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Why does a fuse blow?

As mentioned in the other answers, the fuse blows due to too much current flowing. After the fuse has blown the circuit becomes open so a voltage develops across the fuse (usually the supply voltage like mains voltage or the battery voltage). The fuse must be able to withstand that voltage and keep the circuit open.

What if a fuse is not blown?

You (wrongly) protect this circuit with a 1 A, 50 V fuse. When the fuse is intact (not blown) there is no issue, no more than 0.5 A flows through the fuse so it does not blow. Then a fault develops in the circuit making more current flow and blowing the fuse.

Why does a 240V fuse blow?

If you apply 240V to a circuit designed for 6V, the fuse will probably blow because the current in the circuit would be multiplied by 40 (240V/6V), thus the fuse would dissipate 40^2=1600 times more power. Well, if you want to get pedantic, ultimately it's heat that causes it to blow.

Is voltage a function of a fuse?

But, it's not the supply voltage or rated voltage of the fuse, it's the Voltage drop across the fuse along the R/Temp curve. Perhaps I'm being pedantic, but a fuse's usefulness really depends on it's non-linear R combined with the fundamental Power transfer law in a series circuit and power is a function of Voltage and Current.

How much voltage does a fuse have?

The fuse has no idea how much voltage is involved. There's only a small fraction of a volt drop across the fuse. The fuse has no terminals connected to common, neutral, ground or any other voltage ref. The entire fuse floats at supply voltage. Until the fuse blows; then it has working voltage across it as long as the switch is on.

Will a fuse blow if rated current is too high?

Nobody has yet pointed out clearly that the rated current of a fuse is the maximum current that it will happily pass forever. Over that and it will blow, the greater the overcurrent the quicker it blows, depending on slow- or quick-blow design, etc. The likelihood of a fuse blowing is based mainly on how inconvenient it will be.

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