Since the introduction of electricity in 19th century, there has been a growing demand for electrical energy. Higher voltages led the researchers to develop new types of insulator. Many natural insulators, which were widely used at the beginning of this century have been replaced by ceramic or porcelain materials. Many of these suffer low impact strength, brittleness, inflexibility, cracking during the manufacturing process, etc. New polymeric materials are advantages in such cases. Polyester resin mixed with other materials is becoming used throughout the electrical industry. However as with other polymeric insulating materials under abnormal long term stress conditions it suffers from several breakdown mechanisms such as treeing, surface tracking or erosion. Several test methods have been developed to examine the performance of polymeric materials, however usually they exhibit too many random factors associated with them, which reduce their reliability. This paper includes a knowledge of these factors and methods and tries to improve the accuracy and repeatibility of the results.