DC arc fault fire - have you had one?

[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?


I have seen some alarming examples on Youtube of how these fires develop (but these were onshore PV systems and battery storage.

[GordMay]

DC arc faults can occur wherever there are ‘bad’ joints in the DC cables, a breakdown of cable continuity, or a breakdown of insulation.
DC arc faults only occur on systems that have [unprotected] DC voltage circuits above ± 80 Volts DC.

Originally Posted by National Electrical Code Article 690.11

Arc-Fault Circuit Protection (Direct Current).
Photovoltaic systems with dc source circuits, dc output circuits, or both, on or penetrating a building operating at a PV system maximum system voltage of 80 volts or greater, shall be protected by a listed (dc) arc-fault circuit interrupter, PV type, or other system components listed to provide equivalent protection. The PV arc-fault protection means shall comply with the following requirements:
(1) The system shall detect and interrupt arcing faults resulting from a failure in the intended continuity of a conductor, connection, module, or other system component in the dc PV source and output circuits.
(2) The system shall disable or disconnect one of the following:
(a) Inverters or charge controllers connected to the fault circuit when the fault is detected
(b) System components within the arcing circuit
(3) The system shall require that the disabled or disconnected equipment be manually restarted.
(4) The system shall have an annunciator that provides a visual indication that the circuit interrupter has operated. This indication shall not reset automatically...

“Low Cost Arc Fault Detection and Protection for PV Systems”
➥ https://www.nrel.gov/docs/fy14osti/60660.pdf

[Robint]

Originally Posted by GordMay

DC arc faults only occur on systems that have [unprotected] DC voltage circuits above ± 80 Volts DC.

“Low Cost Arc Fault Detection and Protection for PV Systems”
➥ https://www.nrel.gov/docs/fy14osti/60660.pdf

well said Gord, this relates to the implementation of Arc fault detection which is where a loose or corroded joint makes an irregular arcing connection at the same time generating heat which can start a fire. Im a bit surprised at the 80V criteria but I wont ever be operating at that level. This type of fault can happen in trucks/cars but is usually the result of amateur interference or deliberate overloading or old age corrosion



IMHO 24V is so much safer and better in many ways. We have well proven fusing systems on car(12v) and trucks (24v). But notice that they are not nearly so reliable in 48v PV systems


My OP is concerned with an Arc flash event (like a stick welder arc), once struck, it doesnt self extinguish and happens under fault current with a fuseholder or with a circuit breaker thats opened under full load, or more and cannot breaker the circuit. here's a good vid with many design mistakes because is self taught DIY

https://www.youtube.com/watch?v=H5jYRlLm_VI

some clips of circuit breaker fires.

[Kettlewell]

I've had a couple of shorts that were pretty scary. One time while on my first trip onboard a small sailboat we noticed a lot of smoke coming up from down below. The boatyard had done some engine work and had left an unfused wire laying across the engine block, which eventually melted through the insulation due to the heat. Can't remember why the line was unfused, but it created a lot of smoke when the wire shorted and then burnt off its insulation. Another time I was drilling a hole to install something and the drill bit went through the bulkhead and managed to short out a big battery cable, which created instant clouds of acrid smoke. I held my breath and grabbed a heavy piece of rubber hose, looped it under the battery cable, and then ripped it off the battery. In both of these incidents my takeaway was that you can't believe how much smoke is created how quickly, making it very difficult to even see what the problem is. We also knew some people that went diving and came back to find that their boat had burnt to the waterline and sunk in deep water. Someone rescued their dog swimming around the harbor. They had a big powerful wind generator, lots of solar, and a big battery bank. Onboard electrical fires are one of your biggest threats on any cruising boat.

[GordMay]

There's considerable literature, on the toxicity of burning wire insulation:

“Hazards Related to Electric Cable Insulation in Case of Fire”
➥ https://www.marineinsight.com/marine...-case-of-fire/

"Effects of Toxic Gases Emitted by Burning Electrical Insulation”
➥ https://www.ecmag.com/magazine/artic...cal-insulation

“PVC Sheathed Electrical Cable Fire Smoke Toxicity”
➥ https://eprints.whiterose.ac.uk/1573...ity-k19-89.pdf

[boatpoker]

Originally Posted by GordMay

DC arc faults only occur on systems that have [unprotected] DC voltage circuits above ± 80 Volts DC.

Where do you get this info Gord ? Everything I have seen from BoatUS and underwriters data is pretty consistent that about 35% of inboard small craft fires start in the DC wiring in the engine compartment ... 99.9% of which is 12V.

[rslifkin]

Originally Posted by boatpoker

Where do you get this info Gord ? Everything I have seen from BoatUS and underwriters data is pretty consistent that about 35% of inboard small craft fires start in the DC wiring in the engine compartment ... 99.9% of which is 12V.

I'd say the majority of DC system fires wouldn't be caused by an arc fault, but by other failures. So that statistic is both not surprising and doesn't disagree with Gord's info.

