Electrical Engineering ⇒ Topic : Breakdown Voltage or Dielectric Strength

Breakdown Voltage or Dielectric Strength

In an insulator or dielectric, the valence electrons are tightly bound so that no free electrons are available for current conduction. However, when voltage applied to a dielectric is gradually increased, a point is reached when these electrons are torn away, a large current (much larger thanthe usual leakage current) flows through the dielectric and the material loses its insulating properties.Usually, a *spark or arc occurs which burns up the material. The minimum voltage required to break down a dielectric is called breakdown voltage or dielectric strength.

The maximum voltage which a unit thickness of a dielectric can withstand without being punctured by a spark discharge is called **dielectric strength of the material. The dielectric strength (or breakdown voltage) is generally measured in kV/cm or kV/mm For example, air has a dielectric strength of 30kV/cm. It means that maximum p.d. which 1 cm thickness of air can withstand across it without breaking down is 30kV. If p.d. exceeds this value, the breakdown of air insulation will occur; allowing a large cun-ent to flow through it. Below is given the table showing dielectric constant and dielectric strength of some common insulators or dielectrics :

The following points may be noted :

• The value of dielectric strength of an insulator (or dielectric) depends upon temperature, moisture content, shape etc.
• The electric intensity, potential gradient and dielectric strength are numerically equal i.e.
•              Electric intensity = Potential gradient = Dielectric strength
  
• The breakdown of solid insulating material (dielectric) usually renders it unfit for further use by puncturing, burning, cracking or otherwise damaging it. Gaseous and liquid dielectrics are self-healing and may be used repeatedly following breakdown.
• For reasons of safety, electric field applied to a dielectric is only 10% of the dielectric strength of the dielectric material.

Note. To avoid electric breakdown of dielectric, capacitors are rated according to their working voltage, meaning the maximum safe voltage that can be applied to the capacitor