Polarization switching in triglycine sulfate crystals with one of the polar surfaces covered by a thin dielectric film (1 μm) has been found to be highly diminished in a 50 Hz electric field. We assume that the depolarization field caused by the film significantly moderates the process of the nucleation and (or) sideways motion of 180° domain walls. When crystals with both conductive electrodes were connected to the Sawyer-Tower circuit through the capacitor, which was equivalent to the dielectric film, the polarization reversal did occur. We assume that in the case of the conductive electrode, a nonuniform distribution of the electrical charge at the electrode/ferroelectric interface and an injection current through the interface trigger off polarization reversal. Analysis of the energetic state of the crystals has been performed.