Re: tracks as fuses ?
A track would be a very poor candidate for a fuse - as far as I know there is no study that will tell you width versus current required to burn the trace open, there are too many variables - if the trace is on the surface you would not know exactly how much copper had been plated on it, and on the inside of the board the factors such as foil vendor, thickness of oxide layer, or the amount of roughness on the laminated side facing the core material. A fair amount of damage would be caused due to the trace heating with carbon being formed in the area, that would probably render the pcb unreliable if not useless. Pcb fuses are very inexpensive- that is what should be used.
SiGiNT