A Critical Review of Nonconventional Approaches to Lightning Protection

DECEMBER 2002 AMERICAN METEOROLOGICAL SOCIETY | CONVENTIONAL SYSTEMS. Properly designed conventional lightning protection systems for ground-based structures serve to provide lightning attachment points and paths for the lightning current to follow from the attachment points into the ground without harm to the protected structure. Such systems are basically composed of three elements: 1) “air terminals” at appropriate points on the structure to intercept the lightning, 2) “down conductors” to carry the lightning current from the air terminals toward the ground, and 3) “grounding electrodes” to pass the lightning current into the earth. The three system components must be electrically well connected. Many national and international standards describe conventional lightning protection systems (e.g., NFPA 1997, hereafter NFPA 780), and the efficacy of the conventional approach has been well demonstrated in practice (e.g., Harris 1843, 140–156; Symons 1882; Lodge 1892; Peters 1915; Covert 1930; Keller 1939; Szpor 1959). The classic text on the conventional lightning protection of structures is Golde (1973). The theoretical justification of the conventional approach is fairly crude, in part due to our incomplete understanding of lightning’s attachment to ground-based objects. Hence, the fact that conventional systems have a history of success in preventing or minimizing damage to structures is the primary justification for their use. It is nevertheless instructive to review the current understanding of the lightning processes, this understanding being consistent with the experience gained from the use of conventional structural lightning protection systems. The lightning stepped leader, the process that initiates a cloud-to-ground flash, begins in the cloud charge region (near 5-km height in temperate summer for the typical flash that lowers negative charge) and propagates toward Earth at a typical average speed of 105 m s−1. The charge on the leader channel (effectively drained from the cloud charge source) produces an electric field near the earth’s surface that is enhanced by objects projecting above the surface such as trees and grounded air terminals on strucA CRITICAL REVIEW OF NONCONVENTIONAL APPROACHES TO LIGHTNING PROTECTION