9.1 Introduction

The very impressive natural phenomenon of lightning has been considered from the oldest times of humanity as a sign of power and as an instrument of punishment in the hands of the gods. The ancient Greeks imagined Zeus the ruler of the gods in Olympus as being the master of lightning which he could use as a symbol of his supremacy. Similar stories can be found in the mythology of almost all cultures.

For many centuries lightning has been a danger during thunderstorms for people, buildings and storage places where, for instance, wheat was stored. It can be assumed by extrapolating modern statistics that fires due to lightning took place often and that natural features, like a high tree in the proximity of a construction, were not always efficient protection.

When he began to use electricity man had to build overhead lines to transport energy and information, lines which were exposed to lightning strokes causing disruption in the supply of energy or of the information. In the last case, direct danger for persons could occur due to the overvoltages which, when the insulation level is inadequate, arrive on telephonic handsets and potentially kill people. The probability of direct impacts is however much higher for power lines due to their height over the ground. This explains the fact that practically from the beginning of the twentieth century to the 1960s the attention of scientists was concentrated on protecting this kind of installation. The solution of installing a...