ABSTRACT
Earthing and lightning protection systems protect people from the danger of electrocution
and equipment from over-voltages by dissipating the excess energy into the earth, and at
the same time reducing the potential difference between equipment and earth. This study
investigated the effect of lightning strikes on people, buildings, communication towers
and equipment by determining the number of lightning strikes on the structures
depending on their location and height as well as the earthing resistances. With the aid of
data obtained from April 1995 to February 2003, from the National Aeronautic and Space
Agency (NASA), Lightning Imaging Sensor (LIS) Science Team and from NASA’s
Optical Transient Detector and colour coded onto a map, the annual flash rate for Zaria
was obtained. Based on this, the number of lightning strikes on two structures; the control
tower of the Zaria Aerodrome and the communication tower of the Zaria Nitel Exchange;
were determined. The Fall-of-Potential Method (also referred to as Earth-Spike Method)
was method used for the earth resistance measurement. The null-balance analogue type
Megger Earth Tester was used as the test instrument to determine the earth resistance of
the following locations within Zaria: Department of Electrical Engineering, Zaria
Aerodrome Control Tower, Commercial Complex, No. 8 Sokoto Road and Nitel
Exchange Communication Tower. The earth resistance measurements taken for the
various locations were in conformity with the general standards: “The resistance to earth
of the earth electrode subsystem should not exceed 10Ω at fixed permanent facilities”.
All the earthing resistances measured around the ring earth of the Nitel Communication
Tower were within NITEL recommended resistance of less than 1Ω.
TABLE OF CONTENTS
TITLE PAGE i
DECLARATION ii
CERTIFICATION iii
DEDICATION iv
ACKNOWLEDGEMENTS v
TABLE OF CONTENTS vi
LIST OF FIGURES viii
LIST OF TABLES ix
LIST OF SYMBOLS x
ABSTRACT xi
CHAPTER ONE: INTRODUCTION
1.1 INTRODUCTION 1
1.2 MOTIVATION FOR STUDY 2
1.3 EARTHING 4
1.3.1 OBJECTIVES OF EARTHING 4
1.4 LITERATURE REVIEW 5
1.5 RESEARCH OBJECTIVES AND METHODOLOGY 7
1.6 THESIS OUTLINE 8
CHAPTER TWO: THEORETICAL BACKGROUND
2.1 EARTHING 9
2.1.1 THE ROLE OF THE EARTHING SYSTEM 13
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2.1.2 EARTH RESISTANCE MEASUREMENT 14
2.2 LIGHTNING 15
2.2.1 OCCURRENCE OF LIGHTNING 17
2.2.2 GROUND-TO-CLOUD LIGHTNING 21
2:2.3 EFFECTS OF LIGHTNING 22
2.3 LIGHTNING PROTECTION 24
2.4 SURGE PROTECTION DEVICES AND THE GROUNDING SYSTEM 25
CHAPTER THREE: METHODOLOGY
3.1 INTRODUCTION 26
3.2 DETERMINATION OF NUMBER OF LIGHTNING STRIKES 28
3.3 EARTHING RESISTANCE MEASUREMENT 32
3.4 FINDINGS OF THE QUESTIONAIRE 39
CHAPTER FOUR: RESULTS AND ANALYSIS
4.1 INTRODUCTION 40
4.2 DETERMINATION OF LIGHTNING STRIKES 40
4.3 EARTHING RESISTANCE MEASUREMENT 42
4.4 ANALYSIS OF RESULTS 43
CHAPTER FIVE: CONCLUSIONS AND SUGGESTIONS FOR FURTHER
WORK
5.1 INTRODUCTION 47
5.2 CONCLUSIONS 48
5.3 LIMITATIONS 59
5.4 SUGGESTIONS FOR FURTHER WORK 50
REFERENCES 52
APPENDIX A: SAMPLE QUESTIONNAIRE 55
CHAPTER ONE
INTRODUCTION
1.1 INTRODUCTION
Electricity today is playing an ever increasing role in the lives of everyone in the civilized
world. The high in demand on use of electricity has resulted in the increased danger to
human beings and their properties. Not only defects at consumer’s premises, but even at
supply authority’s premises can electrocute a customer at his own premises. Proper
earthing protects humans from this danger of electrocution [1].
