(a) When does a fuse cut off current? How does it do it?
(b) What is the maximum number of 60 W bulbs that can be run from the mains supply off 220 volts if you do not want to overload a 5 A fuse?

(a) An electric current is cut off by a fuse when the current exceeds a safe value (due to short-circuiting or overloading). 

When the current exceeds a safe value or becomes large, it heats the fuse wire too much. Since, the melting point of fuse wire is much lower than other conducting wires, therefore, the fuse wire melts and breaks the circuit. Thus, the current supply in the circuit automatically gets switched off.

(b) Given,

Voltage, V = 220 V

Current, I = 5 A

Power, P = 60 W

Number of bulb = ?

Let the maximum number of bulbs be $x$.

Power of one bulb = 60 W

So, power of $x$ bulbs, $P=60\times x\ watts$

We know that:

Substituting the given values in the formula we get-

Thus, 18 bulbs can be run in the given case.