This is the virtue of a material resists flow of electric current. It should be as high as possible. When material is subjected to an electrical pressure then will always be a flow of current however extremely a small it is, as insulation resistance is never infinite. An insulator to which a voltage, V, is applied will have a small current I flow through it .The insulation resistance R is then given by:

                              R=V/I

Insulation resistance is of two types:

(A)Volume resistance

(B)Surface resistance

If a potential difference is applied across the faces A and B a current will flow. This current is likely to take path, straight through the material and around the material over the surface. The current that flow over the surface is denoted by I_s. The possibility of current flowing over the surface of material is due to the presence of moisture or other atmospheric or surrounding impurities.