DC arc fault fire - have you had one?

[paul2884]

I think the tern ARC is probably misleading in this context, as said before you really need a higher voltage to get Arcing problems but a corroded joint or undersized/shorted cable will still melt and catch fire without arcing.

The fires on PV systems most likley start with a loose or corroded joint then start arcing due to the higher voltages present but they may well have cought fire anyway.

In many cases the protecrive device (fuse or MCB) wont even see a problem, if you have a 100a protective device and a poor/loose connection you could easily generate enough heat to start a fire with a current way less than 100amp!

[Robint]

Yes Paul, I was referring particularly to circuit breaker switches used to interrupt a fault current. Thats where you can get an arc flash, cos the distance of separation is not big enough to stop a 24v arc continuing.


whatever floats your boat.

[dfelsent]

It’s tough at 12 v to maintain an arc but it can be done. At 24 v not so hard. Look at battery powered stick welders. That’s what they are.
I’ve been fortunate to never see a sustained arc on a boat.
Ashore I’ve seen some real doozies as I used to work with high voltage high power systems.

[Shaneesprit]

I wouldn’t really describe 48VDC as Arc flash, unfortunately there was a big AC flash over today. Just by luck I wasn’t there personally. No one injured.
God there have been so many over the years - not many were my fault, hopefully none, well except a bit of control wire, when I was less experienced oh and an Engine harness, but I kind of did that one on purpose.after someone spent a year replacing blown fuses. It was one of about 100 issues I had to solve.
A few capacitors failed to smoke and fire. I rebuilt a VW after a 12VDC Battery fire (might have been aided by a fuel leak) Gasoline and batteries don’t mix, but there was nothing left to decide where it started. I seem to remember the steering wheel and the gearbox survived. Engines often have the fuel directly above the starter motor, which is helpful.
Solar like a battery there is no way to turn it off without a fuse and often a fuse will be further away from the source than the fault.
Protection also doesn’t always work as planned either as the fault might not go directly to full fault current. and so a slow burn begins
Ideally the cables are in an environment where they and the surroundings don’t readily add fuel.
I am sure I can list many many more, just that’s kind of the environment I have lived and breathed for the last 30 years 99.99% of everything goes as planned.

I would say the 80VDC is mentioned in the Regs because it’s below the point where DC voltage becomes an issue to your heart, ability to pass 50ma across your heart which can equal death. Normally considered to only happen above 80VDC

Less than 80VDC is Extra Low voltage,
above 80VDC is Low Voltage
above 1500VDC is High Voltage

[Na Mara]

The nearest I have come to meeting my maker along with the other 4 soles aboard at the time was when the fuse holders for the main fuses on my boat melted without the fuses blowing due to old batteries failing whilst the alternator was trying to charge them. There was a bit of smoke and some bad smells before the fuses finally melted open the circuit.

This happened at 11pm in the middle of the Northsea in a force 7.

Since that day I do NOT f-around with DC onboard. Everything is fused, everything is overrated except the fuses, everything is top-quality victron or equivalent and everything is monitored up the ying yang. I will not make the same mistake again.

[wholybee]

I think we are conflating an arc flash with other electrical issues and fires. An arc flash would be two conductors, not touching each other, that arc. That requires a much higher voltage than a fire caused by a short, failed component, or a fuse that blows but doesn't stop the current.

Less than 80VDC is Extra Low voltage,
above 80VDC is Low Voltage
above 1500VDC is High Voltage

That will vary depending on the standard applied. I don't think any standard that considers 80V low voltage would be applied to a yacht.

From wikipedia:

In electrical power distribution, the US National Electrical Code (NEC), NFPA 70, article 725 (2005), defines low distribution system voltage (LDSV) as 0 to 49 V..

The NFPA standard 79 article 6.4.1.1[4] defines distribution protected extra-low voltage (PELV) as nominal voltage of 30 Vrms or 60 V dc ripple-free for dry locations, and 6 Vrms or 15 V dc in all other cases.

Standard NFPA 70E, Article 130, 2021 Edition,[5] omits energized electrical conductors and circuit parts operating at less than 50 V from its safety requirements of work involving electrical hazards when an electrically safe work condition cannot be established.

