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To amplify upon Phil's remarks, there are two problems. Th first is the uncertainty of the heat conduction away from the trace. The other is the manufacturing tolerances for very narrow traces.
I think that you also have problem with wider traces because the PCB manufacture has to guarantee a minimum thickness and has generous maximum tolerance.
I check this once when I was doing a shunt resistor for current measurement and got the response that the tolerance was something like –0% and +50% on the thickness.
IMHO pcb trace can be used as a fuse, if the requirements are not very precise. In other words, diffrence between operating current and fault current are very wide, and the purpose is primarly to prevent fire than protect some other components. For instance it mightbe good enough in a car environment, where the battery for sure delivers plenty of current in case of a short circuit, and while the load to be protected is something simple with quite predictable failure modes (short to chassis or similar).
One big trouble is that the PCB is seriously damaged when the fuse blows.
I would in most cases recommend a pcb solderable chip fuse, or other well specified "real" fuse.
Btw. be aware that the specification for nominal current can be according to several standards, and they are VERY different! The humble fuse can be a though thing to specify.
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