This paper deals with the novel idea of controlling the electric stress in a hybrid air–solid dielectric insulation. In a parallel-plate electrode system, the electric stress can be reduced if the electrodes are covered with thick non-conducting dielectric coatings. Free charges are generated by microdischarges developing between the electrodes and are deposited at the dielectric surfaces. As a consequence, a counteracting electric field component results, which causes a reduction of the electric field in the air gap and an enhancement of the field in the dielectric coatings; i.e. the electric stress is forced into the dielectric coatings by the charges. A computation of the threshold voltage of the microdischarges is presented. The charge simulation technique is used for field calculation, irrespective of the thickness of the dielectric layer and the values of the charges deposited on the dielectric surfaces. The calculated threshold voltages are compared with those estimated before and those measured for different gap lengths.