All about lightning and thunder
ALL ABOUT LIGHTNING
AND THUNDER
Through all history, thunder, lightning and lightning strike were natural phenomena which fascinated and frightened man. His efforts to protect himself from efects made by that phenomena were not successfull because of his unuderstanding of phisics. Noticeable progress hapened in 18th century when Benjamin Franklin with his experiments discoverd that lightning strike is actually discharge of static electricity gathered in storm clouds. Today the need that we know more abouth this phenomena has increased, and the modern devices made that possible.
Lightning strike is still a fascinant and not all the way explained phenomena. Protection from lightning stike has various definitions through history, but we still don't know how to completly explain all influences which determine certain lightning strike. But that doesn't mean that we don't know anything about it, on the contrary, there are few less or more complex theories which explain mechanisam of spreading of thunder.
We tried to explain most significant theories with llanguage that everyone can understand, to describe the effects of lightning strike and explain the ways of protecting from it. Every theme is detaly examined in that way we started from global and simple to complex and concrete things, trying to enable everyone to stop on the level which suits him.
Where ever it is possible, text is ilustrated
with pictures and more or less interactive animations, which list can be found
here. After every theme you can check your knowledge with a small quiz.
Like one of most interesting natural phenomenons, lightning, through most parts of history, has fascinated and frightened man, kept his imagination busy, but remained unexplained until recently. Even today this effect isn` t clearly explained and demistified. Scientists still have their hands full of work to understand and explain some effects about lightning strike.
In the Antic era, in beginnings of science,
and there was more gods than people in some bigger village. Almost to all nations
and cultures, lightning and thunder were divine signs.
To old Greeks thunder represented one of Zeusse´s weapons which were made for
him by Minerva, goddess of wisdom.
Both, Greeks and Romans observed the sky, were frightened by a thunder, as a
sign of gods' bad will. To get gods trust, they adored and were afraid of them.
They build their temples on the locations where the lightning stroke, which
because of that were holy to them. By sharing similar believings, Astec's tried
to get gods trust by sacrificing virgins. Similar customs could be found in
many cultures and religions of old nations, and various superstitions remained
until today. In some areas even today there are believs that the sound of church
bells can make lightning strikes go away, and the Santa´s slaides are pulled
by Donner (Thunder) and Blitzen (Lightning).
Turnover in examining the phenomenon of thunder has hapened in the middle of 18th century thanks to work and fascinating experiments of Benjamin Franklin. With helf of these experimants he proved that thunder is an electrical phenomenon, and he constructed a device that could protect objects and people within that device.That was surely great advance, but it needed to pass a lot of time until his ideas were accepted in science and everyday life.
Next bigger advance hapened on the end of 19th century, when scientists could for their research use fotogrphic and spectrographic devices. First man who succesfully measured electric current of thunder was german scientist Pockles. He measured strenght of magnetic field caused by thunder, and indirectly from that information calculated strenght of electric current of thunder (1897 - 1900).
Modern researches began with work of C.T.R.
Wilson who first measured electric field to determine the structure of charge
in clouds that participate in atmospheric discharges. With his work Wilson significantly
contributed to todays understanding of thease phenomenous, and for invention
of "Cloud Chamber", he was recieved Nobel's prize.
Science has furthermore advanced in small steps until until rapid advance of
technology and measuring technics and instruments in 60's of 20th century. That
improvement brought new possibilties of study, but also need for more efficent
protection for objects and vehicles (airplanes and space shuttles), and various
technical (electrotechnical) devices sensitive on high voltage which can accure
as result of lightning strike.
FOR THOSE WHO WANT TO KNOW
MORE
1. Lightning and politics
2. How to attract or send away lightning?
1. Lightning and politics
In ancient times people often tried to
read the Gods will observing the nature and looking for some special signs.
Although major part of these believes disappeared, some of them exist even today,
usually as a custom or tradition, but sometimes as a belief too.
In Ancient Rome, members of the College of Augurs tried to read the will of
the gods by observing the southern sky, looking for birds, lightning and shooting
stars. If they saw a lightning bolt passing from left to right they thought
it was a good sign. On the other hand, if they saw a lightning bolt passing
from right to left, they considered it a sign that Jove did not approve actual
political events. Whenever the augurs reported any sign of lightning, judges
could cancel all public assemblies for the following day. Those reports became
a useful political resource to delay unwanted meetings, the passage of laws
and to forbid some pre-election assemblies.
