kiev wrote:All that soot and molten metal [pic# 21]looks to me that the OBC was operating with a split snubber [pic# 11] that was arcing and eroding over some length of time.
Look at the size of that gap! That looks like arc erosion has just eaten out the guts and spread it inside the box.
I agree, and was thinking similar thoughts, but not in as much detail.
Perhaps one capacitor failed first, and unusually failed not shorted but medium impedance. Operating at pack voltage, it would get hot. Amazingly, it stayed that way, eventually boiling its contents and depositing it on the PCB above and on surrounding components. The medium impedance was not enough to blow the MCU fuse.
I find it harder to explain the soot or more likely metal deposited on the waffle plate. Perhaps as you suggest @Kiev, there was arcing under the board as a result of extra long pigtails. Maybe arcing under the board happened first, heating the capacitor above and tipping it towards failure.
Finally, the second capacitor failed, and as is far more common, it failed shorted, blowing the MCU fuse and alerting the owner that there was a problem. Perhaps it was affected by the same heat that fried the first one, but failed later as it was further away from the source of the heat (either the arcing below the board, the first capacitor failing medium impedance, or both).
> And the metal spheres and soot on the waffle plate looks like there may have been arcing between the un-cut or extra-long leads at the solder joint of the snubber cap [pic# 25].
Well spotted, I didn't notice the metal spheres. It seems unlikely to me that extra long pigtails would be present from the factory; it depends on how they buy the capacitors. So this might have resulted from a botched repair job. Possibly performed from above, so there was no opportunity to do the obvious tidying up, including clipping the pigtails. The metal spheres might even be solder, resulting from inserting and soldering from above.
BTW, the numbering of the pictures was just my way of keeping track of them when reordering them. The numbers are actually in front of the img tags, so refer to the image to the right or below. But no matter, I'm glad that they served another purpose.
Edit: the numbers indicate the order of the original images. Pity it's now such a pain to find a particular image (use search I guess).