Breakdown Voltage and Dielectric Strength

Before discussing the concept about the breakdown voltage and dielectric strength we need to recall our basic knowledge of insulators. Electrically, there are three types of material available. These are conductors, semiconductors, and insulators. A conductor can carry electricity very easily. Whereas, a semiconductor has a moderate value of conductance. But an insulator is not at all capable of conducting electricity. Rather say, it is a very poor conductor of electricity.

The cause of conducting electricity by a conductor and a semiconductor is the presence of free electrons. But the concepts of free electrons in a pure conductor and a semiconductor are different. Although, free electrons present in both of the materials.

But in an insulator, there are no free electrons available in it. Although this is an ideal condition. If there are free electrons in the insulator, the number of such free electrons is very few. Practically, we can consider it as the zero. This is the reason, for which an insulator can not conduct electricity through it.

Voltage Stress

There is always voltage stress on the insulator of a live system. When this voltage exceeds a certain value, there may be a breakdown in the molecular structures of the insulator. Hence, this may produce plenty of free electrons in the insulator. As a result, there may be a huge current flowing through the insulator. This current then may produce excessive heat energy. This heat energy may cause punctures and creaks in the insulator. And the voltage above which this phenomenon of breakdown occurs is the breakdown voltage.

Definition of Breakdown Voltage

In other words, the breakdown voltage is the minimum voltage which can electrically breakdown an insulator.

Flashover

A flashover generally accompanies the breakdown of an insulator. The flashover means the breakdown of the air surrounds an insulator during disruptive discharge. The disruptive conduction is luminous.

Definition of Dielectric Strength

Whenever we apply a voltage across an insulator, a voltage gradient appears in it. The voltage gradient is the voltage per unit thickness of the insulator. If we go on increasing the applied voltage, the voltage gradient also increases. But after a certain voltage gradient, the breakdown occurs in the insulator. The maximum voltage gradient that one insulator can withstand without rupturing, is the dielectric strength of the insulator material. The unit of the voltage gradient is in volts per meter or volts per mm. So, we express the dielectric strength in the same units.

For example, the dielectric strength of air is 3KV/mm means 1 mm thick air can withstand maximum 3KV voltage.

The dielectric strength is a very essential parameter for designing the insulation system, especially for high voltage applications.

The dielectric strength of an insulating medium depends on many factors. The first and foremost factor is the material which makes the insulator. Because different materials have different dielectric strength. Also, the temperature, impurities, moisture contents of an insulator seriously affect the value of the dielectric strength. Although, we express the dielectric strength in voltage per unit thickness. But still, this parameter depends on the thickness of the insulator.

For example, the breakdown voltage of a 1 mm thick air is 3KV. But the voltage withstanding capacity of 2 mm thick layer of air is not 3X2 or 6KV. It is somewhat different. The below-mentioned formula governs it.

Where V is the breakdown voltage, t is the thickness of the insulation and A is a constant. We know this constant as the Baur’s constant.

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