In 1753, Benjamin Franklin published the description of the lightning rod for
the first time. Soon after that, many co-called "Franklin rods" were
installed on buildings in America and some British colonies. They proved themselves
useful and lot of them was soon installed in all parts of the world. Despite
that success, Franklin rod had its opponents. They thought that those devices
actually attract lightning strikes to objects instead of protecting them. Some
scientists preferred blunt-ended lightning rods. They thought that blunt-ended
rods would not attract lightning to the object, but still would conduct away
any lightning if it strikes.
Debate over pointed versus blunt-ended rods soon became a question of politics,
rather than science. Identifying pointed rods with rebellious American colonies,
English king George III preferred blunt ended rods. That kind of political rather
than scientific thinking caused East India Company to replace all lightning
rods on its powder magazines in Sumatra and soon lightning destroyed one of
them.
2. How to attract or send away lightning?
We don't need to look far into history
to discover strange and sometimes even tragic customs and treatments which people
used to prevent lightning strike or "call" the rain.
At the beginning of 19th century during the dry times people attempted to "call"
the rain with this ritual: three men would climb on the tree, first one would
tie up two flammable ribbons which symbolised lightning strike. Second one would
pour water over branches imitating rain and the third one would ring the bells
trying to attract lightning strike. Were they successful we don't know.
In the medieval in Europe custom was to ring church bells during the thunderstorm.
That supposed to send away lightning strike from churches and church objects,
which often were high so they attracted the lightning strikes. The sign on churches
"Fulgura Frango" means "I break thunders" date from that
time.
Often that wasn't successful so we have information that only in France about
1753 and 1786 lightning stroked 386 church towers. Lightning current has killed
103 bellmans. 1786 governments have finally forbidden that attempts.
In 18th century military often used churches for stockpiling military materials. That was very dangerous combination of high risk from lightning strike and explosive padding. That combination often was fatal so when the lightning stroked the church tower of St. Nazaire back in 1769, result was an explosion that killed about 3000 people. However that hasn't stopped that practice so the similar accidents happened even later in history.
Before Franklin started his scientific
experiments, it was thought that electricity consisted of two opposing forces.
For Franklin electricity was like a fluid, he could change its direction as
he wanted. Exactly on that theory were created accumulators with "+"
and "-" as we know them today. From Franklin's time we know that the
positive direction of current is from area (pole) of larger to area (pole) of
smaller electricity. Plus and minus pole, conductor, armature are just some
of the words Franklin used so his theory could subsist. For lots of those words
substitution isn't found yet.
EXPERIMENT WITH THE KITE
For proving his theory that electricity is created during lightning strike Benjamin
Franklin had a lot of ideas. First idea was to draw electricity to the top of
the church tower in Philadelphia. For constructing a tower a lot of time was
needed so he had the idea that it would be easier to get close to clouds that
are "rich" of lightning strikes.
For that occasion Franklin constructed a kite. A wire was desired on the top of that kite (which should draw electricity) and on edges thread of hemp. After he constructed the kite, Franklin traveled to areas well known for lightning strikes. As he was afraid of mocking and sneering, he didn't tell anybody about his plans. He and his son went to a field and during the storm he let the kite fly. They hid in an arbor to protect themselves. In the beginning results of experiment weren't worth attention. In the moment he started to doubt in his theory he saw that the hemp threads were corrupted. They looked like every one of them was connected to a separate conductor. As it is later said in his manuscript, the feeling in that moment that lived in his body he never forgot.
To confirm his theory of electricity on
the end of the thread he attached a key and he raised the kite closer to "lightning"
clouds. After certain time on the end of the key appeared evident and clearly
visible electric spark. In the moment when the rain wet the thread and the key
they accumulated great amount of electricity.
In June of 1752 Franklin proved constancy of his theory. However, it stayed
mysterious and foggy for a long time, because Franklin didn't publish his notes.
Everything that was noted that June day, J. Presteley published 15 years later
using Franklin's notes.
FOR THOSE WHO WANT TO KNOW MORE
Besides the experiments with the kite, Franklin tried to prove his theory in
some other ways. On the top of his own houses chimney he attached iron rod to
attract the lightning. Iron rod was about 2,80 meters long. On the bottom of
rod he attached wire and put it into glass pipe. Other end of wire was attached
to the bell in the hallway. The other bell was about 15 centimeters from the
first one and between them he put small brass ball attached to the silk string.
