A fuse blows when the current flowing through it exceeds the safe limit for that particular fuse. The purpose of a fuse is to protect the circuit from excessive current, which could cause damage to the wiring or potentially start a fire. Fuses are made of a metal that has a low melting point, so when the current flowing through the fuse becomes too high, the metal will melt and break the circuit. This interrupts the flow of current and protects the circuit from further damage. Fuses are designed to blow before they cause any damage to the wiring, so if your fuse blows, it's important to find out what caused the excessive current in the first place and correct the problem before replacing the fuse. Otherwise, you'll just end up blowing another fuse.