Why to use Fuse?

Any electrical or electronic appliance is designed to take specified current. Under some circumstances, there may be drawing of larger currents by then then what they are rated for.

For instance too many appliances connected simultaneously in a circuit causing the total current drawn to be higher than can be handled by wires. This is called overloading.

The higher current due to short circuits or overloading can damage the appliances and the electric wires may even catch fire. To avoid this risk, a safety device called "Fuse" is used in series with the circuit.

A fuse is a short piece of wire of low melting point which is connected in series between Mains and the Appliance. When the current rises above the safe limit the fuse wire gets melted and makes the circuit open.

Design aspects of fuse like: Material of  wire, Dimensions etc are selected based on the safe limiting of current required.

                  

Electric power & Energy

Electric Power

Electric power is the electric work done per unit time.

1 Watt = 1 Joule / 1 Second

Though Watt is unit of power, it is too small for measuring domestic consumption of electricity. Thus we use a larger unit of power called Kilowatt.

Electrical Energy

It is usually measured in a unit called Kilowatt hour(KWh) which is amount of electrical energy consumed when an electrical appliance with a power rating of 1 KiloWatt is used for 1 hour.

1 KWh= 3.6 x 10 ^ 6 Joules

Power Rating of Domestic Appliances

The power rating of an electrical appliance tells us the rate at which the appliance uses electrical energy.
An electric iron box marked 220V, 1000W has a power consumption of 1000W at 220V AC, i.e it consumes electrical energy at the rate of 1000Joules per second when a potential difference of 220V is applied across it (which is potential difference at which electricity is supplied to our homes).  

The electrical energy consumed (in KWh) by an electrical appliance is given by the product of its power rating (in KW) and the time for which it is used(in hrs).