Oil Breakdown at Large Spacings

Much work has been done on the breakdown of insulating oil at small spacings between electrodes. Information for electrode separations of several inches is not as complete. It has been found that sources of ionization external to the gap influence the gap breakdown, so that the design of electrode supports and parts is of great importance. Data on several sizes of spherically terminated rods or cylinders are presented. Short-time breakdown tests are shown to be quite erratic and a form of long-time test schedule was developed which gives more consistent results. The final test used is called a tenminute-hold and yields values for a given condition representing the maximum voltage that can be held consistently. This is of special interest in design. The empirical curves of oil breakdown are analyzed by mathematical methods. A general equation for breakdown voltage in terms of electrode diameter and separation is developed which agrees quite well with the experimental data. Evidence is presented to show that water in globular form suspended in oil may increase the breakdown potential considerably with spherical electrodes if the separation is several times the diameter.