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Electrical Properties of Plastics
Dielectric Strength is an indication of the electrical strength of a plastic as an insulator. The dielectric strength of an insulating material is the voltage gradient at which electric failure or breakdown occurs as a continuous arc (the electrical property analogous to tensile strength in mechanical properties). The dielectric strength of plastics varies greatly with several conditions, such as humidity and geometry, and it is not possible to directly apply the standard test values to field use unless all conditions, including specimen dimension, are the same. Because of this, the dielectric strength test results are of relative rather than absolute value as a specification guide.
The dielectric strength of polyethylenes is usually around 500 volts/mil. The value will drop sharply if holes, bubbles, or contaminants are present in the specimen being tested. Dielectric strength varies inversely with the thickness of the specimen.
A specimen is placed between heavy cylindrical brass electrodes which carry electric current during the test. There are two ways of running this test for dielectric strength:
1) Short-Time: the voltage is increased from zero to breakdown at a uniform rate, 0.5 to 1.0 kc/sec. The precise rate of voltage rise is specified in governing material specifications.
2) Step-By-Step: the initial volt age applied is 50% of breakdown voltage shown by the short-time test. It is increased at rates specified for each type of material and the break down level noted.
Breakdown by these tests means passage of sudden excessive current through the specimen and can be verified by instruments and visible damage to the specimen. ASTM D-149.