The dielectric strength of thin insulating materials

The object of this paper is to record the results of an investigation which has been made at the Electrical Testing Laboratories to determine the effect of electrode area on the apparent dielectric strength of insulating materials in the form of thin sheets. The following tests were made with two, similar, flat, circular, electrodes ranging from 1/64 to 6, 10, and 15 in. (0.39 mm. to 15.2, 25.4 and 38 cm.) in diameter and placed directly opposite each other: 1. On insulating cloth and thin sheet hard rubber in air. 2. On insulating cloth in air and in transformer oil. 3. On transformer oil with various spacings. 4. On air with various spacings. The results of the various tests show very conclusively that the apparent dielectric strength of insulating materials in thin sheet form is materially higher with small electrodes than with large ones. This probably applies generally to all dielectrics, gaseous, liquid and solid, although the amount of variation differs widely with different materials. The tests emphasize the need for standard specifications for testing insulating materials, especially when in the form of thin sheets. It does not seem probable that the dielectric strength of an insulating material, under all working conditions, can ever be predicted with exactness, but at least we can have standard methods of rating such materials so that a value for the dielectric strength will have the same significance to the manufacturer, the purchaser and the designing engineer.