Wire inside a glass breaks when subjected to high current, doesn’t let the rest of the circuit suffer what it has been through.
Imagine fuse like a bridge between supply and “circuit town”. All the components need the daily dose of current to work, BUT the overdose might kill them (or just make them sick). So this bridge is made in such a way that when a heavy dose of current comes through, the bridge collapses. The fuse sacrifices itself to protect the rest of the circuit.
This over-current doesn’t always come from the power source, sometimes heavy machinery pull more current than what other components can handle.
What is a fuse:
Fuse is a single piece of wire that breaks because it can’t handle the excessive current, it just can’t! Some say its being over-dramatic, but that’s exactly what we want.
Some fuses break suddenly. Some take a dramatic pause before breaking, these ones are called “slow blow” fuse because they blow slow.
Length of the wire matters too. If too short; after blowing,high voltage may create a spark and jump from one end to other, completing the circuit again.
Why it works:
Example: Fuse rated 10A (amperes) is subjected to 15A of current, more current moves through the wire than its meant to handle. Movement creates friction. Friction creates heat. Heat causes melting. Every material has its melting temperature. When the wire in the fuse reaches that temperature, BOOM!
The wire lives inside a safety sheathe to minimize the affect of blast when wire burns open with violent force and tries to destroy the universe (
although its not that powerful).
Fuses that resurrect:
Circuit breakers are like fuse + switch. When subjected to “over-current”, the switch turns off.
They have different mechanism than fuses (so they aren’t exactly “fuses” that resurrect). Some use two metal strips connected back to back that break connection because of heat. Some use magnetism.
They can be turned on manually and used again.
For detailed information: http://www.allaboutcircuits.com/textbook/direct-current/chpt-12/fuses/