The sechond bell was grounded with wire on the water pump in his back yard.
Every time the lightning striked, the iron rod on the top of the roof would
become electricly charged and he could hear bells. Sometimes the quantity of
electricity was enough to iluminate the whole hallway so he said that you could
find a needle. Bells rang often what we can see from his wifes letter, which
begs him to explain her how to disconnect that "thing" because it
rings during almost every thunderstorm.
Recent research of Lightning can be divided
in several directions.
One of them is developing a network for lightning detection. That kind of network
is established in lot of developed countries. It consists of sensors sensitive
for changes of electric and magnetic field caused by lightning. Those sensors
are connected and using them it is possible to observe the large area from one
place.
There are different applications of that network. One of them is to observe
the area and developing thunderstorms to assure that neccesary measures (in
electroenergetics, traffic etc.) can be taken on time. Using that network we
can collect precise information about distribution of number of lightning strikes
on some area annualy and that is valuable information for designing lightning
protection.
The other imprtant direction of research is that scientists are still trying
to record all phenomena that occurs during the lightning and to describe them
mathematically. In that efforts they used experiments with "triggered lightning".
That means that they provoked the lightning strike fireing a rocket grounded
with thin wire. Usually lightning strikes the rocket or the wire and that gives
a chance for scientists to make different kinds of measurements.
Due to the optical effects caused by atmospheric
discharge, lightning strike was called by various names among the people. If
the discharge is visible to our eye as a line that branches close to the ground
we call it "the line lightning" and if there are more such lines we
call it "ribbon lightning". Sometimes we see the discharge in form
of small glowing balls that follow one another. We call that "the pearl
lightning", and if the balls are bigger with longer tail then we have so
called "ball lightning". Sometimes we don't hear thunder, and all
we can see is light. That kind of lightning we call "the blitz lightning"
or "luster of time". Sometimes it will get other way around, we don't
see anything, but we hear the thunder. That is so called "dark lightning".
Light of lightning strike sometimes seams to be "flickering". That
happens when several lightning strikes pass through same channel in a very short
time.
Today, when physical characteristics of lightning are more or less researched,
we can say that its effects are actually a radiation of hot plasma (about 3000
deg. C). The optical effects of atmospheric discharge, which human eye can see,
can lead us to wrong conclusions about path of lightning and about its real
dimensions. Development of photography enabled us to find out more about that.
By taking pictures of lightning we found out that its light radius is 5 - 30
cm depending on which film and exposure we had, and the current of lightning
is concentrated in 1 mm diameter, as was concluded in laboratory researches.
In the channel in which discharge current
flows dominate high temperature and high pressure which contain "conductive
plasma ". This condition lasts while current flows through channel. The
current is serried by forces of its magnetic field on a small cross-section.
When current flow is stopped the pressure of plasma starts breaks free and spreads
radially.
This air pressure affects our ears and we say that we hear the thunder. In the
past people didn't understand phenomenon of thunder and lightning and they thought
that it was sign from Gods. For example, the old German god Donar represented
with the hammer in his hands was incarnation of thunder.
Pressure wave moves. Highest pressure is
in the area close to the lightning, only few centimeters far from channel of
lightning, then getting lower on some distance. Thunder, which is acoustical
effect of this pressure, can be heard on some distance of thunder impact like
detonation with rumble. If we are far from the impact, we hear some noise. On
distance of 10 km we don't hear anything. If we are very close to lightning
impact we could lose every feeling about this act. This is happens because of
the high pressure which rule in this area.
FOR THOSE WHO WANT TO KNOW MORE
Distance from lightning strike.
We believe that many people at least once
in their life asked themselves a question "How close to me lightning stroked?".
We can simply calculate that. When we see lightning we start to count seconds
until we hear thunder. We multiply number of seconds with speed of sound in
air, which is about 340 meters in second. Result is exactly what we are looking
for, distance from the lightning strike (in meters). Speed of light, which is
about 300 000 kilometres in second, is ignored because it has a lot bigger value
than speed of sound.
Example:
We see lightning. Counting 1, 2, 3, and 4. We hear thunder and multiply 340
with 4 what is equal 1360 and matches what we are looking for (distance in meters).
