The maximum value of safe current allowed to flow through a fuse wire before it melts is called fuse rating. Fuse rating depends upon the thickness and length of fuse wire.

A fuse of proper rating as per the appliance need should be used in circuit. If a fuse of higher rating is used, it will not blow and protection of the circuit will be achieved. If a fuse of lower rating is used, it will blow before the circuit is fully loaded. 
In a live wire, we should use a fuse of rating slightly higher than the maximum current that can flow through it. A fuse of 5 A rating is used in power line meant for bulbs, fans, etc. 
A fuse of 15 A rating is used in line required to feed appliances of power 1000 watt or more. 

To understand the working of an electric fuse, take a thin strip of aluminium foil about 5 cm in length. Fix its two ends on the tips of the two iron nails placed vertically on the table. Connect the two nails to a battery through a plug key and a small bulb, as shown in Fig.

Fig. Action of electric fuse
As we switch on the circuit, the aluminium strip burns out immediately. The circuit breaks and current stops flowing. This is how an electric fuse works using the heating effect of current.

Electric fuse commonly used in domestic circuits are of cut-out type shown in Fig.(a) while those used in electric appliances such as TV, computers, music systems etc., are of cartridge type shown in Fig. (b). The circuit symbol of an electric fuse is shown in Fig.(c).

Fig. Electric fuses