Electrical Engineering ⇒ Topic : Breakdown Voltage or Dielectric Strength
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Sunita
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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 :
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 | |
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