Question

what is a fuse? can we use 5 ampere fuse in 15 amp wire or vice versa ?why? name commonly used materials for making fuse.

Answer 1

fuse wire is made up of 75% of lead and 25% of tin it has low melting point and high resistivity fuse wire is used in series to control the current in a circuit
fuse wire used in a circuit has slightly higher current capacity than the calculated value
..... now I believe you've got all your answers

Answer 2

Fuse is a wire of appropriate melting point (copper etc.)connected in series.It protects the appliances by preventing passage of unduly high current by melting and thus breaking the circuit.
We cannot use 5A fuse in 15A wire as requirement of wire is more than what the fuse can handle. thus the fuse will melt even though a current less than the required value is passed in the circuit. thus a fuse should be having a slightly higher current capacity than the appliance/wire