Lightning is one of nature’s most unpredictable events. It strikes at random, damaging
buildings, endangering lives, corrupting data, creating expensive repairs and costly down
time . Lightning strikes the earth, on average, one hundred times per second (as there are
over eight million lightning strikes per day around the world [2]). The tallest structures
(communications towers and masts) in cities across the world are most vulnerable to
lightning strikes due to the induction effect between a charged cloud and them. Many of
the world’s historic and heritage rated buildings, like the Eiffel Tower as shown in Figure
1.1, are at risk because of their construction methods and the lack of sophisticated fire
protection systems [2].
Proper earthing provides return paths for large number of electronic equipment or radio
frequency (RF) antennas, etc. Earthing is necessary for proper functioning and protection
of certain equipment. Some of the equipment suffer physical damages caused by induced
transients on service wiring running inside or entering the structure. Engineers, designers,
consultants and managers have a responsibility to provide a safe environment for
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employees, patrons and microelectronic computer and communication systems as a
correctly designed and installed earthing system will safeguard both lives and equipment
[3].
Figure 1.1: Lightning Striking the Eiffel Tower
1.2 MOTIVATION FOR STUDY
Rainstorms with high thunderstorm activities have been known to have caused the losses
of lives, electrical gadgets damage, total failure of electrical and/or telecommunications
installations in Nigeria.
According to the Daily Trust of 24th of September, 2006, a residential building located at
Kontagora town of Niger State was hit by a lightning strike as a result of which the
building caught fire killing the occupants of the building (a woman and her six children).
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The server room of the Ahmadu Bello University Zaria Network located at the seventh
floor of the Senate building in which digital video broadcasting (DVB) satellite receiver,
low noise block (LNB), Satellite transmitter and other equipment worth over two million
Naira (N2,000,000) were lost as a result of lightning strike in 2006. This was attributed to
poor earthing of the VSAT equipment
Several cases of electronic gadgets have been reported damaged and/or lost due to
lightning strikes. Catastrophic failures of telecommunications equipment are particularly
common in regions of high thunderstorm activities. A typical example is the case of the
Nigerian Telecommunications Limited (NITEL) Digital Exchange in Jos in which the
AXE-10 Ericsson equipment and most of its components got damaged on the 13th of
September, 1997. The cause of the failure was a lightning strike that hit the tower which
was believed to have been poorly earthed. Because the poor earthing system was not
immediately identified and corrected, there was a repeat incidence at the Exchange in
July, 1998 [4].
All the electronic printed cards inside the Motorola Base Radio Communication
equipment at the Power Holding Company of Nigeria (PHCN) Mubi got burnt after the
tower was hit by lightning on 21st October 2004. Another case is that of the Nigeria
Television Authority (NTA) Yola transmission tower, which was hit by thunderstorm and
lightning on 6th April 1992. The tower was damaged along with some vital transmission
equipment [4].
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In August 1995, during the intermediate approach in a thunderstorm into an airport in
Denmark, an A320Airbus was hit by lightning strike – according to the crew, no aircraft
systems were affected. However, the approach had to be interrupted with a subsequent
diversion to Copenhagen because the destination airport was hit by lightning as well,
which caused a complete electricity supply outage including loss of Instrument Landing
System (ILS) and communication [5].
These examples emphasize the need for buildings, towers and equipment to be properly
earthed by professionals according to standard practices in order to minimize damage and
loss of lives and properties as a result of lightning strikes during thunderstorms.
1.3 EARTHING
Earthing may be defined as a system of electrical connections to the general mass of earth
[6]. Earthing a building, tower or equipment implies connecting it to general mass of
earth by means of a metal plate or rod or electrical conductor. Earthing protects human
beings from danger of electrocution by dissipating the excess energy into the earth and at
the same time reduces the potential difference between equipment and earth. A correctly
designed and installed earthing system will safeguard both lives and equipment [4, 7]
1.3.1 Objectives of Earthing
The objectives of an earthing system include the following:
i) To provide safety to personnel during normal and fault conditions by limiting step
and touch potential;
ii) To guarantee correct operation of electrical and electronics devices;
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iii) To prevent damage to electrical and electronic apparatus;
iv) To dissipate lightning strikes;
v) To stabilize voltage during transient conditions; and
vi) To divert stray radio frequency (RF) energy from sensitive audio, video, control and
computer equipment [8].
