The ability of shield wires installed in overhead
lines to mitigate lightning-induced overvoltages has been extensively investigated. Unfortunately, these studies came to different results, sometimes contradicting each other: some
authors found that shield wires produce a significant overvoltage reduction, while others found the reduction negligible; conflicting
results also pertain to the role played by the various parameters involved, such as the relative height of the shield wires compared to the phase conductors. This paper aims to clarify this topic. Thepaper is organi zed in two parts: Part I, which starts from the analysis of the theory behind the mitigation effect, is devoted to
establishing a more solid base to the topic. Two fundamental improvements are proposed: the first is the distinction between internal and external of the parameters involved: current
literature makes an indiscriminate grouping of all of them; the second is concerned with the point along the line where the mitigation effect needs to be assessed. Thanks to this new approach, we show that this effect can be precisely quantified. The analysis in this Part I is limited to the basic case of a single grounding point of the shield wire, which represents an
unrealistic case. Part II is devoted to completing the study, by applying the proposed approach to more realistic and practical cases.