[GordMay]

Originally Posted by boatpoker

Where do you get this info Gord ? Everything I have seen from BoatUS and underwriters data is pretty consistent that about 35% of inboard small craft fires start in the DC wiring in the engine compartment ... 99.9% of which is 12V.

True.
I was only speaking about arc-faults in photovoltaic systems, as referenced in the National Electrical Code.

[wholybee]

I think what the OP might be getting at is the AIC rating of a fuse. Particularly with LiFePO4, the interrupt capacity of a fuse is less than what might occur during a direct short.

[SailingHarmonie]

While a "real" arc fault doesn't happen at typical DC voltages experienced on a small boat, the hazard of a high resistance connection in a high current circuit is just as real, and every bit as dangerous.

These can happen even in the best designed systems, with all the safety features that you can have, although they are much more common in systems "homebrewed" by people without the knowledge to do it right.

My favorite first hand experience of this was on a new client's boat. An inverter was installed by a previous owner. The high amperage ring terminals were installed on top of a penolic safety cover where the power studs stuck out. The ONLY contact between the stud and the wire terminal was the inner diameter of the ring! For a 3000 W inverter. When the inverter was worked hard the terminal glowed red hot. Fortunately the phenolic cover was of the fireproof variety. "Oh? Is that what that funny smell was?" And, No, the surveyor did not notice.

This is the kind of cluelessness that has some insurance companies insisting that new Lithium battery systems only be installed by "professionals".

But in any event, no circuit breaker could have solved this. No arc-fault interrupter. Nothing. Similar problems occur with loose connections, or connections that get salt water splashed and corrode. Without a good inspection and maintenance program, they are never found until the fire starts. This is the reason that insurance data shows DC power causes FAR more boat fires than propane. Yet people still think they are "safer" by swapping a propane cooker for an electric one.

I personally have seen four serious boat fires. Three were DC electrical, one was leaking gasoline. Several close calls from AC shore power inlet cords. Hardly enough data to draw a conclusion, but the trend is real...

[SV Confianza]

Only electrical fire we have had was when a custom made board by the manufacturer (privilege) got wet. The irony was, this board's purpose was essentially for fusing, but it drew its own power, and itself wasn't fused to the battery.

Hit it with a dry powder (class c) extinguisher. Made the boat smell for months while we tried to find all the crevices it went into.

[GordMay]

Analyzing Onboard Fire Claims ~ BoatUS
1. ➥ https://www.boatus.com/expert-advice...rd-fire-claims
2. ➥ https://www.boatus.com/expert-advice...fires-on-boats

Originally Posted by 2

”We analyzed five years of BoatUS Marine Insurance claim files and found that the boat's DC electrical system — batteries, lights, wiring, and so on — cause more than a third of all fires.
The boat's AC shore power system contributes to another 9 percent.
More than half of DC electrical fires, or 19 percent of all fires originating on boats, were associated with either the engine or the batteries, both of which tend to be in the engine room. That's because there are so many things that can burn in the engine room — fuel, oil from a slow leak, or even, in the case of a gasoline engine, gas fumes — once a DC wiring problem creates enough heat. In addition, that's where a lot of high-load devices are — things like the engine starter and battery chargers. These higher amperage loads create more heat where there are undersized wires, loose or corroded connections, or intermittent shorts. Finally, the vibration from the engine increases the likelihood of chafe in such vulnerable areas as the wiring harness and connections to the alternator and the starter...”

1.


2.

[Kettlewell]

I misread the post and wasn't thinking "arc fault fire." I was thinking "ground fault."

[edventuredad]

I had an electrical fire get started from the wiring harness on my old Yanmar engine. At the time, the starter wire ran up to key rather than having a solenoid. Over the years, the heat produced in that wire from starting the engine eventually broke down the insulation. Then, when an overheat happened to trigger the overheat alarm light to turn on, that circuit closed and made a direct from the battery/alternator to ground through the temp sensor switch. My son noticed that the engine didn't sound right and shut down. I pulled the housing off to find acrid smoke billowing out. It didn't take long to find the wire... it was smoking, sizzling, and nearly glowing. I pulled it off the sensor with some handy pliers. We managed to carry on without the heat sensor until I was able to rewire the whole thing, putting a solenoid in so that high current wasn't going up to the key, and properly fusing the lines. In retrospect, switching off the battery switch would have been the better, faster emergency fix than trying to pull the hot wire... it could have melted other wires in there, but the only return to ground was through that sensor. Everything is a teaching moment with a boat. I think the big lesson is that one shouldn't trust the wiring on an old boat just because it's been working fine for all these years... check things out and rewire it properly according to the latest standards!

[Robint]

Yup, fire afloat is a worst nightmare and as Capt Shakespear said familiarity breeds contempt.


A cheap battery isolator switch cost $10 and should work fine on 12vdc.. Every boat should have one. 24v needs a big one as it can start an arc flash IMHO 0r otherwise keep a pair of cable cutters handy in oiled zip bag tagged on nearby