Why should a 5-ampere fuse never be replaced with a 20-ampere fuse?

Fuses are carefully sized to match the safe current-carrying capacity of the wiring and components in a specific circuit. If a piece of equipment is designed with a 5-ampere fuse, it means the internal wires and components are only rated to safely handle a maximum of 5 amps without overheating. Replacing that 5-amp fuse with a 20-amp fuse fundamentally defeats the safety mechanism. It allows a faulty circuit to draw up to 20 amps before the new fuse blows. This massive four-fold increase in current will rapidly overheat the thin wiring, melt the surrounding insulation, and very likely start a fire.