The unit dielectric strength of oil

IN DESIGNING FOR optimum dimensions and shapes in oil circuit breakers and other oil-insulated equipment, it has been found advantageous to determine, by means of the electrolytic field analyzer, the distribution and magnitudes of the electrical stresses which exist as a result of the application of high voltage. To make use of this information, the designer must know with engineering accuracy the unit dielectric strength of the oil; that is, the maximum electrical stress that the oil will withstand anywhere in the gap without puncture. The best generally available data on this subject have consisted of a set of empirical curves published by F. W. Peek in 1915 for oil adjacent to spheres and cylinders of different radii. The fundamental reason that oil would withstand much higher unit electrical stress when adjacent to some shapes than to others has not been understood. As a consequence, when faced with a practical shape which was neither a precise sphere nor a precise cylinder, one has not known what value of dielectric strength to use, within a range of two or three to one.