The intrinsic electric strength of polythene and its variation with temperature

The development of a method of measuring the electric strength of polythene is described. Details are given of apparatus for moulding recessed specimens and for measuring the test thickness at the base of the recess. Systematic investigation of the factors involved in electric-strength measurements¿thickness of specimen, time of application of voltage, radius of curvature of the recess, polarity, ambient medium, and electrode material¿shows that the method gives a value of d.c. electric strength that is an intrinsic property of the polythene. The d.c. electric strength is substantially independent of specimen thickness over the range tested (0-0.008 in). The variation of d.c. electric strength with temperature shows a measure of agreement with the electronic theory of disruptive discharge developed by Frohlich and Von Hippel. At 25° C and below, it is about 17 MV/in, changing little with temperature down to ¿ 200° C; above 25° C it falls steadily to a value of about 2 MV/in at the melting point (c. 115° C). A preliminary investigation of the factors involved in the breakdown of polythene at power frequencies is described. Using the same electrode system as for the d.c. tests, the observed a.c. electric strength decreases with increasing specimen thickness and time of application of voltage. A.C. values are lower than d.c. values at corresponding temperatures, the difference being less at low than at high temperatures. It is suggested that these differences between a.c. and d.c. breakdown may be associated with internal heating due to ionic discharges or dielectric loss.