Abstract: A lightning protection circuit involves the grounding of the neutral center point of a direct current circuit carrying current to electrolytic reduction cells in a pot room of an aluminum reduction plant. The grounding is accomplished to earth ground via buried grounding rods which are connected to the center point via an interruptible grounding circuit. A safety circuit arrangement is provided which is responsive to direct current and effects the opening of the grounding circuit whenever the current in the grounding circuit exceeds a predetermined value. The safety circuit arrangement also involves a voltage sensing path responsive to the magnitude of the voltage between plant ground and the center point of the direct current circuit which effects an interruption of the grounding circuit whenever this voltage exceeds a predetermined value in either the positive or negative direction.