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Lightning Protection Principles

Lightning, this beautiful and yet dangerous phenomenon, is one of the signs of the Creator on the Earth. Lightning is an electrostatic atmospheric discharge between clouds and the Earth which hits a point on earth or objects on it, and causes physical changes in the struck point.

An average of about 45 discharge per second occurs in the Earth's atmosphere, the total annual discharge reaches to the number of one billion, four hundred million.

Fortunately, only 25 percent of this total number is the lightning between the cloud and the earth, but still this amount is considerable and it does not decrease the importance of protection. According to the existing statistics, maximum (peak) lightning currents are in the range between 2 and 200 kA, and in 50 percent of the strikes it is more than 30 kA. This amount of current can transfer approximately 500 MJ of energy.

The risk of lightning is not unknown to people. The fear of lightning has always been in the human's mind from previous era. One of the most important events that has ever recorded so far, is the disaster of explosion of hundreds of gunpowder which took place in 1769 in Brescia, Italy. Gunpowder which were stored beneath the dome in a church, was struck by lightning. More than one sixth of city was devastated and approximately 3000 people were killed.

Today, in addition to the lives of humans and animals, sensitive electrical and electronic equipment, telecommunication systems, explosive materials, buildings and structures are at risk of strike or effects of lightning. Elevated structures, communication towers and buildings located in the open wide plains or high levels, are most likely at the risk of lightning strikes .

Benjamin Franklin first used Lightning rod in 1749 and actually he is the inventor of lightning protector. What Franklin invented was firstly "Lightning Attractor" and later it was called "Franklin rod". Years later in the early-20th century, American physicist Nikola Tesla, that his researches were mostly in the field of electricity, performed broad investigations on the subject of lightning protection.

By the following, we will briefly discuss about basic Lightning Protection Principles.


Definition: An elector-atmospheric flash between cloud and earth which includes one or more return strike. This flash hits the earth or subjects on it and causes considerable physical damage in the struck point.

Lightning formation: Thunderstorm can accumulate electrical charges(often negative) in the specific mass of clouds, by movement and atmospheric activities. When a cloud gets a large amount of electrical charge, it induces an electric charge on the surface of the earth with same quantity but opposite polarity. This process creates a strong electrical field in the medium between cloud and earth.

Due to this electrical field, ionized channels called "Downward Leaders"  or Stepped leaders are formed from cloud down to earth. Downward leaders try to lunge abruptly toward opposite polarity side, the earth. Number of downward leaders are usually limited to a few channels (one or two). Same procedure happens on the surface of the earth with a little difference and makes "Upward Streamers". All the objects on the earth are capable of emitting upward streamer.

When thunderstorm moves atop the earth, electric charge is non-uniformly distributed on the surface of the earth. Since high density charge concentration usually takes place on the sharp objects, tall buildings, high-level antennas, towers, altitudes and lightning air terminals have stronger and longer streamers. Both ionizing channels move toward their directions with abrupt lunges. Each channel in its movement protrudes one or more branch channels. Length of each protrusion can reach up to 40 or 50 meters. Both leader finally contact each other in a point, and consequently air's dielectric resistance suddenly breaks down and a channel of air is formed to conduct electric charge form cloud to earth in a very short time. Therefore lightning is formed.

Lightning flash produces heat and sound. The emitted light is the result of charge transfer plasma and the thunder sound is heard as a result of sudden expansion of air due to heat produced. Surrounding air temperature can rise up to 30000 °C, almost five times the sun's surface temperature.

Lightning Protection System (LPS): The system which prevents direct strike to human beings, structures and buildings, and deviates the lightning current path. Note that we can never prevent lightning from striking the earth, we can only protect and make area safe by installing LPS system.

Benjamin Franklin invented the LPS for the first time. Each LPS consists three parts; Air terminal, Down conductor and Earth termination. Air terminal is the safe striking point. It absorbs the lightning current and down conductor conveys it to earth terminal. The current is then dissipated to surrounding earth.

Depending on its use and applied technology, air terminal is different. There are two general systems; Active (ESE) system and Passive system. Passive system does not have any reaction to lightning and therefore is named simple or passive. While in active system lightning rod is equipped with an Early Streamer Emission (ESE) system which has reaction against the lightning. different models of ESEs are distinguished with Triggering Advance time (Δ). This advance time is compared with a simple rod in an equal conditions in the laboratory. It makes the ESE rod to have an extended volume of protection and outstanding function in comparison to passive rod. Apart form air termination, there is no difference between two systems in other parts.

