Where cause is unknown I closely inspect fuses to gauge whether
mechanical/aging or thermal failure and then an idea of rupture current.
This ELU 20mm 500mA F fuse had no trace of the wire , no smoke stain or
hazing at all in the glass tube. I cracked the glass expecting to see
remnants of wires or powder or something in the ends but nothing, all
"evaporated". Looks like the construction is "festoon bulb" ie totally
glass , sealed envelope with metal end caps added, something to do with it?
, I've not ground-off or heated off the end caps yet.