Benjamin Franklin was responsible for invention of the “lightning rod” over two centuries ago. In the intervening years, lightning rods have come to be known as air terminals, but the basic principles and the physics of lightning protection have not appreciably changed. Ongoing research utilizing modern electronics and high speed film photography have improved our understanding of the mechanics of a lightning stroke. This has resulted in improved understanding of the zones of protection provided by a single air terminal or by an array of multiple air terminals. Over the past two decades, the concept of a zone of protection falling outside of the surface of a rolling ball with a radius of 50 meters has become a fairly common practice. Some designers work with as small as a 30 meter radius rolling ball model. This results in a further reduction in the probability of a direct lightning stoke beneath or close to an array of air terminals, but at considerably higher cost. We do not believe that the higher cost of a 30 meter radius rolling ball model is justified for most systems.
Modern lightning phenomena theory proposes that the buildup of charges leading to a stroke is caused by friction associated with convective activity in clouds above the freezing level. During the mature stages of a thunder storm, very significant charges build up in different regions of the clouds. Lightning stroke activity can be cloud to cloud, cloud to ground or ground to cloud. The strokes result in a large energy transfer from a negatively charged region to a positively charged region. A considerable number of data presented in the literature in the 1930s indicate that the ground end of at least 95 percent of cloud-to-transmission-line strokes are positive. (1, 2) This would result from the majority of the lower surface of the cloud base being negatively charged. For the purposes of this limited discussion, we will assume a negatively charged cloud base with a corresponding positive charge on the surface of the earth underneath the electrical shadow of the cloud.
As the electrical stress between the cloud and the earth becomes higher and higher, conditions finally reach a point where a leader starts its way down to the earth picking the best path from instant to instant. The leader will rarely, if ever exceed a vertical distance of 50 meters before branching or side-stepping, then will continue downward again. As this step leader comes closer to the earth, electrical charges in the earth move to a point underneath the down coming leader until the stress on the earth’s surface becomes so great that the air is broken down and upward leading streamers form on the earth's surface. At this point, a step leader rises from the earth's surface to meet the down coming step leader from the cloud. Up to this point, the lightning activity is not visible to the naked eye. When the two step leaders meet, there is an immediate charge transfer from cloud to ground. This charge transfer heats and ionizes the air molecules in its path, creating the familiar "lightning bolt." The heated air, or plasma expands quickly outward, cooling rapidly. When the expanding gasses cool and run out of energy, the atmosphere rushes in to collapse the vacuum, resulting in the thunder clap.
A lightning strike may be a single stroke or there may be multiple strokes beginning with a return stroke from the earth's surface to the cloud. Because of the tremendously high voltages present in a lightning stroke, it is not possible to insulate or isolate structures from ground in order to prevent lightning activity. Published data in the literature indicates that direct strokes may have current capabilities ranging from 5.5 to 160 kA, with some indications of discharge currents as high as 310kA in rare instances. Stroke energies of these magnitudes can certainly wreak havoc with electrical equipment and control systems. The principal or function of overhead ground wires (shields) and air terminals (lightning rods) is to intercept the lightning stroke and conduct the current safely to ground without sufficient potential developed to cause flashover on protected electrical equipment. Electrical equipment within the defined "zone of protection" is thus considered to be shielded from a direct stroke. One should recognize that there are very significant electromagnetic fields associated with these lightning strokes. Therefore, unshielded or ineffectively shielded conductors in control systems are susceptible to induced voltages high enough to damage or destroy circuit components. The best that we can hope for in a power plant, electrical substation, or in an industrial environment is to shield sensitive equipment from a direct stroke and to minimize resulting potential gradients within the grounding grid. This shielding may be supplemented with lightning arrestors, where appropriate. Surge suppressors or transient voltage surge suppressors (TVSS) on lower voltage power lines and telephone line are also helpful.
The above discussion clearly demonstrates that the surface of the earth behaves as the grounded plate of a very large capacitor. As such, the charge buildup, preliminary to a lightning stroke, will exist as a shallow electrostatic field at the surface of the earth. Topology of the charge distribution across the earth’s surface is a reflection of the charge distribution in the cloud. With an irregular cloud base, the charge distribution at the earth’s surface may be similarly irregular. When a lightning strike occurs, there is a massive instantaneous charge transfer to the site of the stroke where potential is being neutralized between cloud and ground. The resulting current flows at the surface of the earth produce tremendous IR Drop voltage gradients in the earth normal to the stroke(s). These gradients may be on the order of hundreds of volts per meter, or even per foot. Where a stationary reference electrode (SRE) is located 100 or more feet away from a well grounded terminal, the voltage gradient in the earth would be measured in kilovolts. Earth potential gradients between SREs and the terminal or between two or more SREs that occur during a near stroke to a terminal or a direct stroke to earth are capable of destroying any type of printed circuit board.
There is no “magic bullet” for lightning protection, just basic principles and practices (rooted in fairly well understood grounding and shielding electro-physics) that must be initiated and maintained system wide. If any segments of the system are left unshielded, unprotected from surges and spikes, or ineffectively grounded, they will remain vulnerable.
1. Lewis, W. W., and Foust, C. M. Gen. Elec. Rev., 1936, p. 543.
