I. Introduction
Overvoltages induced by nearby lightning ground flashes (GF) must be considered in the design of the insulation for overhead distribution lines with operating voltages up to at least 22 kV. In recent years, there has been a considerable upsurge of interest in the effects of ground resistivity on the magnitudes and waveshapes of induced voltages [1]–[5], [30]–[32]. The field experiments to measure induced voltages on test lines [6], [7] were in areas with variable soils of variable resistivity and so do not provide data which can be used to clarify the role of . It is necessary therefore to utilize the results from analytical models that account for realistic values of usually in the range up to 1000 -m. An examination of such models shows that they are sophisticated and are rather more complex than is required for line design. In part, this is so because the models have been developed to cater for GF at various orientations to the line and for GF-to-line distances up to at least 10 km, and these are much greater than the 50 to 500 m of relevance to distribution lines. The simplest and most widely used in line design is the Rusck model [8], [9] because it reduces the calculation of maximum induced voltage from a GF transverse to a long line to a simple formula. However, the Rusck model assumes a step-function current waveshape and a perfectly conducting ground , and it is known from theoretical studies that increasing values of increase the magnitudes of the maximum induced voltages.