Chapter 2 Literature Review 2.1 Introduction Despite of the

Chapter 2

 

 

 

Literature
Review

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2.1  Introduction

 

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-62305 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.

 

2.4.1.1 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.

 

2.4.1.2   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.

 

 

 

2.6 Conventional
and Non- Conventional lightning rods

 

The conventional
Lightning protection system comprise of three fundamental parts: an ordinary
lightning conductor bar at a suitable points or areas on the structure to block
the lightning, a conductor fitted between the terminal and the grounding system
to pass the lightning current into the earth, and  the down conductor convey the lightning
current into the earth. With legitimate installation, this system is said can
give lightning connection point and ways for the lightning current to take
after from the connection points into the ground without making any damage to
the structure.

 

The conventional rod and commonly known as franklin
rods are the most conventional Lightning protection system with spikes on top
in various shape and configuration. Those spikes on top are highly conductive
metal and mainly made of copper of stainless steel .These types of devices are
passive in nature and act as sacrificial device. The conventional rod act in
accordance with standards. However, the major drawbacks of these type of
conventional rods is, it only provides small protection zone. Hence, more than
one conventional rods are required to give optimum protection to structures
with greater in width, length and height.

 

The examination for further developed terminal was
sought after because of the downsides of FR, for example, extremely constrained
scope zone, which is conditioned, just by the height and needed protection
level and for greater measurement installation number of bars with mesh network
must be worked out. As needs be, present day Lightning protection system
practices have followed and modified Franklin’s guideline in order to increase
the protection zone.

 

 

 

 

 

These days, a new approach suggested by scientist and
researchers, which is known as Early Streamer Emission (ESE) LP system has been
presented in advertising for around ten years. This system as per lab thinks
about completed in numerous nations is said to give a more compelling
protection against a lightning strike. The inventors and manufacturers had
additionally made this as a claim. Nonetheless, solid quantitative data about
the relative execution of ESE lightning rod contrasted with Conventional
Lightning Rod (CLR) under comparable conditions are as yet inadequate. In this
way, the adequacy of these advancements still stays open to being researched
more.

 

ESE types of non-conventional terminals are available
in the market in different shapes and configurations. Figure shows few examples
of nonconventional lightning rods (such as ESE type of terminal, Dynasphere and
DAS) that are manufactured by different manufacturers.

 

Nowadays Non-Conventional terminals are widely
accessible in global market in various shapes and designs. Figure …. shows few types of nonconventional lightning
rods, (such as ESE sort of terminal, Dynasphere and DAS) that are fabricated by
various designers:

 

 

                         

 

2.7 Lightning Parameters

 

The data about lightning parameters are essential in
deciding lightning protection level (LPL) for a lightning protection system
(LPS). Lightning current and lightning ground flash density
are two key factors in determining lightning protection level. Lightning current is utilized to decide the
measurement of lightning protection system since the peak value of the
lightning current is identified with impacts of lightning strike. Besides, the
gradient of lightning current head influences the induced voltage because of
lightning strike. In the meantime, ground flash density is number of lightning
flashes to the ground for a unit zone in a unit of time and communicates in
term of ground stroke per kilometer square every year.

 

The magnitude of the lightning current is a critical
parameter in planning a lightning protection system despite the fact that total
protection against the lightning strike is unpredictable because of high
electric potential and quick ascent time of a lightning strike. Lightning protection
system will make a way for high lightning current that makes harm to the
buildings channels to ground. Basically, there are four lightning protection
levels and each of them relates to various estimations of the lighting
parameter. The need or requirement for the structure to be ensured will
influence by the choice of lightning protection level. The maximum tolerable
hazard should constantly more than the risk of harm to the structure because of
an immediate lightning strike. The yearly recurrence of the direct lightning
strike to the structure, Nd will be utilized to assess the danger of harm to
the structure.

