Some limitations of current electrical insulation test methods

The standard comparative and absolute dielectric tests in the U.S.A. for solids, liquids and gases are those of the ASTM.¹ These are comprehensive and delineate the significant tests which should be carried out to specify the relevant electrical characteristics for a wide range of conditions of voltage, application and measurement, conditioning, surrounding medium and exposure to coronas, arcs, moisture and gaseous, liquid and solid contaminants. However, recent developments in high power pulse technology and the extension of generation and transmission voltages towards the megavolt region have brought to attention some limitations in these test methods. This paper is concerned with just two; namely, area of electrodes and volume of the dielectric stressed to greater than 90% of the test voltage. The main problem is that recent tests have shown that the dielectric strength of liquids and solids is a function of both of these parameters and, therefore, meaningful extrapolations become difficult, particularly if the relationship varied from one dielectric to another.