This paper examines the role of interfaces in the polymeric insulating films under the scope of their dielectric strength. PET (Polyethylene terephthalate) films of different thickness are put together forming a planar sample of a typical thickness of $200~\mu$m and subjected to AC ramp voltage, until the dielectric breakdown. The role of interface as a potential barrier for electrons was examined as well as the influence of interface number for a standard thickness. Experiments were also executed for investigating the role of the position of the first, the second and the last interface in the sample. All the experiments were executed with the same voltage rising rate, the breakdown AC voltage Vb was measured and the dielectric strength Fb was calculated. It was shown that interfaces act as deep trapping centers for electrons giving rise to the sample dielectric strength for sort time experiments. Moreover the thinner the first film, the higher the dielectric strength for the sample. It was also found that the position of the second interface does not play an important role in the process.