The mechanism of electrical degradation and breakdown of polymericinsulating films is examined by this paper as a two-stage process.Electron injection by Fowler-Nordheim tunnelling, provides theenergy for the creation of the first cavity near to the injectioncontact (the beginning of the formation of the so-calledlow-density region) where impact ionisation of molecules and henceelectron avalanche, can occur. This stage is assisted by intrinsictunnelling of electrons through local potential barriers. Thesecond stage, the macroion bond scission and the creation ofanother macroion and a free radical is a thermofluctuationalprocess which involves the action of the stretching force by thelocal electric field. From the other hand breakdown initiationstarts when an electron following a Poole-Frenkel hoppingmechanism, is accelerated in a hole with sufficiently largedimensions. The role of the applied electric field F a and itsrelation to the local electric field F l in both stages isexamined. Experiments were executed for measuring the breakdown ACvoltage V b and dielectric strength F b , with voltagerising rate 3 KV/s, in order to examine their relation withspecimen thickness d and hence to derive a steady statebreakdown criterion. Measurements of PET films life-time were alsotaken in order to examine the degradation mechanism and thepolymer's lifetime.