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Experiments were conducted to measure the ac breakdown strength of epoxy alumina nanocomposites with different filler loadings of 0.1, 1 and 5 wt%. The experiments were performed as per the ASTM D 149 standard on samples of thickness 0.5 mm, 1 mm and 3 mm in order to study the effect of thickness on the ac breakdown strength of epoxy nanocomposites. In the case of epoxy alumina nanocomposites it was observed that the ac breakdown strength was marginally lower for 0.1 wt% and 1 wt% filler loadings and then increased at 5 wt% filler loading as compared to the unfilled epoxy. The Weibull shape parameter (β) increased with the addition of nanoparticles to epoxy as well as with the increasing sample thickness for all the filler loadings considered. DSC analysis was done to study the material properties at the filler resin interface in order to understand the effect of the filler loading and thereby the influence of the interface on the ac breakdown strength of epoxy nanocomposites. It was also observed that the decrease in ac electric breakdown strength with an increase in sample thickness follows an inverse power-law dependence. In addition, the ac breakdown strength of epoxy silica nanocomposites have also been studied in order to understand the influence of the filler type on the breakdown strength.