Sounds like a fuse fail at the link to ferrule joint/connection. Gets hot as hell from the arcing (internal) heats the spring clips, deteriorates them eventually, then the wire/lug heat & deteriorate and poof...a meltdown.
An IR scanner (cheap ones available now) shows most hot spots, and is a good investment isf you can get the clients to do it.
BTW, Bussman may be interested in the "failed" fuse if you have it. I have a Bussman Fuseologist around here, and he loves to get burnouts for his lab.