Beside the acoustical effects of the lightning there are noticeable mechanical effects. It is noticed that strike can damage a part of some building (roof, chimney…), destroy a tree or some wooden post but that is not all. It is noticed that some metal parts that conducted the lightning current were damaged and misshapen. It means that when we talk about the current of lightning we must pay attention to its mechanical effects that can be very important in certain circumstances.
It is determined that the force can be caused by two reasons. First reason is a great enhancement of pressure that happens when lightning passes trough some media and heats it. That pressure is especially destructive when it is caught in some closed area, not in the open air. If lightning current passes through some crack in the wall of the house, pressure increases so those walls can't stand that. It especially happens if those cracks are humid and water in them transforms into steam. Same thing happens if lightning current is passing through the capillary of some tree. Usually the pressure is very high and the tree explodes.
Second reason for appearance of force is
purely electromechanical. When current passes through some metal part put in
magnetic field, mechanical force acts and "tries to pull the part out of
the field". Force can be determined from next expression:
P = B * ig * l
P stands for mechanical force,
B stands for magnetic induction (magnetic field),
ig stands for current and
l stands for length of the conductor.
It is also known that every current produces
its own magnetic field. For example if current passes through two wires not
far one from another, every wire produces its magnetic field and every wire
is in the field made by another wire. It means that mechanical force appears
and affects both wires. Direction of force depends on directions of currents
in wires.
For example, if the currents are parallel and have the same direction, than
the force is trying to separate them. Oppositely, if currents are parallel but
have the opposite direction, than the force is trying to bring them close to
each other.
That force can be determined from this expression in wich P stands for force, uo stands for permeability of media between wires, ig stands for current, a stands for distance between wires and l for lebght of wires.
Almost all damages and deformations of electrical installations are result of this kind of forces. If the house doesn't have appropriate protection and lightning strikes into the house or transmission lines nearby, these damages could be huge.
The highest temperature in a channel of thunder is achieved on the position where electric current enters the metal surface, that is, on the bottom of the channel. Because of that on these places the surface is melted, but that is so small that the diameter of these melted parts of surface isn't greater than 5-20 mm. That's why these holes are hard to find in nature or any other place that isn't specially observed. That is explained by the fact that the major part of the energy that the lightning produces is dispersed through surroundings by radiation or pressure, and only smaller part of that energy is spent on heating and melting of the metal surface. That expense of energy on the surface for middle sized lightning is around 150 Ws, and for larger lightning up to 7000 Ws. You can expect that strong lightning can burn through steel plate 0.5 mm thick and make a hole about 20 mm in diameter.
This thermal effect of lightning (melting of material, surface or conductor) causes high pressures that can be the source of mechanical destruction, especially if these conductors, through which electricity flows, are in the walls of buildings. Eventual fires and explosions can be explained as a consequence of sparks made by metal parts on the places of input or output of electricity, or the overheating on the places where the lightning meets a high transient resistance due to bad contact or other reasons.
Very often the flow of lightning after
the hit continues through the earth. Because lightning produces heat, after
the contact with the ground we can find burnt sand in a shape of a flower. Those
are so called fulurities.
DEVELOPMENT
OF LIGHTNING STRIKE
Till now we learned that lightning strike is a discharge of static electricity accumulated in clouds but we didn't say anything about its development. The typical storm cloud responsible for lightning is "cumulonimbus" - the big, heavy cloud, ascending 15 km upward. Its base is 2 - 3 km above ground. Usually they are appearing in periods of year when the ground is warmer and the warm air is heading towards the cold atmosphere. At certain speed of that wind the drops directed to the ground are combining into smaller and bigger drops. Smaller drops are negatively charged and bigger drops are positively charged. The smaller and lighter drops are taken in higher part of cloud by the wind and then the cloud becomes negatively charged. The lower part of cloud is positively charged. That process is very complicated and the distribution of charges in cloud can be various, so it's hard to say which part of cloud will be positively or negatively charged. In that way electrified cloud could provoke the increasing of electric field near ground. If the value of electric field is between 15 and 20 kV/m it will come to breakdown. We can approximately calculate the difference between earth potential and lower part of cloud which could be a few dozen of million of volts.
The most often lightning strikes are negatively
polarised. They are starting in clouds and ending on earth. That's why we will
work out an example for this case. A negative charge from clouds is starting
to move to the earth when intensity of electric field near the clouds exceeds
the permeable strength of air and water drops (500 - 1000kV/m). The permeable
strength of some medium is the largest value of electric field when it still
doesn't come to breakdown. Further, the charge is advancing towards the ground
in steps. The individual skips of about 50 m are happening every 40 - 100 ms.
