Lightning Discharge and Fundamentals of Lightning Protection

A review of lightning protection concepts introduced by Benjamin Franklin (18th century) and James Clerk Maxwell (19th century) is given. Modern approaches to lightning protection of various structures and systems are discussed. In particular, the widely used electrogeometrical model (one version of which is the Rolling Sphere Method) and the topological shielding are presented. Bonding requirements, needed to avoid side flashes (in air or in the soil), are discussed. Lightning parameters important for lightning protection are reviewed. 1 GENERAL PRINCIPLES Systematic studies of thunderstorm electricity can be traced back to May 10, 1752 in the village of Marly-la-Ville, near Paris. On that day, in the presence of a nearby storm, a retired French dragoon, acting on instructions from Thomas-Francois Dalibard, drew sparks from a tall iron rod that was insulated from ground by wine bottles. The results of this experiment, proposed by Benjamin Franklin, provided the first direct proof that thunderclouds contain electricity. Even before the experiment at Marly, Franklin had proposed the use of grounded rods for lightning protection. Originally, he thought that the lightning rod would silently discharge a thundercloud and thereby would prevent the initiation of lightning. Later, Franklin stated that the lightning rod had a dual purpose: if it cannot prevent the occurrence of lightning, it offers a preferred attachment point for lightning and then a safe path for the lightning current to ground. It is in the latter manner that lightning rods, often referred to as Franklin rods, actually work. There are generally two aspects of lightning protection design: 1) diversion and shielding, primarily intended for structural protection but also serving to reduce the lightning electric and magnetic fields within the structure, and 2) the limiting of currents and voltages on electronic, power, and communication systems via surge protection. Primarily the first aspect will be considered here. Properly designed structural 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. The efficacy of this so-called conventional approach to lightning protection has been well demonstrated in practice. Neither data nor theory supports claims that non-conventional approaches, including “lightning elimination” and “early streamer emission” techniques, are superior to the conventional one. Lightning protection system for houses proposed in 1778 is shown in Fig. 1. Modern structural lightning protection is illustrated in Fig. 2. Note that metallic roofs whose thickness is 4.8 mm (3/16 in.) or greater do not require air terminals (NFPA 780 [3]). In 1876, James Clerk Maxwell suggested that Franklin rod systems attracted more lightning strikes than the surrounding area. He proposed that a gunpowder building be completely enclosed with metal of sufficient thickness, forming what is now referred to as a Faraday cage. If lightning were to strike a metal-enclosed