Electric Properties of Plastic

  • Electric Properties of Plastic
  • ARC RESISTANCE

    Arc Resistance shows the ability of a plastic to resist the action of an arc of high voltage and low current close to the surface of the insulation in tending to form a conducting path therein. Arc resistance values are of relative value only in distinguishing plastics of nearly identical composition, such as for quality control, development and identification. ASTM D-495.

  • DIELECTRIC CONSTANT

    Dielectric Constant is the ratio of the capacity of a condenser made with a particular dielectric to the capacity of the same condenser with air as the dielectric. For a plastic used to support and insulate components of an electrical network from each other and ground, generally it is desirable to have a low level of dielectric constant. For a material to function as the dielectric of a capacitor, on the other hand, it is desirable to have a high value of dielectric constant, so the capacitor may be physically as small as possible. ASTM D-150.

  • DIELECTRIC STRENGTH

    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.

  • DISSIPATION FACTOR

    Dissipation Factor is the ratio of the real power (in phase power) to the reactive power (power 90 deg out of phase). It is also defined as the ratio of conductance of a capacitor in which the material is the dielectric to its susceptance or the ratio of its parallel reactance to its parallel resistance. It is the tangent of the loss angle and the cotangent of the phase angle. Dissipation factor is a measure of the conversion of reactive power to real power, showing as heat. ASTM D-150.5.

  • ELECTRICAL RESISTANCE

    Two electrodes are placed on or embedded in the surface of a test specimen. The following properties are calculated:

    Insulation resistance is the ratio of direct voltage applied to the electrodes to the total current between them; dependent upon both volume and surface resistance of the specimen. In materials used to insulate and support components of an electrical network, generally it is desirable to have insulation resistance as high as possible.

    Volume resistivity is the ratio of the potential gradient parallel to the current in the material to the current density.

    Surface resistivity is the ratio of the potential gradient parallel to the current along its surface to the current per unit width of the surface.

    Knowing the volume and surface resistivity of an insulating material makes it possible to design an insulator for a specific application.

  • LOSS FACTOR

    Loss Factor is the product of the dielectric constant and the power factor and is a measure of total losses in the dielectric material. ASTM D-150.

  • VOLUME RESISTIVITY

    Volume Resistivity is the ratio of direct voltage applied to the electrodes to that portion of current between them that is distributed through the volume of the specimen.

    Surface resistance is the ratio of the direct voltage applied to the electrodes to that portion of the current between them which is in a thin layer of moisture or other semi-conducting material that may be deposited on the surface.

    High volume and surface resistance are desirable in order to limit the current leakage of the conductor which is being insulated. ASTM D-257. Based on "Standard Tests on Plastics/Bulletin GlC: 4th Edition," published by Celanese Plastics Co., Division of Celanese Corp., Newark, N. J.