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An unconventional technique to improve the lightning performance of transmission lines, consisting of the addition of grounded wires properly positioned below the phase conductors, is assessed. Overvoltages developed across insulator strings of existing 230-kV lines in response to direct strikes to the towers were simulated using an electromagnetic model. Minimum reductions of these voltages in the ranges of 19%-32% and 26%-44% were achieved for 20- to 80-Ω tower-footing grounding resistances due to the addition of one and two underbuilt wires, respectively. This practice was shown to be more efficient than reducing the tower grounding resistance to its half value.