Despite of the fact that occurrence of lightning phenomenon has discussed in chapter 1 earlier, yet it still impotent of explaining in detail about many of the basic concept of lightning stoke. Hence this chapter presents a detailed literature review about modern day lightning protection system. The objective of this chapter is to provide brief explanation on the methods that have been using worldwide to protect the structure and safety of occupants from lightning strikes. All the methods that will be discuss in this section comply the IEC-63205 standards. The chapter also covers the placement of air terminals and ground electrodes to provide less impedance path to discharge at ground.
2.2 Lightning Phenomena
As a matter of fact, it has been known for decades about the conditions for event of lightning however the correct procedure of lightning development has never been confirmed. There are a lot of myth and misconceptions of lightning occurrence. Modern science have acknowledged speculations are about partition of electric charge inside a cloud and creating electric field. Moreover, recent researches prove that ice, hail and semi solidified water drops also contribute during the formation of lightning. However, the modern day science are still incapable of predicting the time and location the lightning strike.
There is an unstable environment in a thunderstorm. When there is a storm, solid updrafts and downdrafts frequently happened and proximately near each other. The updrafts bring little fluid water beads from the lower district of tempest up to stature of 70000 feet, far over the solidifying level while downdrafts will cut down ice and hail from the upper area of the tempest. The warmth discharging will happen when solidified water droplets slam into hail and ice. The surface of hail will keep slightly hotter than its encompassing condition because of the warmth and a delicate hail will be developed.
At the point when the fragile hail crashes into ice particles and water droplets, the electrons accumulate on the dropping particles and shared off the climbing particles. From that point onward, a thundercloud with positively charged at upper part and negatively charged at bottom part was accumulate since the electrons bring negative charges.
From electricity perspective, the unlike charges will pull in each other and minimize the insulation gap. An electric field will be delivered between upper and lower portion of the thundercloud when the positive and negative charges inside a same cloud start to separate. As the partition separate between the two charges grows bigger, the electric field strength increase gradually. In any case, a lot of charges are expected to form lightning since atmospheric layer is a decent insulator which counteracts stream of electric. Lightning will be form just if the quality of electric field is adequate to overpower the insulation quality of atmospheric layer.
At the point when a thundercloud is moving, it will gather another pool of molecule with positive charge along the ground that is in its way of movement since the earth is positively charged. The height of object from the earth surface influences the positive charge of the ground. The structures with greater heights such as tall building and skyscrapers have more positive charges.
Then, a channel of negative charge otherwise called stepped leader will be formed from lower part of the thunderstorm toward the ground. It advances toward the ground in a progression of quick advances that happens so quick and not visible to human eyes. As stepped leader comprise of negative charges approaches the earth, positive charges additionally accumulate in the ground or in the any objects near to the ground.
Then, the upward rising positive charge known as streamer will begin to move upward into the air through building or tree from ground. It approaches the stepped leader in the air from decent distance from ground and they may collide each other at a height roughly equivalent to the length of a football field. An entire directing pathway is formed once there is a contact between stepped leader from the thundercloud and streamer starting from the earliest stage. In this way, a return stroke is formed which carries a huge current (bright bolt) and this underlying strike is trailed by a couple of secondary strikes.
2.3 Types of lightning
Researches shows that, there are a few types of lightning and the most well-known kinds of lightning are cloud to ground, intra-cloud, between cloud and ball lightning.
2.3.1 Cloud to ground lightning
Cloud to ground lightning is one of the most dangerous and risky kind of lightning that can make serious harm to the objects or even put human life into risk. It happens when there is a discharge from bottom part of the thundercloud which is negatively charged and approached the ground which contains positive charges. This kind of lightning generally tall structures, for instance, tower, building, media transmission radio wire, and tall tree. It has shown a lot of light threating evidence. Since it can cause serious damage to objects and human life, cloud to ground lightning is the most dangerous type of lightning of all.
2.3.2 Intra-cloud lightning
Intra-cloud lightning occurs due to inverse charges between area in a similar thundercloud and it will release from pool with negative accuses to the pool of positive charges. In any case, it can cause visible lightning flash that indistinguishable as cloud to ground kind when it surpasses the limit of the cloud.
