Dielectric breakdown of ceramics is obviously an important failure in the levels of equipment requiring some insulation safety or to ensure their proper functioning. The manifested microscopic damage processes under electrical stress are indisputably related to charge trapping and detrapping and thus related to energy localization on defects. This work enabled the development of a technique (Influence Current Method) using a specific arrangement in a Scanning Electron Microscope chamber, allowing to measure separately and simultaneously the influence and conduction currents. This permits subsequently tracing back to the trapped charge dynamic during and after electron irradiation. The purpose of this paper is to study firstly the stability of trapped charge in ZrO2/Y2O3 ceramic and secondly to investigate the grains size effect on charging and discharging processes. Via this results the microstructure - dielectric strength correlations are well justified. Instability of trapped charge was found; the majority was evacuated from the irradiated volume. This entrusts to it a conductive insulator character. It has been found that more the grains size decreases more the amount of stabilized trapped charge decreases. This is due to the eventual increase in the grains boundary density in which the oxygen vacancies are concentrated. Note that the increase of grains size improves the dielectric strength.