Lightning Protection. General Principles and Standards

In the XVIII ^e century, a period that brought new rigor to theories and experiments, it became clear that lightning was not a weapon at the disposal of a thundering god, but a natural phenomenon governed by the laws of physics. Without knowing too much about these laws, the researchers of the period demonstrated that it was possible to disrupt its path and trace a route that it could follow without danger to others, thanks to lightning protection systems made up of lightning conductors and down conductors connected to earth. From then on, efforts were made to find the best method of installing the lightning protection system, in order to improve its efficiency.

For some time, power transmission lines in the USA were protected on the basis of a 30° angle to the vertical. In the mid-1950s, when transmission switched to higher voltage levels (345 kV) using taller towers, this protection became less reliable. The outage rate due to lightning strikes turned out to be higher than expected. Based on statistical data on lightning strikes on power grids, an empirical model known as the "electrogeometric model" was developed. It can be used to determine the extent of a lightning conductor's protection zone as a function of its height and the lightning current it can intercept.

Today, knowledge of lightning physics has improved, thanks in particular to experiments on large air gaps and artificial lightning triggering experiments. We know that lightning interception results from the increase in the electric field produced by a descending tracer from a thundercloud. Indeed, the approach of a downward leader induces and amplifies an electric field in the vicinity of an asperity in the ground, resulting in the formation of an upward leader. Lightning strikes when these two tracers meet.

Furthermore, it's not enough to simply attract the upward tracer and direct the lightning current to the ground to protect a structure. It is also necessary to properly dimension the entire protection system, and to take into account the electromagnetic field induced by the lightning current, in order to protect against it as well. For this reason, international standards now provide various recommendations for optimizing lightning protection, with the aim of providing a comprehensive and increasingly reliable solution.

This article reviews the current state of the art in lightning protection. In order to fully understand the concepts used today, it is first necessary to define the different phases in the formation of a lightning strike on the ground. Given that around 90% of lightning strikes on flat terrain are of the negative downward type, we will focus only on those models that apply...