Nonpressurized air is used extensively as a basic insulation medium in medium/high voltage equipment. An inherent problem of air-insulated designs is that the systems tend to become physically large. Application of dielectric barriers can increase the breakdown voltage and thereby decrease the size of the equipment. In this paper, the impact of polytetrafluoroethylene (PTFE) and polyvinyl chloride (PVC) as dielectric barriers on breakdown voltage for DC and AC voltages were investigated by developing two geometric models. In the first model, it was assumed that a U-shaped electrode was covered with PVC dielectric. In the second model, it was assumed that PTFE dielectric was inserted in the air gap between electrodes. Both models were simulated using COMSOL Multiphysics software. The simulation results were verified by experimentation in the lab. The results show that a layer of PTFE and PVC dielectrics behaved as a mechanical obstacle, and they increased the voltage breakdown channel. In addition, the residual charges over the barrier changed the electric field distribution, resulting in a significant increase in the breakdown voltage.