Solid dielectrics

For measuring the breakdown voltage of solid dielectric materials, the electrode shapes shown in  fig. 1 can be used. The thickness of the sample dielectric plate is 3mm (fig. 1a). If the original sample is thicker then the excess material should be milled away from the center part of the sample (fig.1b). The electrodes are made of brass in the form of cylinders. The height of the electrodes should be at least 15mm.

The measurement of the dielectric strength of solid dielectrics can be done with the electrodes surrounded either with air or with a liquid dielectric. Surrounding with a liquid dielectric diminishes the possibility of sparks through the surrounding medium or on the surface of the solid dielectric sample.

During the measurement high voltage is applied to the electrodes and slowly increased until the dielectric breaks down and a sudden increase of current occurs. The effect of breakdown on the dielectric can be visually observed after the sample is removed in the form of an opening created by the sudden rush of current. The dielectric strength is calculated as the quotient of the measured breakdown voltage and the thickness of the sample dielectric: E_br=U_br/d. The measurement is usually repeated 5 times and the final result is calculated as the arithmetic mean value of the 5 measurement results.

Liquid or gaseous dielectrics

In the case of liquid or gaseous dielectrics the shape of the electrodes is shown in  fig. 2. The electrodes in this case can be made of copper, brass, bronze or stainless steel. The shape of the electrodes can be spherical (with a diameter of 12.5 – 13 mm), or a sphere segment as shown in  fig. 2.

During the measurement the voltage is continually increased with a speed of about 2 kV/s. The measurement is repeated 6 times using the same sample of liquid dielectric. After each breakdown the liquid between the electrodes should be mixed slightly with a dry glass stick while avoiding the creation of bubbles. The final result is calculated as the arithmetic mean value of the 6 measured results.

Created by Zoltan Losonc (feprinciples@on.mailshell.com) on 27 Jun 2003. Last updated on 6 September 2003.

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