A fuse protects a circuit by deliberately destroying itself
Inside a fuse sits a thin strip of metal designed to be the weakest link. Normal current barely warms it. Too much current heats it until it melts, breaking the circuit before wires overheat or equipment fails. The fuse is gone afterwards, but everything else survives.
A fuse guards against overcurrent. Its heart is a wire or strip, narrower than the circuit's own conductors, fixed between two terminals and usually sealed in a housing that will not burn. Wired in series, it carries every bit of current flowing to the protected equipment. Its own resistance generates heat, and its size is chosen by experiment so ordinary loads leave it cool. Excess current drives the temperature up until the element melts outright or melts a soldered joint inside, opening the circuit. After that it must be replaced or rewired.
The idea is old. Louis Breguet suggested using thinner sections of conductor to shield telegraph stations from lightning, the slim wire melting to save the equipment indoors. Fusible wires and foils were protecting telegraph cables and lighting by 1864, and Thomas Edison patented a fuse in 1890 as part of his power distribution system.
Making one behave predictably is subtle. Elements are made of zinc, copper, silver, aluminium or alloys, and ideally carry their rated current forever yet melt fast on a small overload. They must shrug off harmless surges and not corrode or drift over years of service. Some are shaped to concentrate heat; large ones split the load across several strips. A dual-element fuse combines a strip that vanishes instantly in a short circuit with a low-melting solder joint that responds to smaller, prolonged overloads. Springs can yank the fragments apart faster, and silica sand or insulating liquid helps snuff the arc.
Timing depends on how much current flows. At double its rating, a standard fuse opens in about a second, a fast-blow type in a tenth of a second, and a slow-blow type only after tens of seconds. Thousands of designs exist today. Circuit breakers have taken over many jobs, but fuses remain the choice where space, robustness or cost count, and wiring rules usually cap the rating allowed on a given circuit.
Source: Fuse (electrical)
