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Alright, back in business... Borealdreams (talk) 02:20, 15 January 2013 (UTC)

Lightning formation - a brief overview


A thundercloud, is a naturally occurring electric field on a large scale. The highly complex electrical dynamics within the cloud, and their formation are not important to this discussion, however it is well accepted in the scientific community that the bottom of the thundercloud is generally understood to carry a relative negative charge. This in turn induces a net positive charge on the Earth's surface in the thundercloud's shadow, in equal but opposite polarity to the intensity of the charge on the cloud surface, thereby establishing an electric field within the air. Lightning, an electrical spark, occurs once the intensity of the electrical field exceeds the conductive resistance of air through a complex process described simplistically below, and is a momentary, spatial equalization of the opposite charges on both sides of the lightning flash.

In a negative cloud-to-ground lightning flash, two events occur prior to one seeing the actual “bolt” of lightning. First, a downward (stepped) leader composed of ionic plasma, generally a concentration of the negative charges, comes down from the clouds in a zigzagged manner that may branch resulting in multiple leaders near the earth. Second, as the stepped leader(s) approaches the ground, the positive charge on the surface concentrates and forms multiple plasma upward streamers, which rise from surface features offering connection points for the downward leader. Once any of the downward leaders connect to any of the upward streamers available, in a process referred to as attachment, a single ionic channel is created and an electrical pathway/circuit is formed. The visible portion of lightning then occurs, which is the discharge or temporary equalization of the opposite charges between the thundercloud and earth.

The exact physics behind the attachment process is not fully understood and there is still some debate in the scientific community.

Charge neutralization and electric field dynamics
As described above, the lightning flash is a temporary equalization of charge between the cloud and ground through the ionic channel. During the milliseconds of time within which the equalization occurs, there are rapid changes to the electric field. These field dynamics affect everything at both the termination point and in general proximity to the flash channel.

Near instantaneous current fluctuations release an enormous quantity of heat, measured upwards of 50,000 degrees F. As an ignition source, to both natural objects and man-made structures, it was the main concern of lightning protection systems prior to modern times, and is referred to as primary effects. Injury, or even death, to both humans or livestock through a direct strike or the subsequent fires they caused, made it imperative for humanity to address this uncontrollable natural phenomenon and allow for the advancement of society as a whole. The development of the lightning rod system in the mid 18th century was the answer to the primary effects of lighting terminations, and it has been adopted world-wide with only slight modifications to the initial designs.

However, with the electrification of the world in the 20th century, and the importance of electrical systems and micro-electronics in almost every aspect of today's technological world, dynamic electric field fluctuations or secondary effects of a lightning termination are equally destructive, although often not as openly apparent as was a building set on fire in the past. Types of secondary effects that can cause damages not directly attributable to the direct lightning strike include; atmospheric transients, earth current transients, electro-magnetic pulses and ground rise potential.

Strike collection versus charge transfer technology
The primary element of the traditional LPS or Franklin system, in reference to its inventor, is the lightning rod or air terminal. When a thundercloud is present, a lightning rod efficiently begins the process of point ionization when bonded to earth and exposed to the electric field. A lightning rod reaching an ionization threshold initiates an upward streamer to which an incoming downward stepped leader can attach to, thus the lightning is said to be "collected" and routed to ground through a network of conductors.

Charge transfer, in the genre of lightning protection systems, is an application of point ionization used to reduce the localized electric field potential through a concentration of multiple points. This is done by transferring the thundercloud induced electric charge, available on the grounded structures under the cloud, into the air through point ionization, en mass. The reduced electric potential retards streamer development thereby lowering the probability of leader attachment and subsequent lightning strike.

Strike protection and generalized standards


Benjamin Franklin's famous electricity experiments in the 18th century set the stage for a world-wide standardized approach to addressing the need to protect structures from damages caused by lightning. Over the intervening 250 plus years, both scientific investigation and trial and error approaches have been used in an attempt to best understand the myriad complexities of a lightning strike and its interaction with ground based structures. Traditional standards commonly used to define a lightning protection system (LPS) focus on that interaction and the primary effects of lightning through strike collection only. The standards do not take into consideration the secondary effects of lightning involved in nearby terminations or the strike collection itself.

The British Standards Institution (BSI), International Electrotechnical Commission (IEC), the National Fire Protection Association (NFPA) & Underwriters Laboratories (UL) have implemented best practices, BS EN 62305-1:2011, IEC 62305-1 , NFPA 780 & UL 96A , respectively, to standardize the application of traditional air terminal systems.