In many ancient mythologies, lightning and the associated thunder crash were seen as mechanisms for divine retribution. Though we are well beyond that view, the public still has many misconceptions about lightning. In fact, there is much that is not known about lightning initiation. It begins abruptly and is over in an instant, so for that reason it is difficult to pinpoint all the details.
It is an established fact, however, that lightning rods, properly known as air terminals, are effective in preventing lightning damage to buildings. Placed at correct intervals along the roof, they limit the risk of fire and electric shock.
In the popular narrative, lightning bolts strike the rods before actually reaching the building, whereupon the electrical energy is harmlessly diverted to ground. That is not how it is supposed to work although it may happen like that as a sort of fallback mechanism.
The prelude to a lightning bolt is a build-up of static charges between different regions in the atmosphere. There are very high voltage differentials between the regions. Discharges may be point-to-point or point-to-ground. The air is an effective insulator preventing discharge until the rising voltage reaches a certain level depending upon the distance between charged locations and the temperature and amount of moisture in the air. Then, like any insulator, the air breaks down because of high voltage stress. An ionized (conductive) path is abruptly established between the two regions, and a very high current follows the path. It is a consequence of the quantized nature of the ionization that the arc characteristically follows a zigzag and sometimes branching route.
Lightning rods, which are connected by low-impedance conductors to the ground electrode system, are designed to continually bleed the charge from the region above the roof into the ground, so there is no sudden arc. What makes lightning rods work is that the top end of the rod is a very sharp point. The charge continuously bleeds out of the surrounding region rather than building to a high level, ionizing the air and arcing abruptly as would happen if the lightning rod took the form of a flat plate facing upward.
Designing and installing a lightning protection system, especially for a high-rise building that has a large electrical service and extensive data cabling, is exacting work and any errors can be costly. Lightning protection design and installation is governed in most jurisdictions by specific lightning protection mandates, although some aspects are covered by the National Electrical Code.
A central requirement is for low-impedance grounding electrode systems for the power and light wiring as well as for all data and communication subsystems. All of these must be interconnected to each other to prevent any dangerous voltage differentials. Many home and business owners will contend that this is a dangerous practice because it will call lightning into the main power system, but the opposite is actually true.
The ground electrode conductor should be run as straight as possible, without short-radius bends. Otherwise, the electrical energy may leave the conductor causing a fire hazard. Many people figure that the inability of electrical energy to navigate bends in conductors is due to centrifugal force, as when a train derails taking a curve too fast. That is not the case. Electrons have very little mass and do not necessarily tend to go straight.
The true explanation is that lightning energy has a fast rise time when it arcs and thus resembles a high-frequency wave. The bend in the conductor is like a part turn in an inductive coil, increasing the ac reactance. The fault current follows the path of least impedance, exiting the bent conductor. Air-gap lightning arrestors work in this way also.
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