On Sep 28, 12:39*am, "N_Cook" wrote:
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...

If the fuse wire was tin, it may have evaporated and then oxidized.
Tin oxide is transparent.