 

 

 

 

 

 

 

 

2.8 Fixed Angle method

 

There is an assumption been made by the researchers
and scientists, which is the air terminal can capture all the lightning strike
from making a direct contact with the buildings, only if the air terminal can
keep up a specific geometrical connection by methods for partition separate and
differ in term of height to the protected structure. The easiest technique to apply those presumption is
utilizing fixed angle method where the edge, ? is portrays as a slanted line
that secure all objects underneath it. Both 30°
and 45° are the most commonly used for beta angle, ?. The downside of this fixed angle method is, it’s only
applicable for structures with symmetrical dimension or roof –mounted
structures.

 

 

 

                            

 

 

 

 

 

 

 

 

2.9 Rolling sphere method

 

In 1977, Ralph H Lee built up a rolling sphere
strategy to be connected in protecting substation and structures before
additionally enhanced for application in the design of substation by J.T.
Orrell. Whitehead turned out with fundamental standards and speculations that
being utilized as a part of current rolling sphere technique.

 

Rolling sphere method is favored in deciding the
position of air terminal since it appropriate for a wide range of structure
neither straightforward structure nor complex structure and furthermore, can be
utilized to get the optimum zone of protection for structure. The idea of this
strategy is utilizing a radius of an imaginary sphere, R that moves over the
protected structure. A structure is thought to be ensured in the safe zone,
only if it stays beneath the bent surface of the imaginary sphere.

 

 

Protection Level

Radius

                 
I

20m

                
II

30m

               
III

45m

               
IV

60m

 

 

 

The lightning protection level relies on the fluctuation
of the radius of an imaginary sphere. In order to decide the protection zone of
the structure, the whole structures including top and side of the structure
should be rolled by those imaginary spheres. Subsequently, air terminals should
be installed in the unprotected areas where the sphere touches the structures.

 

 

The striking distance is influenced by the magnitude
of a lightning strike which corresponds to the radius of the rolling sphere.
The object which is in the range of striking distance will get the strike. The
striking distance of the lightning assumed to be similar as the radius of the
imaginary sphere.

 

 

 

 

 

2.10  Earth termination system

 

The grounding
system also is known as earth termination system is a piece of external
lightning protection system which channels the huge lightning current to pass
through via the down conductor in order to disperse it into the ground without
harming the structure. There are several factors influence the performance of
grounding system, however remote earth (earth resistance) and resistivity of
local soil ( soil resistivity) are two major parameters which must be considered.
Data about this parameters are vital to plan an effective and reliable
grounding system.

 

 

 

 

2.11 Earth
Resistance

 

In theory, the estimation of earth resistance should
be low to discharge lightning current in a shorter period to the ground and
manage the effective grounding system. Normally, an entire grounding system may
have just an earth electrodes or coordinated earth electrodes. There are a couple
of components that can influence the estimation of earth resistance, for
example, the relation between earth electrode and resistivity of nearby soil.
In standard, the estimation of earth resistance should be 10 ? or less to
guarantee that the system can work effectively.

 

In any case, if the value of earth resistance is more
than that, there are a few choices that can be implemented to drop it down. The
length of earth electrode can be expanded to put it deeper into the ground or
multi earth terminals can be utilized. Since the soil resistivity additionally
influences the estimation of earth resistance, an improving material, for
example, salt can be utilized to treat the soil since it can diminish the
resistivity of the soil.

 

2.12 Soil Resistivity

 

As stated before, the soil resistivity is another real
parameter that offers impact to the value of earth resistance and it is
estimated either in ?-m or ?-cm. The data with respect to soil resistivity can
add to give successful grounding system since it can decide the size of earth
electrode, burial depth of earth terminal into the ground and best area for
situating the earth electrode. Because of these reasons, it is key to have possess
adequate knowledge regarding soil resistivity.

 

Sort of soil gives noteworthy impact to the
resistivity of the soil. Note that, dampness content in the soil influences the
resistivity since drier soil has higher resistivity. Other than that, the
substance of minerals in the soil, for example, salt or synthetic compound
additionally can change the soil resistivity. For instance, soil that with less
salt substance has reduced the resistance. Soil resistivity can be influenced
by temperature on the grounds, due to high temperature, the resistance of the
soil turned out to be low. Estimation of earth resistance can differ by time
since many factors like moisture content; mineral substance and temperature
likewise change the soil resistivity. But the value should be kept inside
standard value to guarantee the unwavering quality of grounding system.