Electromagnetic Computation Methods for Lightning Surge Protection Studies

2Lightning

2.1 Introduction

The primary source of lightning discharge is a thundercloud, in which the electrification of various cloud particles occurs. Lightning discharges are classified into two categories: cloud-to-ground discharges and cloud discharges. Cloud-to-ground discharges are of primary interest when studying and designing lightning protection of manmade structures located on or below the ground surface. Cloud-to-ground discharges are classified, on the basis of the polarity of the charge effectively transferred to ground and the direction of the initial leader that creates a conducting path between the thundercloud and ground prior to high-current return strokes, into four types: downward negative lightning, upward negative lightning, downward positive lightning, and upward positive lightning. It is believed that about 90% of all cloud-to-ground lightning are downward negative lightning discharges and about 10% are downward positive lightning discharges. It is thought that upward lightning discharges occur only from tall objects higher than about 100 m or from objects of moderate height located on mountaintops. The shapes of electric and magnetic fields generated by a lightning return stroke depend on the distance of the observation point from the lightning return-stroke channel, the ground condition such as ground conductivity, the spatial and temporal distribution of current along the channel, and other factors. Transient voltages or currents generated by lightning ...