A substance, which will not allow the flow of electric current to pass through it is called the insulator. The conductance and conductivity is zero in insulators. Insulators are used to isolate the electric current from neighbouring parts. Insulators will not allow the leakage of current, short-circuiting current, shock to the operator and isolates the electric current safely with out any diversion to any other place.
Qualities of good insulator
a. It should be flexible
b. It should have good mechanical strength
c. It should easily moulded into any shape
d. It should not be effected by acid
e. It should be non-inflammable
f. It should have very high specific resistance to prevent leakage current
g. It should be withstand high temperature. Because insulators posses negative temperature coefficient of resistance. That is resistance decreases with increasing temperature
h. It should have high dielectric strength