The commonly used insulating materials, satisfying the above mentioned properties are,
1. Porcelain
2. Glass
3. Synthetic resin
1. Porcelain
This is the most commonly used material for the insulators. It is a ceramic material. It is manufactured from the china clay. The plastic clay is mixed with silicon and feldspar. The fine powdered mixture of clay, silicon and feldspar is processed in the mills. It is heated at the controlled temperature. It has been given a particular shape and it is covered with glaze. The cast iron with galvanizing is used for the metal parts inside the insulators.
The porcelain is free from cracks, holes, laminations etc. its insulation resistance is very high. Porcelain is heated at the temperatures such that the insulators become mechanically strong and it also remains nonporous. The rough surface catches the dust and the moisture very quickly. Hence it is important to provide glazed surface to the insulators so that it remains clean from dust and moisture.
The dielectric strength the porcelain insulator is about 60 kV/cm.
2. Glass
The glass also can be used instead of porcelain. The glass is made tough by heat treatment which is called annealing. The glass insulators have following advantages :
1. As transparent, cracks, bubbles and defects in the insulator can be easily detected by inspection.
2. The dielectric strength is very high.
3. Low coefficient of thermal expansion hence less affected by the temperature changes.
4. Cheaper than the porcelain.
5. The resistivity is very high.
6. Simple design is possible.
7. Higher compressive strength than the porcelain.
8. Quite homogenous and withstand high compression stresses as compared to porcelain.
Still the glass insulators have the following disadvantages :
1. Chances of moisture condensation on the surface are higher which can cause higher leakage current.
2. Less stronger than the porcelain.
3. In high tension systems, the heavy mass of insulator can cause internal strain.
4. Cannot be moulded in irregular shapes.
3. Synthetic Resin
The synthetic resin insulators are manufactured from the compounds of silicon, rubber, resin etc. these insulators have the following advantages :
1. The tensile strength is high.
2. The weight is low.
3. Comparatively cheaper.
But high leakage current and short life are the main limitation of these insulators. The indoor applications and bushings are the application areas of the synthetic resin insulators.