Article | REF: C3307 V3

Lightning protection of buildings - physics

Author: Alain ROUSSEAU

Publication date: July 10, 2020, Review date: November 4, 2021

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ABSTRACT

Lightning protection of a structure depends on the lightning severity of the site given in number of strike points to ground per km² and per year. These data are obtained through specialized detection networks. Lightning detection also allows for prevention in order, to keep people in a safe area or to delay a potentially dangerous industrial process.

It is important to understand the physics of lightning in order to protect properly, and in particular to differentiate upward and downward flashes as well as polarity. The effects of lightning flashes are also to be taken into account with high frequency aspect of the lightning current and the energy associated with the impulses. Finally, models are needed to determine the locations of the structure that are likely to be impacted.

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AUTHOR

  • Alain ROUSSEAU: Engineer, École Centrale de Lyon – DEA in Electrical Engineering - President SEFTIM – Chairman of AFNOR and CENELEC Lightning Protection Standardization Committees. Chairman, IEC Lightning Arrester Standardization Committee - SEFTIM, Vincennes (France) - This article is the updated version of article C 3 307v2 entitled "Lightning and building protection", written by Alain ROUSSEAU, Claude GARY and Gérard BERGER in 2000.

 INTRODUCTION

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....

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KEYWORDS

risk   |   lightning rod   |   lightning   |   fire protections   |   collective protection   |   individual protection


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Lightning and building protection – Physics