When lightning strikes a phase conductor, no other object shares in carrying the lightning current. Most flashes to an unprotected phase conductor are therefore capable of producing flashovers.
OHGWs may intercept the stroke and shunt the current to the ground through the tower impedance and footing resistance if they are properly located. The resultant voltages across the transmission-line insulation, and the likelihood of flashover are substantially reduced.
Shield wires or static wires are normally employed on transmission and distribution lines, and are located high above the phase conductors to shield the phase conductors from direct lightning strokes.
The shield wires intercept most direct strokes and allow them to be conducted harmlessly to ground, that is providing low resistance path to ground.
On distribution systems, the grounded neutral conductor sometimes is located above the conductors to act as shield conductors.
One important task of transmission-line designers is to locate the OHGWs. Well-planned geometry will reduce the probability of lightning striking the phase conductors to an acceptable level.
The proper placement of the OHGW around the phase conductors is usually deÞned by the shielding angle. The shielding angle is negative if the OHGWs are horizontally disposed outside the phase conductors.
Before about 1951, a shielding angle of 30° was usually employed for transmission lines. This produced acceptable lightning performance on existing lines of voltages up to 230 kV.
In the mid 1950s, 345 kV lines were introduced and tall double-circuit lines were constructed. The lightning performances of these lines were considerably worse than expected.
After extensive theoretical, field, and laboratory investigations, a general agreement was reached that the usual 30° shielding angle should be decreased as the height of the transmission-line structures increased.