an opinion would pick up on this and say 'that's not right' much as we all know helpful folk who will tell you that your car has a problem - even if as in  my case you already know.

Thus the wider educated collective can correct

I do take that point in general. But that doesn't appear to be happening with Solar & Battery installations.

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).

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. PV panels, unlike mains or large battery supplies, are incapable of producing large currents (it's limited by sunlight) - even into a dead short the maximum expected is 125% of normal output current. Oversize your PV d.c. cables by a little and no overcurrent protection is needed in a simple system.

Introducing batteries does introduce the potential for much nastier problems of course, but the charge controller would usually prevent backk-flow from the battery to the panel circuit.

   - Andy.

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:

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?

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).

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?

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..