Lightning Protection

Electrical discharges generated in the Earth’s atmosphere by cumulonimbus clouds, volcanic eruptions, dust storms and snow storms are usually referred to as lightning discharges. In this chapter we confine ourselves to the lightning discharges produced by cumulonimbus clouds. Lightning discharges can be separated into two main categories, ground flashes and cloud flashes. Lightning discharges that make contact with ground are referred to as ground flashes and the rest are referred to as cloud flashes. Cloud flashes in turn can be divided into three types: intracloud flashes, air ischarges and intercloud discharges. These different categories of lightning flashes are illustrated in Figure 2.1a. A ground flash can be divided into four categories based on the polarity of charge it brings to the ground and its point of initiation. These four categories are illustrated in Figure 2.1b: downward negative ground lashes, downward positive ground flashes, upward positive ground flashes and upward negative ground flashes. The polarity of the flash, i.e. negative or positive, is based on the polarity of the charge brought to the ground from the cloud. Upward lightning flashes are usually initiated by tall objects of heights more than ~ 100 m or structures of moderate heights located on mountain and hill tops. The basic features of the mechanism of lightning ground flashes, summarized next, are given in Reference 1. Electromagnetic field measurements show that a downward negative ground flash is initiated by an electrical breakdown process in the cloud called the preliminary breakdown. This process leads to the creation of a column of charge, called the stepped leader, which travels from cloud to ground in a stepped manner. Some researchers use the term preliminary breakdown to refer to both the initial electrical activity inside the cloud and the subsequent stepped leader stage. On its way towards the ground a stepped leader may give rise to several branches. As the stepped leader approaches the ground, the electric field at ground level increases steadily. When the stepped leader reaches a height of about a few hundred metres or less above ground, the electric field at the tip of the grounded structures increases to such a level that electrical discharges are initiated from them. These discharges,