The Institute of Electrical and Electronic Engineer (IEEE) Guide for Safety in Substation
Earthing [8] states that a safe grounding design has two objectives:
i) To provide means to carry currents in to the earth under normal and fault
conditions without exceeding any operating and equipment limits or adversely
affecting the Main Distribution Frame (MDF) [8]. However, the path followed by
these diverted currents (within the grounding system) can cause dangerous overvoltages
capable of damaging existing equipment, depending on the exact
discharge path and the parameters of the lightning strike. Consequently, Surge
Protection Devices (SPDs) should always divert the current to the same reference
of the protected current and should do so through the most direct path to avoid
potential difference within the circuit.
ii) To assure that a person in the vicinity of grounded facilities is not exposed to the
danger of critical electric shock.
1.4 LITERATURE REVIEW
There are more than a thousand thunderstorms around the Earth causing some 6,000
flashes of lightning strikes every minute and each large thunderstorm has more total
energy than that of an atomic bomb. Even an individual lightning charge may contain 30
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million volts at 100,000 amperes and can cause serious damage to properties and even
loss of lives [9].
According to Conroy et al [9], a study was carried out in 1996 to determine the most
effective method of installing low resistance earth grounding. It was determined that
deep-driven ground rods would offer the best solution, if full rod contact could be
maintained. By 1998, a new process was developed for installing deep-driven ground
rods. This new process helped in overcoming the problems associated with installing
deep ground rods.
Schmitt et al [10] carried out a study on the simulation of lightning over-voltages in
electrical power systems. Lightning strikes which directly strike towers were found to
increase the potential of the towers affected and can, dependent on the level of the tower
footing resistance and the electric strength of the overhead line insulators, lead to
backward flashovers from the tower to an overhead line conductor. These backward
flashovers cause traveling waves which propagate via the overhead line towards the
switchgear where they cause over-voltages which can pose a risk to any items of
equipment connected, such as cables or transformers. They concluded that with the use of
metal-oxide arresters, these over-voltages are reduced to values that are within an
adequate safety margin.
Dzara [4] carried out a study to determine the soil resistivity and earth resistance for the
Nigeria Telecommunications Limited (NITEL) Digital Exchange Mubi and concluded
that the earthing plan had been implemented according to NITEL specifications. Average
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earth resistance values obtained ranged from 0.23Ω to 0.56Ω. These values lie within the
Nitel recommended value being less than 1Ω.
This work is aimed at investigating the effect of lightning strikes on people, buildings,
communication towers and equipment as a consequence of the earthing standard
employed. This is based on the fact that most buildings do not adhere to standard building
and electrical installation practices as non professionals are involved in the building
construction, electrical earthing and lightning protection systems. The investigation
involved the use of questionnaire, determining the number of lightning strikes and the
earthing resistance of certain locations around Zaria.
1.5 RESEARCH OBJECTIVES AND METHODOLOGY
Electricity and Electronics play a vital role in the life of everyone. Modern electronic
systems, such as computers, radar and telecommunication equipment and/or installations
are highly sensitive and susceptible to lightning and electrical surges and other transient
pulses received through power or signal lines. Proper grounding and bonding through low
impedance ground is essential to facilitate dissipation of these unwanted signals. The
objectives of this study are, among others to:
i) Determine the extent of exposure of electrical and electronics gadgets in buildings
and structures and their occupants within Zaria to the danger of direct hit by
lightning strikes. The possible number of lightning strikes will also be
determined;
ii) Determine the earthing resistance of some buildings around Zaria and compare
the measured earthing resistance to the standard specified earthing resistance; and
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iii) Determine the earthing resistance of Zaria NITEL exchange and compare the
value obtained with the standard specified earthing resistance of the industry.
The methodology adopted in carrying out this work involves the following:
i) Use of questionnaire (Appendix A1) to determine the extent of damage caused by
lightning strikes on such persons, equipment, buildings and towers within and
outside the Ahmadu Bello University Main Campus;
ii) Measurement of earth resistance using the Megger equipment on some buildings
and towers earthing points. Some of these buildings and towers include NITEL
Exchange Zaria, Control Tower at the Zaria Aerodrome, Electrical Engineering
Department building etc; and
iii) Comparison of the measured earthing resistance of the various earthing points
taken in ii) to the recommended earthing resistance for such buildings and/or
towers.
1.6 THESIS OUTLINE
The thesis consists of five chapters. Chapter one contains the introductory material
together with the literature review. Chapter two contains the theoretical background of
the work. Chapter three describes the methodology used in carrying out the study while
Chapter four describes the analysis of the results obtained. Chapter five presents the
conclusions and recommendations for further work.
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