UL standard 508A, article 43 (table 43.1) defines 0 to 20 V peak / 5 A or 20.1 to 42.4 V peak / 100 VA as low-voltage limited energy (LVLE) circuits.

[GordMay]

The NEC only requires an ARC FAULT detector [for buildings] on all Photo-Voltaic systems, operating at 80VDC, or more.
I presume that most DC faults, below 80VDC, are "BOLTED" faults [short circuits]; not arcing faults.
I could very well be wrong.


“NEC 690 Photovoltaic Power Systems” ~ Institute of Electrical and Electronics Engineers [IEEE], 26 May 2022

”... Arc fault protection is required for any PV system with PV circuits operating at 80 VDC or greater (between any two conductors) [690.11]. ...”

More ➥ https://events.vtools.ieee.org/m/314475

“Article 690 -- Solar Photovoltaic Systems, Part 2" ~ by Mark Lamendola*, Based on the 2020 NEC
➥ National Electrical Code Tips: Article 690, Solar Photovoltaic Systems, Part 2

[Shrew]

Originally Posted by Robint

Im just floating this topic to hear about anyone's experience with a dc fault current fire (ie where the fuse or circuit breaker failed to extinguish the fire) in your system (hope you had some cutters at hand to cut through the cable).
Maybe you had a large flash which vapourised the fuse holder?

The assumption here is that the short is supplying the fire. It is not.

The wiring heats for really two basic reasons:

Arc (unexpected short through contact)

Heat (overloading)

I'd hope that either would trip a breaker.

However, the arcing or the heat is the catalyst that starts the fire. The actual fire isn't electricity, nor is electricity the fuel. What is burning is the adjacent plactic sheathing, wood, and plastic. Cutting the electrical circuit no more puts out the fire than blowing out the match would.

Step #1, Cut off the power to the circuit

Step #2: Put out the fire.

[nfbr]

We are 48v and I setup the 56v alternator and WS500 with control / sense wire fusing as per design. Mistake. I fused as close to the current source as possible using waterproof inline spade fuses, then blew one with a dumb error. Thr arc did quench, but the fuse holder died in the process. Melted and badly carbonised. Could have been worse.
Most spade fuses are not suitable at 48v. I switched to glass inline, which I normally avoid but seem best for this low current application.

When I redesigned I changed the wiring design to significantly reduce these possibilities.

The dual alt WS diagram is good
The default diagram I don’t like.

[BayouSailing]

It doesn't fit in any of these categories, but it makes for a interesting story:

YEARS AGO ... (25+), my overland sailboat tender was a GMC Yukon. Kind of like a suburban but shorter, only two doors up front.

Of course, I tended to carry a bunch of stuff in the back depending on which project I was working on. Usually had a fire extinguisher in there too, harkening back to even longer ago when I did volunteer fire/rescue/emt work.

On this particular day, one of the items in the back was a Group 27 or Group 31 battery, either AGM or Gel Cell, no matter. Probably 900~1000 CCA and 12V. Pretty standard.

Also back there, very near the battery, was a can of WD-40. The good stuff. The old kind, propelled by propane not CO2.

And on a pile next to that, was a trusty hacksaw.

Many miles were safely navigated, until I turned into my buddy's dirt/sand/lumpy driveway. About half way down, I heard arcing in the back, and quickly saw in rear view mirror the shatter of sparks. It was immediately followed by a whoosh, and large ball of flame.

Suffice it to say, I did not park by his barn, nor by his house, but rather in the middle of a field. Quickly. Popping the rear lid, first out was the WD-40 torch which had melted it's lid and was spewing flame. Threw that 30-40yds in a direction. Next came the battery, also on fire (plastic), which was tossed a considerable distance considering it's weight; more than I ever thought I could toss something that heavy, in the opposite direction. A few quick pats extinguished the smoldering upholstery, which is still in the truck today.

And there laid the hacksaw, it's blade severed as if a fuse (which, in fact, it was). But not before melting the nozzle on the WD-40, which blew against the battery case, until both could be tossed.

I remember walking into the joint (it was a bed and breakfast), where the staff was just sitting down for 9AM breakfast after service for guests. I was, of course, always invited to join, and the same this day; a warm greeting, "would you like breakfast". Without a word, I stepped behind the bar, and poured something strong...and lived to tell this tale again!

RESPECT EVEN 12VDC