Failure of insulators, resulting in a interruption in the continuity of power supply, is a matter of serious concern to the power engineers. It may occur due to cracking of porcelain, porosity, puncture, mechanical stresses, flash-over etc.
1. Cracking of Insulators.
This failure is very common in case of pin type insulators and the cemented-cap type suspension insulators.
This occurs due to unequal expansion of steel, porcelain and cement during the varying conditions of cold and heat and dryness and dampness.
This develops high stresses in the porcelain near the joint and it results in tension failure. This can be avoided to some extent by using elastic cushions between the shells.
2. Porosity of Material.
Porosity in the porcelain which may be due to under-firing or other causes, always leads to failure after a comparatively short period of service.
The pores usually absorb moisture from the atmosphere or the cement, thereby decreasing the insulation resistivity of the material.
This gives rise to leakage current flowing through the porcelain, resulting in a gradual rise in the temperature until the porcelain is punctured. Such a failure can be avoided by glazing the insulator, to some extent.
3. Improper Vitrification.
This is another cause of the puncture of the material and it can be avoided by carrying out suitable routine tests during the course of manufacture,
4. Flash-Over.
The most common cause of the insulator failure is the flash-over, that causes unequal expansion of the porcelain thereby shattering the insulator with big cracks and causing interruption of the supply.
This can be avoided by providing arcing horns or rings which take up the arc and divert it away from the insulators.
5. Mechanical Stresses.
Although the compressive strength of the porcelain is quite high, yet its tensile strength is not adequate and the insulator is always weak in tension and usually fails in that fashion.
Such a failure is very rare because defective pieces are weeded out in the routine factory tests.
6. Short Circuits.
In the case of pin insulators, the birdage is very common. Birdage means the short circuiting of the conductor to earth through the large birds or similar objects.
Such a situation can be avoided by providing bird gaurds near the insulator on the cross arm, by increasing the clearance of the conductor from earthed parts or using suspension insulators, instead of pin type, where the clearance between the earthed structure and the conductor is very large to warrant any birdage.
7. Deposition of Dust.
If the insulator material is not properly glazed, the water will stick over it resulting into deposition of dust etc. over it which is partially conducting and reduces the flash-over distance.
The deposits of dust and like matter e.g. salt, cement, dust etc. on the interior surfaces can actually cause much havoc under the condition of fog and mist. This can be easily avoided by cleaning the insulators periodically.