Effects of thickness and area on the electric strength of polymers

When the short-time electric strength (E_st) of low loss polymers is determined in accordance with IEC 243, breakdown is usually caused by `surface discharges´. Unless there are gross isolated defects, E_st is more dependent on the thickness d than on the area of the samples. E_st is affected by the permittivity and electric strength of the immersion medium and when d>0.1 mm, E_st is usually higher when samples are tested in air than in a clean transformer oil