Lightning protection of buildings - physics

When Benjamin Franklin invented the rod lightning conductor in 1753, he believed that its spike effect would allow the "electrical fluid" contained in the storm cloud to flow to earth, thereby preventing lightning strikes.

This hypothesis does not stand up to the analysis made possible by modern knowledge of ionization phenomena. Even in Franklin's day, this view was quickly disproved: of the many rods he had erected, five were struck by lightning within the first year of their installation.

The second way of explaining the protective role of lightning conductors is to consider their power to attract lightning. It was soon recognized, however, that this power was limited to a relatively small volume, which nevertheless ensures a certain zone of protection around the lightning conductor. Various definitions of this zone, all empirical, have been given, generally in the form of a cone of circular cross-section, with a vertical axis, and whose apex coincides with the tip of the lightning conductor. It was long accepted that the half-angle at the apex of this cone was fixed and of the order of 45 or 60˚.

Unfortunately, this simplified model of protection is imperfect, as it does not provide for certain cases. In many cases, lightning has struck at the foot of a lightning conductor or a high tower, or has struck the tower at mid-height. These observations have been made in the vicinity of television towers, and even seem to show that the concentration of lightning strikes in their vicinity is higher than the average for the region.

The study of the physical phenomena involved in lightning strikes has led to the development of a method for determining the protection zone of a Single Rod Lightning Rod (STL), or horizontally tensioned wires, and for defining the maximum mesh size of a cage. This method is based on an analysis of the lightning impact mechanism, and is implemented by means of a mathematical model known as the electro-geometric model. Although this model is not perfect – many uncertainties remain – it is nevertheless the most practically coherent and straightforward approach to direct lightning protection that has been developed to date. Among other things, it explains why lightning can strike at the foot of a tower, thus explaining certain protection "failures", and shows that the protection zone depends on the peak intensity and polarity of the current flowing through the lightning strike. Other approaches are possible, but they require calculations using simulation software, which is still an obstacle.

But in order to develop this model and specify its applications, it is necessary to study the thunderstorm phenomenon and examine the main parameters that characterize lightning....