The exposure of the distribution line to lightning depends on how much adjacent structures protrude above
the surrounding terrain. Structures located along the top of mountains, ridges, or hills will be more likely
targets for lightning flashes than those shielded by natural features. Trees and buildings may also play a
major role in the lightning performance of distribution lines in level terrain. Trees and buildings may
intercept many lightning flashes which otherwise would have struck a line.
The influence of nearby objects on the number of direct flashes to a distribution line is expressed using a
shielding factor, Sf is defined as the per-unit portion of the distribution line shielded by nearby objects. The
number of strikes to the line is then