Usually after every discharge the direction is changing so the discharge looks
curved. The most advanced and starting part of charge is called leader and channel
it is passing through stays ionised and full of negative charge. As the leader
moves down towards the ground, it attracts positive charge on the ground and
on the top of high objects on it. That attraction is increasing as leader is
approaching the ground. It produces more and more charge on the ground and objects
on it and value of electric field is increasing very fast. When that field reaches
the permeable strength of the air the positive upward leader appears and starts
its way to meet the negative leader. When these leaders meet, a strong avalanche
of positive charge from ground to cloud appears through the ionised channel.
The charge is neutralised and it is called the main discharge. It usually lasts
70-100ms and that is when flash and sound occurs.
FOR THOSE WHO WANT TO KNOW MORE
The whole process of discharge might be over after one cycle but usually it is not. All charge in the cloud is probably not neutralised by one stroke. After a short while (50-100 ms) charge in cloud regroups and new discharge could appear. It follows the same mechanism as the first one except leader follows the ionised channel made in previous discharge. Usually there are several discharges (4-5). The whole phenomenon usually lasts 0.2-1s. That is we don't see the separate discharges but only one flash. Sometimes it is possible to see a kind of flickering in lightning flash and it could be explained with several discharges in one lightning strike.
All occurrences within the lightning strike
are explained in the previous chapter, but we explained them for one kind of
lightning strike only. We assumed that lightning starts in the cloud and that
downward stepped leader caries negative charge. In the areas with moderate climates
that is what usually happens (in 80 or 90 percent of all cases). Other than
that there are other distributions of charges in the clouds and they cause different
kinds of discharge. If we take a charge and a direction into consideration we
could sort them in four groups:
positive descending lightning,
negative descending lightning,
positive ascending lightning and
negative ascending lightning.
Ascending lightning start on some object on the ground and rise towards oppositely charged cloud. They are relatively rare.
Lightning currents are measured for different kinds of lightning and results could be presented with diagrams. They show the changing of current through the time within one discharge cycle.
Negative descending lightning is the most
common. Its current weakens and almost whole occurrence is over after 100ms.
Descending positive lightning is rare. Its current impulse last longer, but
the time to reach its peak value is longer too.
THE
MAIN FEATURES OF LIGHTNING STRIKE
From the standpoint of protection the most
important value is lightning current because it is passing through the hit object.
That current is not constant. It increases rapidly till the largest (peak) value.
The time between the beginning of this appearance and the peak current value
is called forehead of lightning.
The negative lightning hit produces current waves that can be relatively different.
During the first negative discharge the length of forehead is equal 10-15 Ms.
With the next discharge (if exists) the duration of forehead becomes shorter.
The peak value of current is smaller than in first discharge.
The positive lightning hits are usually made of one discharge which has duration
0,1-0,2 s. The duration of forehead is relatively long and it's value is somewhere
between 20 and 50 ms and peak value of positive current can grow even larger
than 1000 kA.
PEAK VALUE OF LIGHTNING CURRENT
Peak value of lightning current is the most important value, because with it we can calculate the drop of voltage that it makes passing through some object on the earth: U=I*R. Because of that it is important for calculating the protection from lightning strike. Of course we don't know in advance what value of current we can expect so we can't construct the protection. However we know the possibility of appearance of certain current of lightning and that information we can use in calculating the protection. More about that in chapter Probability of lightning strike.
STEP OF THE LIGHTNING CURRENT
Steep of the lightning current is actually
the speed of achieving peak value of lightning current. We calculate it by dividing
the peak value of current with duration of wave forehead. It is important to
us because these sudden changes of current in their nearness create changeable
magnetic field. The value of voltage that is induced on objects in that field
directly depends on the speed of variation of magnetic field.
Briefly: the greater the steeps of lightning current the grater are voltages
and current values on objects close to discharge, and they even don't have to
be directly hit by lightning strike. If voltage and electric current do not
damage those objects, electromagnetic forces that are inducted by passing current
still can damage them (If electric current passes through two conductors, a
force is created among them. Direction of force depends on direction of electric
current and its value depends on value of electric current and the distance
between conductors.)
During the calculation of lightning rod protection we have to think about forces
effecting the lightning rod. If they are too strong they could damage lightning
rod itself.
