A fuse consists of either a wire fuse element or a metal strip inside a small cross-section which are attached to circuit conductors. These units are normally mounted between a pair of electrical terminals and usually the fuse is cased inside a non-conducting and non-combustible housing. The fuse is arranged in series capable of carrying all the current passing throughout the protected circuit. The resistance of the element generates heat due to the current flow. The size and the construction of the element is empirically determined to be sure that the heat produced for a standard current does not cause the element to reach a high temperature. In cases where too high of a current flows, the element either rises to a higher temperature and melts a soldered joint inside the fuse that opens the circuit or it melts directly.
If the metal conductor parts, an electric arc is formed between un-melted ends of the fuse. The arc starts to grow until the required voltage to sustain the arc is in fact greater as opposed to the circuits existing voltage. This is what actually causes the current flow to become terminated. Where alternating current circuits are concerned, the current naturally reverses course on each cycle. This particular process greatly improves the speed of fuse interruption. Where current-limiting fuses are concerned, the voltage required to sustain the arc builds up fast enough to essentially stop the fault current prior to the first peak of the AC waveform. This effect tremendously limits damage to downstream protected devices.
The fuse is often made from silver, aluminum, zinc, copper or alloys because these allow for stable and predictable characteristics. The fuse ideally, will carry its current for an undetermined period and melt rapidly on a small excess. It is vital that the element must not become damaged by minor harmless surges of current, and should not oxidize or change its behavior subsequent to potentially years of service.
The fuse elements can be shaped to increase the heating effect. In bigger fuses, the current could be divided among many metal strips, whereas a dual-element fuse may have metal strips that melt right away upon a short-circuit. This type of fuse can also comprise a low-melting solder joint that responds to long-term overload of low values than a short circuit. Fuse elements may be supported by nichrome or steel wires. This ensures that no strain is placed on the element but a spring may be integrated to increase the speed of parting the element fragments.
It is normal for the fuse element to be surrounded by materials that are intended to speed the quenching of the arc. Silica sand, air and non-conducting liquids are some examples.
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