The mandril uses pulling bar made of epoxide resin plastic steel, which has good insulation ability, light mass and high intensity and so on. 2. The outer insulation layer is silicon rubber, which formed by high temperature integration pressing and has good sealing ability and high grade of anti – dust, maintenance free and so on. 3. The metal connection adopts pressing technique, which makes the insulator smaller and no damage on the mandril. It has good sealing ability and is very convenient for installation.