Dielectric Strength

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 Dielectric Strength

Dielectric Strength reflects the electric strength of insulating materials at various power frequencies. Or it can be defined as the measure of dielectric breakdown resistance of a material under an applied voltage and is expressed as Volts per unit thickness. It is an indicator of how good an insulator a material is.

In other words, it is the voltage per unit thickness at which a material will conduct electricity. The higher the value, the more electrically insulating a material is.

It is an important property sought for materials used in applications where electrical field is present and is a vital parameter for electrical industry applications.

Applications include:

  » Development of materials for energy storage applications

  » Dielectric materials for capacitors

  » Thin films in high speed digital circuitry

The dielectric strength depends on:

  » The type of the plastic and electrodes

  » The shape of the plastic and electrodes

  » The rate with which the field is increased, and

  » The medium that surrounds the insulator

How to Measure Dielectric Strength?

The most generally used standard tests to calculate dielectric strength are ASTM D149 or IEC 60243-1 (ofcourse there exist several other methods as well, but they are not discussed here).

The measurement of dielectric strength is usually carried out either by the:

  »  Short-time method

  »  Slow rate-of-rise method

  »  Step-by-Step method

Short-time method

In this method, the voltage is applied across the two electrodes and increased continuously at a uniform rate (500 V/sec) until the breakdown occurs. Breakdown is defined as when an electrical burn- through punctures the sample or decomposition occurs in the sample.

Slow rate-of-rise method

In this test method, the voltage is applied to the test electrodes from the starting voltage 50% of the breakdown voltage until breakdown occurs.

Step-by-Step method

The voltage is applied to the test electrodes at the preferred starting voltage in steps and duration until breakdown occurs.

Specimen Size - The recommended specimen type is a 4 inch plaque or larger. Any specimen thickness can be used.

Dielectric strength is calculated by dividing the breakdown voltage by the thickness of the sample.

Most plastics have good dielectric strengths (in the order of 100 to 300 kV/cm).

Factors Affecting Dielectric Strength

• The dielectric strength of an insulation material usually decreases with increase in temperature 
  It is approximately inversely proportional to the absolute temperature. At the same time, it is equally important to note that below room temperature, dielectric strength is substantially independent of temperature change.


• Mechanical loading has a pronounced effect on dielectric strength
  Since, a mechanical stress may introduce internal flaws which serve as leakage paths, mechanical loaded insulators may show substantially reduced values of dielectric strength.


• Dielectric Strength of an insulating material is influenced by the fabrication details
  For example, flow lines in a compression molding or weld lines in an injection molding may serve as paths of least resistance of leakage currents, this reducing the dielectric strength. Even nearly invisible minute flaws in a plastics insulator may reduce the dielectric strength to one-third this normal value.

Available at https://omnexus.specialchem.com. Acceso el 9 de octubre, 2020.