At the detonation of condensed explosives, on the profile of electric conductivity is observed the area of high values, which is usually associated with the chemical reaction zone. The new interpretation of experimental data on the electrical conductivity allows one to diagnose the influence of the grain size on the charge structure and the reaction zone in the whole range of densities investigated. The reliability of the proposed hypotheses are investigated by the methods of statistical analysis. The level of confidence shows the consistency. The results of this paper are useful for the explosion physics, for the industrial production of nanodiamonds, for the miniaturization of explosive devices.