How does a fuse interrupt excessive current?

A fuse places a deliberately designed conducting element in series with a circuit. Series means the circuit's current passes through that link. During ordinary operation, the element remains intact and the conducting path stays complete.

Current transfers energy to the element through resistance. Excessive current can heat it enough to melt, depending on both current and duration. A fuse therefore does not behave like an ideal switch that opens instantly whenever any current passes one simple number.

As the element separates, an electric arc can briefly conduct through the gap. Complete interruption requires that arc to clear as well. Once clearing is complete, the conducting link is broken, and current cannot continue around the series path.

Once the path is broken, the circuit current can no longer keep delivering energy through the fuse to the load. That limits continued heating from the excessive current. The fuse accomplishes this by changing its own element, rather than asking a reusable switch to open. Its actual clearing behavior depends on its construction, the voltage and how much current the fault can supply.