2.3.3 Inter-cloud lightning
Inter-cloud lightning occurs between two different clouds and it does not touch the ground since it is a cloud to cloud lightning. This type of lightning forms because there are opposite charges between clouds and it travels from point of negative charge at a cloud to point of negative charge in another cloud.
2.3.4 Ball lightning
Ball lightning is a very rare event and it visible once in a while as a reddish lambent ball. However the shading might be differed. It moves horizontally in the air and shows up through sparkling circle only for a few seconds.
2.4 Introduction to Lightning Protection System (LPS)
The lightning studies and Lightning Protection system (LPS) starts in mid-1750 with Benjamin Franklin, who found that the emission from sharp-tipped rod would be able to avoid lightning harm by releasing jolted clouds. In 1753, he understood that a tall metal rod associated with a reasonable down conductor ended in ground could be utilized to conduct lighting strikes (huge current) and dispersed to earth safely with no harm to the structure. From there on that, he held the essential capacity of the sharp-tipped pole was as a preventer of lightning.
The main purpose of a lightning rod is to provide protection to structure by directing the lightning strike (huge current) to discharge at earth. The scientific concept for conventional rod or Franklin rod Lightning protection system are stated in Appendix L of the 1997 Edition of National Fire Protection Association (NFPA) 780 , which explains a system that gives a less resistive way to channel the large current of lightning to dispersed at ground, preventing harm to the structure and it occupants.
2.4.1 Categories of Lightning Protection System (LPS)
The modern day Lightning Protection System (LPS) can be divided into two classes. One provides protection against indirect lightning strike while the second one is to prevent direct lightning strike.
22.214.171.124 Protection against direct lightning strike (Primary Protection)
The most common harms caused by direct lightning strike are the burning of structures and the event of crack on the surface of the building especially at the corner of the building. Therefore, primary protection is essential to secure all structures under its claimed protection zone from being straightforwardly struck by lightning. The primary protection able to avoid causalities or loss of human life by attracting the downward leader before it strikes individuals and other lower points near to the ground.
The primary protection consists of 3 main components such as lightning rod, down conductor and earth electrode. The lightning rods are made to intercept the lightning strikes and act as a lightning receptors, which can be found in various structures, for example, watch wires on High Voltage (HV) overhead lines or air terminal at the highest point of the any structures. They are well earthed to keeping in mind the end goal is to channel the large current to the ground, either by one conductor (regularly a copper strip) or by a few according to the preference. The earth electrode, which must be especially well made to give a low impedance way for the lightning current, is regularly formed by a few, independently buried conductors.
126.96.36.199 Protection against indirect lightning stroke (secondary protection)
The secondary protection components offers protection against the impacts of indirect lightning and sudden increase in power frequency as well. Surge arrestor for Low Voltage (LV) systems, channels and wave absorber to constrain either the impulse voltage or the power transmitted form under this category. The surge arrester is one of the secondary protection segments. It is expected to shield types of well-equipped structures from being influenced by lightning strokes.
Typically, indirect lightning stroke will cause impacts to the overhead cables and transmission lines, for example, the high frequency coaxial cable, power lines and media transmission lines. A voltage surge will be created and spread in the system and may reach connected types of devices in the surrounding. This voltage surge can likewise be created by acceptance because of the electromagnetic radiation of the lighting streak. Indirect lightning stroke can cause a few consequences, for example, pre mature aging of the components, demolition of printed circuit loads up, hardware malfunction, data loss along the line.
Besides the primary and secondary protection as said above, there is other protection type that is identified with the communication and exchanged systems must not be overlooked as well. Both of these system will be influenced by surges like the LV, the main diverse being that the worthy surge level is always lower.
2.5 Lightning Protection methods
Basically lightning protection technique can be categorized in to two, as below:
a) Attracts the lightning stroke towards the rod and it act as a sacrificial device, such as Franklin Rod (FR) and Early Streamer Emission (ESE).
b) Act as lightning prevention air terminal (lightning elimination) such as Charge Transfer System (CTS) Dissipation Array System (DAS).
The Franklin rod is viewed as a lightning collector, as it is the end for a lightning strike in the area. The ESE is likewise thought to be a collector as well since it expands the upward streamer by outline of the end of the bar. The CTS is considered as prevention kind as it keeps the event of lightning strike by neutralizing the charge with multi-pointed electrodes.