robbob2112
Doing more research, mosty harmless
I wondered about this as well:
Looking closely at the shunt, the plates have small cuts in them, I can imagine under high enough current, there could be arcing across this gap quickly creating heat. If the shunt bar were to crack across, but remain close enough for the arc to persist, the breakers may not trip, and the arc under high amperage conditions quickly heats up to ignition temperatures. As @Hedges pointed out, one breaker appears to have been heated to the point it melted, and perhaps couldn't trip.
If all 93 PV panels were connected to the system, there could be 1,000A (51.2v) available to charge the packs. OP noted 300A shunts, and from another thread, issues with breakers shutting off. We can imagine 1,000A across 10 packs = 100A each pack,{OP says highest current 60A, so perhaps less that 93PV panels installed} but imagine a breaker trips, then a second one, a thrid; Now 1,000 Amps is charging six packs, then five, then four with the amperage per pack rapidly increasing.
Eventually something fails, either a crimp, or the shunt - as the OP says in post #1 - or a BMS fails to cut off charging at high current or high cell voltage.
Luckily smoke was noticed, emergency crews were called and responded, property damage was limited, no loss of life. So many of us now have an opportunity to review and learn something.
OP has batrium - should be able to see the individual cell voltages: this would show if any cells were dropping out of charging - ie if a breaker did trip, and if any cells hit over current or over voltage - ie if a BMS failed to protect cells.
Shunts are made by embedding the conductive strips in the end pieces, either by casting around the strips or solder/braze the strips to the ends. Then they make cuts in them to calibrate the resistance to the correct value. For the soldered ones they can reheat and move the end slightly. The strips are a copper alloy called 'manganin'. On the better shunts the ends are solid blocks of copper, on the cheaper ones they are another metal.