a] Derive an expression for the equivalent resistance of three resistors RI, R2 and R3 connected in a parallel.
b] Fuse of 3 A, 5 A and 10 A are available, calculate and select the fuse for operating electric iron of I kW power at 220 V line.

Explanation

b] Given: P = 1 kW, V = 220 V.
P = VI
`rArrI=P/V=1000/220~~4.545A`
Thus, the iron is supposed to function at a current of approximately 4.5 amperes. If the current is much more than 4.5 amperes, the electric iron can get damaged. We cannot use a fuse of 3 A. It would prevent the iron from operating at its desired capacity. We also cannot use a 10 A fuse. It wouldn.t provide protection if the current becomes, say, 7 A or 8 A. Thus, we must use the 5 A fuse. It will protect the iron from voltage and current overloading. Answer is 5 A fuse.