Although a material may have a high electrical resistivity value, its ability to resist transmitting an electrical current will fail once a certain voltage is exceeded. The voltage at failure is called the breakdown voltage. A material’s dielectric strength (KV/mm) is calculated by dividing the breakdown voltage value by the thickness of the material test coupon.
Typically, the dielectric strength test is performed using AC (alternating current) voltage, but the test can also be performed using DC (direct current) voltage. To specify the technique used to obtain the dielectric strength value, the value will often be expressed as AC KV/mm or DC KV/mm. Because AC voltage is a sinusoidal wave of electromotive force delivered over a specific period-of-time, AC voltage is described as the root-mean-square (rms) voltage, or Vac RMS. For example, an AC test that produces two peak 1414 volts during one sinusoidal wave cycle, will be described as 1000 Vac RMS. To produce the same instantaneous level of stress upon the test coupon as would a test at 1000 Vac RMS, a test using DC voltage would need to be performed at 1414 Vdc.
The measurement of dielectric breakdown strength is also very sensitive to test specimen geometry (e.g. thickness) and specimen quality (e.g. surface finish) as well as the test environment (e.g. temperature, humidity). Caution should be exercised when comparing data from different sources.
Regardless of which method is used, a higher dielectric strength value represents a better-quality insulator. The table below contrast various materials used as electrical insulators.
*Typical values at room temperature