The dielectric breakdown voltage of a liquid at power frequencies is affected by the following:
1. Water content of the oil,
2. The temperature of the liquid as it affects the relative saturation level of moisture in solution,
3. The degree of uniformity of the electric field,
4. The area of the electrodes,
5. Volume of the liquid under maximum stress,
6. Insulation on the electrodes in electrical equipment,
7. Size and number of particles in the oil,
8. Length of time for which the liquid is under stress,
9. Gassing tendencies of the liquid under the influence of electric stress,
10. Concentration of dissolved gases if saturation levels are exceeded as a result of a sudden cooling or decrease in pressure, which may cause the formation of gas bubbles,
11. Incompatibility with materials of construction, and
12. Velocity of low.
A decrease in dielectric strength of the liquid can have an accentuated effect on the electric creepage strength of the solid insulating materials immersed in the liquid.
Because of the separate, cumulative, and in some cases, interacting effects of the influences previously listed, the average breakdown voltage of a liquid as determined by this test method cannot be used for design purposes. Alternate procedures utilizing electrode shapes and configurations similar to those in the apparatus and in Test Method D1816 should be used for oil-filled equipment utilizing well-rounded and insulated electrodes.
