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.