I understand that it's a bit startling to think of a fuse as a resistor. I realized that they were the same sort of thing when I was teaching a class one evening and found that there wasn't any other way to explain their operation.
My mention of a 'heat sinked' fuse wire was meant to emphasize that if the wire inside a fuse somehow stays cool, it won't blow.
Fuses aren't rated in power because the load on a fuse varies with the circuit voltage. If you're an electrician in North America and only deal in 120v circuits, then indeed fuses could be rated in watts. A 15 ampere fuse would blow if the power dissipated in the circuit was greater than 15 amperes x 120 volts = 1800 watts. But the very same fuse could be used in a lower voltage circuit: a 15 ampere fuse could be used in a 12v circuit, in which case it would blow if the load dissipated power in excess of 180 watts.
To confuse matters further, power doesn't melt the fuse wire: energy does. Energy = power x time. If the time is very short, then the energy is very small. This means that a very high current that persists for only a brief period will not blow the fuse. Fuse manufacturers provide a 'time-current' plot to let engineers how long it takes the fuse to stop the current for a particular value of overcurrent. When equipment is being designed, the engineer has to figure out how much overcurrent the equipment can handle, and for how long. Then he uses the time-current graph to select a fuse that will blow before the equipment is damaged.
This 'time-current' business is pretty startling in one application. Laboratories that produce artificial lightning use thin (#24, bare, tinned) wire to carry the lightning currents. The currents are very high--in the thousands of amperes (ever hear of a kiloampere?) But the duration of lightning currents is around 100 microseconds. Power x time = energy is thus very small, and these utterly inadequate-looking wires work just fine.
A good example of a truly strange fuse is the old VW fuse. Had a ceramic body with the fuse wire on the outside. Sprayed molten metal on the rest of the fuses, sustained damage from the dust and crud in the car, and generally was horrible. Nobody ever explained why they were made this way, though it's possible that the fuse wires were replaceable. Industrial fuses are often made with replaceable wires.
The unknowns vs. equations business I mentioned in my earlier post was something that you'll run into in some first-year electronics courses.
You're given a roll of fuse wire whose resistance is r/x ohms per meter, and which will melt when its temperature reaches 200 degrees C. You're asked to come up with a fuse that will blow when its current reaches 10 amperes. You have to know the heating characteristics of the wire to solve the problem. Generally it's easier to just experiment.
M Kinsler
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