A fundamental factor controlling the unit dielectric strength of oil [includes discussion]

The theory is offered that the primary factor which determines the unit dielectric strength of commercial oil of controlled quality around electrodes of different geometries is the volume of that oil under stress. The reason is basically statistical. For small volumes of oil, such as test cups, statistical variations are large and unit dielectric strengths are high. For large volumes at high spacings, such as in large power equipment, lower unit dielectric strengths must be recognized, but statistical reliability and reproducibility are greatly improved. In calculations, statistical extrapolations can be eliminated by computing an equivalent volume of oil under stress and obtaining the dielectric strength from test data on a sphere gap with the same equivalent volume. Ignoring everything but the change in dielectric strength with volume, the calculation method checks the fundamental curves in Peek's 1915 empirical data with surprisingly good accuracy. Included are changes in dielectric strength with: 1. the radii of spheres, 2. the separation of spheres, and 3. the changed geometry from spheres to cylinders.