Influence of Independent Towers and Transmission Lines on Lightning Return Stroke Current and Associated Fields

The influence of elevated objects, when they are hit by lightning, on the measured lightning currents and associated fields is studied. For the present study, an electromagnetic model of return strokes with the help of NEC-4 is employed. Cases of strokes to ground, to independent towers, and to a transmission line are compared and the validity of comparison is discussed. The influence of the independent tower of 70 m in height, which simulates Berger´s tower, on the peaks of currents is negligible even for the fast-rising current, which corresponds to a subsequent return-stroke current. The shape of the lightning current at the top of a transmission line is less influenced than that at the top of an independent tower of the same height. Elevated objects significantly decrease the amplitude of vertical electric fields in a close range. If the ground conductivity is infinite, the peak amplitude of electromagnetic field at a distant range will be significantly increased by the presence of an elevated object. In actual cases of finitely conducting ground, however, the increase of the peak field is limited due to the propagation effect.