Power cables are of importance in power transmission and distribution systems. During cable production, the procedures must be followed at every stage of production and quality control must be done precisely to have the cable quality completely by design, without defects. However, during the manufacture and operation of cables, defects may appear in different locations, shapes and sizes in the insulating material. The influence of defects on the electric field distribution is the causes of failure in the cable and electrical system. The change of electric field distribution when the defects are air particles and copper impurities in XLPE cable insulation are analyzed in the paper.