The mechanism of electrical degradation and breakdown of polymeric insulating films is examined by this paper as a two-stage process. Electron injection by Fowler-Nordheim tunnelling, provides the energy for the creation of the first cavity near to the injection contact (the beginning of the formation of the so-called low-density region) where impact ionisation of molecules and hence electron avalanche, can occur. This stage is assisted by intrinsic tunnelling of electrons through local potential barriers. The second stage, the macroion bond scission and the creation of another macroion and a free radical is a thermofluctuational process which involves the action of the stretching force by the local electric field. From the other hand breakdown initiation starts when an electron following a Poole-Frenkel hopping mechanism, is accelerated in a hole with sufficiently large dimensions. The role of the applied electric field Fa and its relation to the local electric field Fl in both stages is examined. Experiments were executed for measuring the breakdown AC voltage Vb and dielectric strength Fb, with voltage rising rate 3 KV/s, in order to examine their relation with specimen thickness d and hence to derive a steady state breakdown criterion. Measurements of PET films life-time were also taken in order to examine the degradation mechanism and the polymer's lifetime.