House burned down

MattiFin said:

Fuse also needs to break only 3 volt potential, again very easy task for even something less capable than class-T.

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Good point here as well 3v doesn't like to arc

Shimmy said:

So in an inverter short scenario how does the battery not get disconnected from the inverter... and why is the fuse considered the ignition source? The fuses are significantly less likely to see currents in excess of their interrupting rating as currents are shared and external resistance comes into play.

For an unmaintained system I get how there could he multiple failures that went unnoticed, but that doesn't seem to be the case here.

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Shimmy said:

why is the fuse considered the ignition source?

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Because fire dept. Guy thinks that was the hottest spot. And that is usually where the fire originates. There is probably some truth to the location, tho, not necessarily the fuse.

Zwy said:

The OP did the right setup but used a fuse between batteries that was not a Class T. I'm quite certain this is the cause of the fire.

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I don't think the fuse is to blame. The OP reported that some of the cells had leaked and he had to replace them. Most likely some cells shorted.

3V could pull a lot of current with the primary resistance being the thick cables we all tend to use. I don't know if it is enough to maintain a plasma across the distance of the gap in these non-T fuses though. Seems like there needed to be some sort of flammable material already present when the fuse might have arced.

Solar254 said:

I don't think the fuse is to blame.

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I look at it this way, if the fuse had not melted and not created and arc, there would not have been a combustion source. I have had leaking LFP batteries before, but they did not self ignite. Yes the cells shorted, and that was what caused the fuse to melt. Shorts themselves cause current to flow but do not necessarily create combustion if a fuse does its job. I may be splitting hairs but from a risk management viewpoint I came to the conclusion a Class T fuse would have mitigated the risk of a short.

SilverbackMP said:

Contactor needs to be in umm contact in order to function. If the fuse blows, then the contactor is pointless. REC manuals and my (former) industrial electrician are in agreement on placement.

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It's a relay. Doesn't matter where in the circuit it is to operate. You shouldn't have anything between the positive of the battery and a class-t but as short as physically possible cable that's sized large enough to carry the current surge that will open the fuse.

Ampster said:

I look at it this way, if the fuse had not melted and not created and arc, there would not have been a combustion source. I have had leaking LFP batteries before, but they did not self ignite. Yes the cells shorted, and that was what caused the fuse to melt. Shorts themselves cause current to flow but do not necessarily create combustion if a fuse does its job.

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I believe it's a chicken/egg thing. We don't know that the fuse was the combustion source. The fuse could have blown because wires melted from the fire and shorted.

Brett V said:

It's a relay. Doesn't matter where in the circuit it is to operate. You shouldn't have anything between the positive of the battery and a class-t but as short as physically possible cable that's sized large enough to carry the current surge that will open the fuse.

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Meh, argue with REC. There’s verbiage besides the schematics but I can’t be arsed to look for it.

So are we really saying that a previously happy and functioning cell just randomly, without abnormal load, shorted itself to an ohm value low enough to raise temp and vent? and after this, the parallel strings fed into this shorted cell and blew the fuse causing a spark?

Im as interested as any in this but, I have dealt with MANY Li Cobalt, Lipo with internal cell shorts. Hundreds of them. I use to go thru old laptop batteries and find the good cells after testing 500 (I charged every one that arrived over 1.5v) cells I took the best in mAh and put them in strings. ALL internal shorts, start very small, almost unnoticeable. The only early indication in my experience is slightly warmer than normal while charging.

Also I was using a Lipo every day for 2-3 years on an ebike and only when I let the battery sit for 2 YEARS UNUSED, did I find out it had a bad cell.

I am not sold on any of these hypothesis

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Yurtdweller said:

I have not read the entire thread. Apologies. My question is, do we know, did the arc start a fire that spread to the batteries? Or did incoming power overload the batteries somehow, and then the battery cells actually initiated the fire? I ask because I travel in a yurt, therefore I have my LiFePo4 cells inside my yurt with me. I cannot have an outbuilding for various reasons. Since the fire started at night, the solar was obviously not feeding excessive power into the system. This fault must have come from the battery outputting excessive power, for some reason, or from an incoming power source feeding massive amps into the system? I have four 12v packs of LiFePo4 cells (Lf280k). Each pack is individually mega-fused (125 amps) before connecting through a Lynx PowerIn bus. Each pack is BMS limited to 100 amps. I am curious as to how large of an arc can be initiated by 100 amps @ 12v, and whether it could arc across that fuse gap? I cannot conceive of any situation that could cause that battery to output enough power to jump such a gap, and since I have no incoming power besides 60 amps from my charge controller, I cannot see any chance of incoming power causing such a situation, either. But I want to try to understand what caused the situation of this fire, just in case I've overlooked something.

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One big difference between your setup and the OP is he was working at 48v verse 12v.

Doing the math -
each battery has internal resistance of 2.72mΩ before a short cell.

Put all of those in parallel and you get 0.388mΩ

Now assume the batteries are at 55.2v (16 * 3.45)

If we just do ohms law and delete one cell from one battery and figure what is left .... Either the figure is 750amps from the other 6 strings or somewhere in the neighborhood of 9000amps for an instant.

If it is 9000amps there is really nothing going to stop instant combustion.



