A fundamental property of a conductor in electrostatic equilibrium is that the net electric field inside the conductor must be zero. If there were an electric field inside, it would exert a force on the free charges (electrons) within the conductor, causing them to move. This movement of charge would mean the conductor is not in equilibrium. Therefore, the charges rearrange themselves on the surface of the conductor in such a way that they cancel out any external electric field within the bulk of the conductor. Regardless of the strength of the electric field just outside the conductor, the field just inside is always zero.