Shielding in High voltage lines

Notice the guard wire on top of the tower
Shielding is a method of lightning protection used in High voltage Transmission lines.  Overhead transmission lines are particularly vulnerable to lightning strikes.  A lightning strike can cause disruption, damage to transmission equipment and generate transients which may damage substation equipment such as transformers.  This may result in considerable downtime. 

Shielding involves running a grounded wire above the line conductors.  The wire shields the line conductors.  Lightning which reaches the shielding wire are discharged to the ground.  This method of shielding minimizes lightning strikes on the power lines to a large extent, though, it does not eliminate the threat completely. 

As the shield wire may carry extremely high voltage in the event of a lightning strike, the shield wire is guided down the tower maintaining adequate clearance with the line conductors.  The shield wire is then connected to a dedicated earth pit.  The earthing resistance of the earth pit needs to be extremely low to enable the lightning charge to quickly get discharged to the earth.  Otherwise, this may result in backflashovers.

Shield wires are used only in high voltage lines as the small clearances in low voltage lines may result in backflashovers during lightning strikes.