CHARGE OF LIGHTNING CURRENT
Charge of lightning current is the charge
that is neutralized during one strike. By itself that isn't so interesting information,
but whole energy freed during the strike depends on it. On that energy depends
melting of lightning rod or part of aluminum plate of plane.
That energy can be calculated as multiplication of charge and cathode voltage
drop: W=Q*U
SQUARE IMPULSE OF LIGHTNING CURRENT
That is a value that is magisterial for calculating the heating of lightning rod installation, which conducts the current of lightning. By definition it is calculated by formula:
where "R" is the resistance of
conductor, "i" is the value of lightning current and "t"
is the time.
THE
MODELS OF LIGHTNING STRIKE IN GENERAL
The models of lightning strike are analytical
tools for making studies about protection from lightning strike in pylons and
air transmission lines. The main use of these models is for making studies about
protection of electric-power lines or easily affected installations (gas installations
for example) from direct strike. They are also in use for determining the places
for setting and dimensioning lightning rod for protection of high buildings.
Why do we use these models?
The reason is simple: Physical characteristics of lightning strike are very
complicated and still partly unexplored and are difficult to describe mathematically.
Even when we could take in consider all characteristics we would get the model,
which would be very complicated for practical use. That is the reason for making
some simplifications and that is how different models were created.
How do we use these models?
Usually these models define a mathematical
term for "last permeable distance" or "pull radius" and
these terms are used similarly. Using geometrical construction we are trying
to determine such place and dimensions of lightning rod to achieve an adequate
protection. The adequate protection is achieved when the descending leader can
not reach the place with distance from the object smaller than the pull radius
and distance from the lightning rod greater than the pull radius in the same
time.
The classic models are based on strike
distance (or last permeable distance) S (I). If head of leader gets inside that
distance, then that object will attract the lightning. The strike distance depends
on quantity of leader's charge, which is proportional to the peak value of strike
current:
In above-said formula the peak current of lightning is inserted in kiloampers
and the last permeable distance is in meters. It is very important relation
because it is connecting the peak current of lightning and last permeable distance.
According to classic models that distance is same for the earth and for objects.
Essentially, an assumption is that when a leader comes close to some object
on permeable distance, lightning will strike into that object. Different authors
have proposed the different values of coefficients in above said formula, and
in the next table we can see some of these values.
| MODEL |  |
 |
| Whitehead et Brown | 6 | 0.8 |
| Whitehead | 6.4 | 0.75 |
| IEEE1993 | 8 | 0.65 |
| Love | 10 | 0.65 |
It is important to say that although this
is the oldest model, it is still in use, especially in protection of objects
where we can be satisfied with lower safety conditions, or in designing smaller
lightning rod installations.
This model is not like earlier models in
which the strike distance is function of current amplitude. This model considers
height of object because we can assume that lightnings will be more attracted
by them and that would match the real situation. It is made with more complex
assumings and the main diference with the classic models is that we took in
consider increasing of electric field near the high objects. Lightning can stike
into the high object only when the top of leader reaches volume above structure
which is determined with radius of attraction Ra. This model gives informations
by regresion: (in every formula distances are in meters, currents in kiloampers,
and the koeficients do not have their measures)
where is:
Radius of attraction Ra is a distance for which all lightning strikes in the
distance smaller than Ra are attracted by object. Radius of earths attraction
is zero and only objects above ground have radius of attraction. Combining experimental
and physical results Eriksson sugessted two formulas for that radius, one for
pylons (Rat) and the other for horizontal electric lines (Rac).
Except that he sugested one more simplified formula which gives simmilar results
and could be used on any objects (no matter dimensions):
where h is the hight of object above ground in meters and R is radius of attraction
in meters. Erikssons model is tryed in practice and showd that he gives better
results than classic models in most cases.
Except Eriksson's model we have other,
more complex, which considered either some specifications for protection of
certain object kinds or tried to take into calculations most of familiar lightning
strike influences.
We can mention two models:
- model of leader in general (Rizko's model),
- model of leader movement
MODEL OF LEADER IN GENERAL (Rizko's model)
This model is made after detail considering
appearance positive leader under negative leader as their later movement. This
model is advisable for configurations where the voltage of corona appearance
smaller than voltage of positive leader appearance. That is the cause that he
finds use in considering chances above overground lines. The final expressions
for calculation of radius of attraction are:
where Rac is radius of attraction for horizontal lines, h is height of line
above ground, I is lightning current and Rat (I, 40) is radius of attraction
for pylons height 40 meters. In formulas we need to insert currents in kiloampers
and height and radius in meters.
