Question

For a system corresponding mainly to a uniform electric field, the flashover voltage is slightly lower than the breakdown voltage of air alone, for the same electrode gaps. The decrease in the flashover voltage is due to the presence of small air pockets between the electrodes and solid dielectric, the influence of moisture and the non-uniformity of the electric field resulting of the difference between the permittivity of both the solid and that of air. The non-uniformity of electric field increases with the voltage because of the surface charging that takes place in this situation.

Answer 1

First you need to know what is flash over voltage and puncture voltage.

Puncture voltage: It is the volatge that appeared inside the insulating material. The arc due to puncture voltage flows inside the insulating material and makes the insulator as conductor by damaging it. So that the insulating properties of the insulator will be lost.

Flash Over Voltage: It is the voltage appeared across the insulating materials that causes arc or flash around the surface of the insulator. It occurs mainly due to the atmpspheric factors like presence of moisture in the air, dust and ice particles forming on the surface of the insulator and the conductors.

Now, let us consider a constant voltage is appeared across the conductor and the insulator. If due to a fault occuring in the system or increase in the moisture between the conductor, then charge carriers freely travels along the surface of the insulator there causes reduction in the dielectric strength between them. The easy flow of charge carriers to spread around the insulators cause a srike in arc. This voltage at which arc appears is known as flash over voltage. As the electric field between insulator and conductor is more non uniform, the more chances are present to occur flash over such that the dielectric strength of the insulator is exceeded.

More the flash over voltage, less are the chances to strike the arc.