When does an electric short circuit occur?

What is the electric potential of the neutral wire in a mains supply cable ?

If fuses of 250 m A, 500 mA, 1 A, 5 A and 10 A were available, which one would be the most suitable for protecting an amplifier rated at 240 V, 180 W?

In which wire in an A.C. housing circuit is the switch introduced to operate the lights?

In household circuits, is a fuse wire connected in series or in parallel?