The behaviour of planar samples of an antiferroelectric liquid crystal under DC electric field has been studied through observations with a polarizing microscope. In the ferroelectric phase the dechiralization lines have not been observed under a vanishing electric field. They appeared with the increasing field. To explain that effect, we show by analysing the anchoring energy, that the structure of the planar helicoidal SmC* sample can exist with π-disclinations on the glass surfaces. The behaviour of such samples under an electric field is also discussed