Electrical

And the perfect answer is.... it depends.

For Low Fault Level:

Breaker opeartes faster than the fuse.

For High Fault Level:

Fuse operates faster than the Breaker.

As my mentor, Mr. Iyer puts it: "At low fault levels, fuse is vulnerable and at high fault levels, breaker is vulnerable"

There used to be current limiting fuses that limited current in less than 1/2 cycle at 60 hertz. These fuses are probably faster than a mechanical circuit breaker. Their problem was that they usually cost more than the device they were protecting.

There are also electronic circuit breakers that open in less than 1/2 cycle.

There are so many types of circuit breakers and fuses that it would be best to compare the circuit breaker and fuse that you are considering using.

The operating speed of the protection devices is inversely proportional to the replacement cost of the equipment it is protecting multiplied by the difficulty in replacing it to the power of the urgency.

In short, the more expensive and difficult the equipment being protected is to replace the slower the protection will operate, and if the repair is urgent <cringe>.

Of course a fuse or breaker is only as good as its curve selection and upstream discrimination, improper selection can easily eliminate any speed or protection level advantage.