Intrinsic electric strength and electromechanical breakdown in polythene

Abstract

The electric strength of polythene in the absence of surface discharges decreases sharply above room temperature, and there is evidence that this is an electromechanical effect. An optical-lever system, suitable for measuring electromechanical strains in recessed polythene specimens at 80° C, showed that true compressive strains were masked by a much larger movement of the whole recess caused by lateral expansion in the compressed polythene. When the movement was suppressed, no electromechanical compressions were detected, and there were indications of a significantly higher electric strength.The existence of an electromechanical effect was confirmed by comparing electric strengths of three grades of polythene over a range of temperatures and detecting differences caused by impulse and slowly applied d.c. electric stresses.