Surface tension as a guide to the prediction of breakdown in liquid dielectrics

In the design of electrical equipment to be operational in dielectric liquids, a knowledge of the effect of impurities (water, solvents), temperature, and pressure on the breakdown of the fluid is needed. The impurities can be introduced through leakage from the outside environment or from degradation and extraction of insulation within the sealed system. The dielectric breakdown of the liquid may be used as a measure of fluid stability. The mechanism of the electrical breakdown of dielectric liquids as given by Krasucki¹ is based upon vapor formation in the liquid and is related to the square root of the surface tension, hydrostatic pressure, and dielectric constant, and the radius of the conducting particle. Temperature will also affect breakdown by its influence on the surface tension and dielectric constant. Breakdown is also sensitive to water impurities^2,3 that affect surface tension. The water impurity does not affect the refractive index of the oils much, so the water content would affect breakdown mainly through changes in surface energy per unit area of the liquid as an aid to bubble formation. Surface energy per unit area can be calculated^4,5 for a pure liquid and requires surface tension versus temperature data (see Figures 6 and 7) where the oils as a first approximation are assumed to be pure fluids.