A "burned fuse" without being blow is a symptom of a high resistance connection between the fuse and its holder. The resistance produced heat even at currents well below the rated
current flow of the fuse. It can get more than hot enough to start a fire, and it is NOT the fault of the fuse or of a fuse that is too large.
This is typical of a a LOT of
cheap fuse holders out there, and/or one that has been exposed to
water and is showing
corrosion.
For ATO or AFC blades style
fuses, 20 Amps should be the MAXIMUM used in an inline fuse holder, higher than that you should switch to a Mega fuse or a proper terminal block fuse holder.