A ground wire is a form of lightning protection employing a conductor well grounded at regular intervals. The protecting lines against direct strokes are by the use of overhead ground wires, This method is most generally accepted and effective. This method of protection is known as shielding method which does not allow an arc path to form between the line conductor and ground. They are made of galvanized steel wires or AC SR conductors. They are provided to shield the lines against direct strokes by attracting the lightning strokes to themselves rather than allowing them to strike the lines (phase conductors). When a ground wire is struck by a direct lightning stroke, the impedance through Which the current flows is very much reduced and correspondingly higher current is required to cause flashover.
In order to provide effective protection to lines against direct strokes, ground wires must satisfy the following requirements.
(a) There should be adequate clearance between the line conductors and the ground or the tower structure.
(b) There should be adequate clearance between the ground wires and line conductors, especially at the midspan, in order to prevent flashover to the line conductors up to the protective voltage level used for the line design.
(c) The tower footing resistance should be as low as economically justifiable. The ground wires should be placed at such a height above the conductors and so located that they should be well out on towers. And they should not be exactly over the conductors in order to avoid any possibility of a short circuit occurring in the event of conductors swinging under ice loading, etc.
The entire ground wire voltage does not show up across the line insulators as a voltage of the same polarity as the ground wire is also induced in the line conductors.