An electric field distribution in a 3D diphasic dielectric structure with inclusions randomly dispersed in a host phase is simulated using a boundary element method (BEM) to mimic real-world system. A dielectric composite is assumed to undergo the intrinsic breakdown, and a percolation model is used to define the initiation of breakdown process. The simulations comprehensively identify the impact of the inclusion proximity, orientation, dielectric contrast, inclusion volume fraction, and role of interfaces on breakdown strength of diphasic composites. The quantitative correlation between all these factors and the breakdown strength of composites has been established.