Question

The fuse wire should be placed in the path of ______ wire.
A) Neutral
B) Phase
C) Earth
D) None of these

Answer 1

Hint: Removing the phase wire completely isolates the entire wiring from the supply voltage whereas removing the neutral wire still keeps all appliances at high voltage.
Complete step by step answer :
A fuse wire is used to prevent short circuits and high currents from damaging the wirings and causing a fire. It also prevents electrical devices from getting damaged during high currents (lightning).
A fuse consists of a wire of low melting point kept in series with the circuit. When a high current flows through the wire, it melts and breaks the circuit.
 Let us look at a normal household wiring as shown in fig1. If all devices worked properly, Only a standard amount of current flows through every wire.

fig1
Now let's say that device B failed at some point and somehow created a short-circuit. This means that the phase and neutral wires of supply are directly connected and there exists only a negligible resistance between them. This creates a very high current flow, which could lead to heating up of wires and eventually fire.
It seems that we could prevent this damage by placing a fuse anywhere in the circuit - either in the phase wire XY or in the neutral wire X'Y'. But if the fuse was placed in the neutral wire, we have two major issues.
Firstly, the neutral wire is just one path for electricity to complete its path. In every household wiring, the ground wire and neutral wire are connected near the main switch to ensure the neutral and ground are at the same potential. So connecting a fuse does not prevent the current flow. It can still flow through the ground as marked in the figure to complete the circuit.
Secondly, say someone accidentally touches the shorted device. If the fuse was on the phase wire, no potential would be present on the device and It would have been safe. But now, the device is at the supply voltage, and touching it could lead to electric shock.
So we see that for overall protection, we need to keep the fuse at the phase wire. So the correct answer is option B.

Note: The same explanation applies to switches as well. A switch should never be installed on the neutral wire because - If the switch was at the neutral end, switching off the bulb would still keep the holder at high potential. If someone accidentally touches it while replacing a bulb, he will be a path for current to flow.