CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 14

Role of the fuse: an electric fuse is a safety device. It is a piece of wire of a metal or an alloy of suitable (low) melting point, connected in series with the appliance (in the live wire), so the whole current of the appliance passes through it. If an unduly high current flows, due to overloading, a short circuit or a sudden rise in the supply voltage, the heat produced in the fuse wire ($H=I^{2}Rt$) melts it. The circuit breaks and the current stops, so the appliance and the wiring are protected from damage and from fire. Why not a larger rating: the rating of a fuse is the largest current it can carry without melting, and it is chosen just above the normal current of the appliance (for example, a $1$ kW iron on $220$ V draws about $4.5$ A, so a $5$ A fuse is used). A fuse of larger rating would not melt when the current rises to a value that is dangerous for the appliance but still below the larger rating (for example $10$ A through a $15$ A fuse). This current would flow through the appliance and the wiring, overheating and damaging them or causing a fire, so the fuse would fail to protect the appliance.