Overhead Line Insulators Interview Questions and Answers:

1. What are the important properties that an overhead line insulator must possess ?

Ans. The most important properties that an overhead line insulator must possess are : High mechanical strength, high relative permittivity, high insulation resistance, high ratio of rupturing strength to flash-over voltage and capability of withstanding temperature variations.

2. What types of insulators are used for overhead transmission and distribution lines ?

Ans. The insulators used for overhead line are pin type, suspension type, strain type and shackle insulators.

3. What are the causes of failure of insulators ?

Ans. The insulator failure may occur due to cracking of porcelain, porosity, puncture, mechanical stresses, flash­over etc.

4. How do the insulators fail ?

Ans. Electric failure of insulators may occur either by puncture or flash-over. In the case of puncture the arc passes through the insulator body while flash-over is caused by an arc discharge between the line conductor and earth through air surrounding the insulator.

5. How are insulators tested to determine flash-over voltages ?

Ans. For determination of flash-over voltage of a given insulator high voltage supply terminals are connected to the pin and insulator and the test voltage is gradually raised to a specified value in approximate 10 seconds and is maintained for one minute. The voltage is then gradually raised till a flash-over takes place. The flash-over voltage depends upon barometric pressure, temperature, humidity etc.

6. What is meant by birdage ?

Ans. Birdage means the short circuiting of the line conductor to earth through the large birds or similar objects.

7. What is string efficiency ?

Ans. The efficiency of an insulator string, called the string efficiency, is the ratio of voltage across the whole string and the product of the number of units and voltage across the unit nearest to the line conductor.

8. How are voltage distribution and the string efficiency affected by rain ?

Ans. More uniform voltage distribution is obtained and the string efficiency is improved because of reduction in value of K [when insulators are wet, mutual capacitance C increases while the shunt capacitance C1 (except for the unit nearest the cross arm) remains constant].

9. What are the methods, that are used for improving string efficiency ?

Ans. Various methods used for improving string efficiency are by using insulators with larger discs or by providing each insulator unit with a metal cap, by using longer cross arms, by capacitance grading or by static shielding.

10. What are the two functions that are performed by grading ring ?

Ans. The grading ring performs two functions viz equalizes the voltage distribution across the insulator units and when used in conjunction with arcing horn fixed at the top end of the string serves the purpose of arcing shield and protects the insulator string from flash-over.