Because of limited radius of protection in passive air terminals, in large buildings and sites with wide open areas, using of passive systems to protect it against lightning will make the project very expensive. So the best advantage of active system is the reduction of costs. On the other hand in active air terminal, the strike takes place in a point higher than its tip, thus the protected equipments and objects are in less danger of lightning strike.

There are different standards and norms in the field of installation, operation and maintenance of lightning protection systems. In the article of active systems the norms NFC17-102 and UNE21186 and regarding to passive systems the norms IEC62305, BS6651, NFPA 780 and VDE 0185 are widely used in most countries. First let us briefly  introduce some general definitions which are in common in most of the norms.

Lightning Current: Lightning current is the combination of some short-term strikes with less than 2ms, and long-term strikes with more than 2ms long. The short-term current wave shape is somewhat like figure 1. According to statistics published by norm BS6651, the maximum (peak) return strike current (I) takes place in the range of 2 to 200 kA intensity. Low peak values have high probability of incidence, so that in 1% of flashes it is more than 200kA, 10% more than 80kA, 50% more than 28kA, 90% more than 8kA and in 99% it is more than 3kA. In figure 1, T2 is less than 2ms.

pic 1

Figure 1: Lightning strike short-term current wave shape

Lightning Protection Level(LPL): In order to achieve standard's purposes, four levels of protection have been defined by IEC62305 and NFAC17-102 criterions. Protection level indicates the level of safety measures or importance of protection required for the project, so that the level I has most and level IV has least value. Therefore various structures depending on their application, need proper level of protection. It is expected that the cost of LPS setting up in higher levels will be greater. However, it should be noted that the different structures and equipments require different LPL. The LPL can be calculated by parameters like structure's application, point of geographical location, civil properties and dimensions, material used and etc .

Table 1 shows the maximum values of lightning current parameters for different LPLs and are used to design lightning protection components (e.g. cross-section of conductors, thickness of metal sheets, current capability of SPDs, separation distance against dangerous sparking) and to define test parameters simulating the effects of lightning on such components.

Protection Volume: Volume or radius of protection defers depending on type of air terminal (active or passive) and its shape (in passive), LPL and height of installation. By the following we briefly discuss about each type of air terminal and corresponding protection volumes.

 

Table 1: maximum values of lightning current parameters in accordance with IEC62305

table 1

 

Passive Air Terminal: There are tree common shapes for this type of air terminal as follow. Norm IEC62305 explains corresponding methods of protection radii calculation:

- Lightning rods

- Suspended wire conductors

- Meshed conductors (Faraday cage)

1- Protection Angle Method: One of the tree methods of protective radius calculation is the angle method. In this method it is supposed that installing of one or more lightning rods with proper height and intervals will protect the area beneath the cone shaped protection volumes. Figure 2 depicts an example of this method for rods and suspended wires. Figure 3 shows also the relationship between LPL, installation height and the protection angle.

 

figure 2: Protection angle in rods and suspended wires air terminals

 

figure 3: Protection angle and relationship with air terminal height and LPL

 

2- Rolling Sphere Method: In this method it is assumed that the electrically charged cloud is in the form of a large sphere mass and electric charge has been uniformly distributed in its volume. It can hit the earth or objects on it to discharge the electric charge. This is the most general method for designing protection system and can be applied to all types of air terminals. Radius of hypothetical sphere depends on supposed LPL. Table 2 reports the predefined radius of rolling sphere for levels of protection. Figure 4 illustrates the rolling sphere design method.

Table 2

figure 4: Rolling sphere method

 

3- Meshed Air terminal Method: In this method, lightning conductors form an equally spaced mesh. This type of air terminal is used in structures and buildings having flat or slopped roof with sensitive equipments inside. According to Faraday's law, density of meshed conductors prevents electrical field from entering into the building. Penetration quantity is not zero, except when we cover the external surface with a metal sheet. Making a whole external covering around structure of course is impractical. So we should use a meshed cage with interconnected conductors which is like a sheet. In practice a negligible quantity of electrical field will pass into the structure. Increasing interconnected conductors will decrease entering electrical field. Figure 5 shows an instance of such air terminal. Table 2 reports corresponding mesh dimensions with each LPL.

figure 5: Protection angle and volume in meshed air terminal method

 

Material and dimensions of passive system: In order to tolerate the electrodynamic forces caused by lightning current and most importantly resist against electrochemical corrosion, material and dimensions of air terminal and down conductor have to be carefully selected. Table 3 reports minimum dimensions for different materials according to IEC62305

 

table 3: minimum dimensions of passive system components for different materials according to IEC62305

 

 
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