Creepage distance -The creepage distance of polymer insulator is the shortest distance along the surface of polymer insulator between two metallic end fittings.
The corrugation below the insulator is for the purpose of obtaining longer creepage path. The more corrugation of polymer insulator, the longer creepage distance, so consequently increasing resistance to the insulator leakage current.
The creepage distance required differently as to different class of pollution:
clean air may be 15 mm per kiloVolt (line voltage)
Medium pollution: 25 mm per kiloVolt (line voltage)
High pollution: 31 mm per kiloVolt (line voltage)
Flashover distance - It is the shortest distance through air between the electrodes of the polymer insulator.
Flashover voltage - The voltage at which the air around polymer insulator breaks down and flashover takes place shorting the insulator.
Puncture voltage - The voltage at which the insulator breaks down and current flows through the inside of polymer insulator.
An polymer insulator may fail caused by excessive electrical stress, excessive thermal and mechanical stress or degradation. Failure due to flashover is generally temporary and self-restoring. But failure caused by insulation puncture is permanent and the polymer insulator is damaged and required to be replaced. An polymer insulator which have internal defects like voids and impurities, reduces the electrical strength of the insulator.
The flashover may results in damage of polymer insulator glaze which can be repaired. In polluted regions contaminants deposit on the surface of the insulator that results in reduction of the flashover voltage of the insulator in wet condition.
The other important electrical parameters of polymer insulator are Electromechanical failing load, lightning withstand voltage and switching impulse withstand voltage etc..
Different design of polymer Insulators are available for different applications.