Hi Snoopy
Fuses are protective devices. They are intended to protect a device that has a voltage applied to it.
The fuse rating needs to be lower than the maximum current rating of the connected device, but higher than the current drawn in normal use.
It would be pointless trying to protect an ESC rated at 15 Amps with a 25 Amp fuse - the ESC would be damaged before the fuse blew.? Pete is correct - if the current drawn by the circuit is less than 10 Amps.
The fuse needs to be positioned in the circuit where it will protect the intended device.
With DC circuits, the fuse will blow when the current passing through it rises sufficiently to melt the fuse media itself. ?The voltages normally used in model boats are not dangerous, but higher DC voltages are far more potentially dangerous than the equivalent AC voltage, simply because the current in the circuit continues to increase until the fuse blows.
Hope this helps.
Regards
Malc