Question

Overheating of an insulator is due to ________ between one metal part to the other. This causes unequal expansion of porcelain, thus shattering the insulator with cracking.
1. porosity
2. mechanical stress
3. short-circuit
4. flash-over

Answer 1

Correct Answer - Option 4 : flash-over

Breakdown of Insulator:

• Insulators are required to withstand both mechanical and electrical stresses.
• The latter type is primarily due to line voltage and may cause the breakdown of the insulator.
• The electrical breakdown of the insulator can occur either by flash-over or puncture.
• In a flashover, an arc occurs between the line conductor and insulator pin (i.e., earth), and the discharge jumps across the air gaps, following the shortest distance. 
• A flashover is an electric discharge over or around the surface of an insulator.
• In electric power transmission, a flashover is an unintended high voltage electric discharge over or around an insulator or sparking between two or more adjacent conductors.
• In case of a puncture, the discharge occurs from the conductor to the pin through the body of the insulator.
• When such a breakdown is involved, the insulator is permanently destroyed due to excessive heat.
• In practice, sufficient thickness of porcelain is provided in the insulator to avoid puncture by the line voltage.
• The ratio of puncture strength to flashover voltage is known as the safety factor.
• For pin-type insulators, the value of the safety factor is about 10.