Now - there are some assmptions here

But this brings to mind recent discussion of where to put fuses in parallel configuration .... i.e. at the bus bar or the battery or both.

In this case he had quit long cables and the fuse closer the bus bar

At any rate if the fuse blew at 0.1 seconds and the wire was 2/0 welding wire and 2 meters long - this makes the wire temp 94c... not to bad, hot but not bad.

Now - add in a megafuse (0.0004ohms) - two bolt on connections of 0.0004 ohms.

This recalculates as 119c or 246f

NOW - the mega fuse 58v data sheet says at 600% of rated amps it will blow between 0.1 sec and 1.0 sec.

Recalculate with time of 1.0 sec

The wire and fuse will be at 966c or 1769f

So the wire was probably someplace in between those numbers

and the fuse plastic if a form of ABS which ignites 349c, if it is a form of PET it ignites around 275c

Wire rated at 105c presumably.



I think this brings us back to the fuse as the hotest point in the wire and the fuse is the fuse itself.

I think I agree with the fire investigator now. The ultimate cause could have been just an out of balance cell verse an actual vent/short.

Tulex said:

The fuse could have blown because wires melted from the fire and shorted.

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Yes, anything is possible. I am just going off what the OP said about the fuse melting and arcing. He did say that the fire inspector referred to that general area as the combustion source. My takeaway from this thread was the importance of Class T fuses in the right places to mitigate the consequences of a short in a cell or in wiring.

Solar254 said:

I don't think the fuse is to blame. The OP reported that some of the cells had leaked and he had to replace them. Most likely some cells shorted.

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Yes, the fuse is to blame for the fire. The cause of the fuse failing is most like due to a cell shorting internally. Follow along with the rest of us.

Class T fuses only. If a Class T was used, and one cell shorted internally, all that would probably have occurred was the Class T fuse would blow, power would be cut off to the battery containing the bad cell. The cell may have vented but it would not have started a fire.

What part of anything I have written in this thread you don't understand? If you believe you don't need a Class T fuse for this application, then do as you desire. My posting is to possibly prevent someone from making a mistake that leads to catastrophic failure.

Zwy said:

Yes, the fuse is to blame for the fire. The cause of the fuse failing is most like due to a cell shorting internally. Follow along with the rest of us.

Class T fuses only. If a Class T was used, and one cell shorted internally, all that would probably have occurred was the Class T fuse would blow, power would be cut off to the battery containing the bad cell. The cell may have vented but it would not have started a fire.

What part of anything I have written in this thread you don't understand? If you believe you don't need a Class T fuse for this application, then do as you desire. My posting is to possibly prevent someone from making a mistake that leads to catastrophic failure.

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What if you used a Class J fuse?

Shimmy said:

The fuse provided an ignition source for sure; it is also likely that it made the fire more difficult to put out.

But in terms of a roof-cause analysis you either need to say cell failures are inevitable, and the lack of BMS string isolation was the root cause, or you say that the cell failure was an extraordinary event and the root cause.

Moreover, with cell failure you have what I understand to be an anomaly in a cell failing shorted to the point that other cells enter thermal runaway. For a cell to fail shorted are we looking at things like dendrites that escalate a small, high-resistance short into a dead short, or some catastrophic event that creates a dead short?

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We don't know what cells were used, whether these were some sub grade cells, how the cells were cycled other than most likely the discharge was not high due to the large size of the bank.

If a Class T was present, it would have blown and any current to that battery with the failed cell would have ceased. Class T blow pretty quick, I blew one today overloading an inverter with a large surge (12V system). No fire, just done in about 1/2 second as the Victron inverter attempted to start the load. No melted wires, no drama, just a blown fuse.

Ya but what if he used a class J fuse?
They blow pretty fast too.....

DPC said:

What if you used a Class J fuse?

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Have you looked at the specs? Interrupt is similar to Class T, but I'm not certain how the Class J is constructed. According to what I have seen, the Class J has a time delay. That may be too long with a very large current flowing.

Zwy said:

Yes, the fuse is to blame for the fire. The cause of the fuse failing is most like due to a cell shorting internally. Follow along with the rest of us.

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You don't know that. The fire inspector said so, but that's a convenient answer. Fuse can blow for many reasons. The reason that caused this fuse to blow could have also been the source of the fire.

Tulex said:

You don't know that. The fire inspector said so, but that's a convenient answer. Fuse can blow for many reasons. The reason that caused this fuse to blow could have also been the source of the fire.

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I'm 99.999% certain it was the fuse.

The 0.001% I'll leave up to you.

I will only add, if you don't think you need Class T fuses for this application and attempting to justify your position because you don't have Class T fuses installed, I can only wish you the best of luck.

DPC said:

What if you used a Class J fuse?

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It is time delay, you have to hit 10x the rated current to get to 1 sec. At rated current you are looking at 10+ minutes.

This is instead of 6x current for 0.01 sec

Zwy said:

I'm 99.999% certain it was the fuse.

The 0.001% I'll leave up to you.

I will only add, if you don't think you need Class T fuses for this application and attempting to justify your position because you don't have Class T fuses installed, I can only wish you the best of luck.

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Where the hell's that coming from? I have class T fuses. That has nothing to do with anything.
The only thing we do know is that the fuse blew and that it arced. We do not know for sure the order of events, nor what caused the fuse to blow.