The shielding of overhead lines against lightning

Most overhead transmission lines carry over-running earth wires, one function of which is to shield phase conductors from direct lightning strokes. Published theories of shielding are incompatible with modern knowledge of lightning. The paper emphasizes the importance of the charges bound on the earth wire and phase conductor by the leader stroke. Equality of charges is taken as a condition under which both wires are equally likely to be struck. For adequate shielding, the charge on the earth wire should exceed that on the conductor. The charges are calculated from the field due to the leader, and it is shown that equality of charges corresponds to both wires lying on the same equipotential of the leader. The limiting position for protection is determined by the slope of the equipotential, and as this depends on the distance between leader and transmission line, the limiting value of which depends on the stroke current, the protection afforded isa function of stroke current. Calculation suggests that shielding is normally adequate for conductors in a wedge of semi-vertical angle 45° of which the apex line is the earth wire.