MODEL OF LEADER MOVEMENT
In creation of this model, beginning was
at the determining of resultant electric field in negative leader to simulate
the charge in cloud. Result is model that doesn't have a formula for radius
of attraction but he has maximal horizontal distance (LD) and maximal protective
distance on the ground level (PDg).
Using this model we can see series of relations for LD and PDg and different
structures of dimensions even on the different configurations of field.
GEOMETRICAL
CONSTRUCTION OF LIGHTNING PROTECTION
When we want to check the functionality
of some lightning rod installation or dimension a new one, then we usually start
with geometrical-electrical model. The classic models can still be used, or
we can use some newer model. That means that we assume that the descending leader
progress in steps. Length of those steps is equal to the length of the last
permeable distance. On the other hand, we assume that it will cause the appearance
of the upward leader on some object when the distance from that object becomes
smaller than the last permeable distance. Before any drawing or calculating
we must determine the minimum value of lightning current from which we are trying
to save the object. It is because lightning that brings less electrical charge
causes smaller current and progress in smaller steps. The last permeable distance
for such lightning is smaller and it is the worse case for designing protection.
By picking the minimum value of current we decide on quality of protection,
because a lightning with smaller current could go around the lightning rod and
hit the object. We are trying to determine the probability of that kind of hit
and make it as small as possible. The recent researches and practice gave us
the next table which shows the smallest last permeable distance for certain
percentage of all lightning strikes.
| Percentage of all lightning strikes [%] | 90 | 95 | 99 |
| Minimum current of lightning [kA] | 7.7 | 3.9 | 1.6 |
| Last permeable distance [m] | 40 | 20 | 10 |
It means that if we use the last permeable
distance 20 m long, we will protect the object from being hit by lightning in
95% cases and current greater then 3.9 kA certainly will not struck the object.
Although, there is still a small possibility that abject can be hit by lightning
with current smaller than 3.9 kA.
After that, we choose the type of protecting installation and its basic dimensions.
Using geometrical construction, we check if the dimensions are appropriate.
If they are not appropriate, we change dimensions or shape of the installation
and check again.
PROBABILITY OF LIGHTNING STRIKE
What probability is for lightning to strike
in some object? Can that be calculated? That should be main consider for setting
lightning rod installation. First step in finding that probability would be
to establish how many times lightning strike in some area. That wasn't easy
before because there were no information's or services which provided that information's.
That's why people started to explore the numbers of lightning strike days in
year for some areas. Based on that information's we have isocheraunic charts.
They contain lines that combine places which have same or similar number of
lightning strike days per year.
Those kinds of charts are maid for different areas. On the isocheraunic chart
of world we can see that the areas around equator have more lightning strike
days than on the far north or south where that number is almost zero.
Scientists have suggested few expressions that give us average number of lightning
strike days in 1 square kilometre per year. One of them is:
In this formula Ns is expected number of lightning strike in 1 square mile per
year. Ni is isocheraunic level (number of lightning strike days per year) and
alpha is geographic width of area. We can see that the results can variate and
could be used for orientation. Today some developed countries use devices which
record lightning strikes in some area. Networks of that devices give us more
precise informations about lightning strikes per year in some areas.
To get probability of lightning strike
in some object we need the information about ground surface on Earth where the
lightning will strike. That information we can get from the geometry of desired
object. How?
We imagine the model of that object and the sphere, which radius matches final
permeable distance for the average lightning strike current in that area. If
we role the sphere around that object and it touches it. We record places where
sphere touches ground and we got the wanted surface.
When we know the average number of lightning strike per year on 1 square kilometre
(Ns) and we he have wanted surface (A), then we divide Ns with A and we get
the average number of lightning strike in some object per year.
PROTECTIVE
MEASURES FOR PEOPLE
In spite many researches for mechanism of lightning and protective measures, the number of accidents increases rapidly. That number is especial high in open area because more and more people try to run away from cities for a wacation in nature. If the lightning strike is direct, more than 60% people dies instantly. Indirect lightning hit causes permanent consequences (some parts of body are paralized, high degree burns etc.) The number of accidents in houses, for example, is small because of lightning rod protection.
DIRECT LIGHTNING HIT
When lightning hits a man, certain value of voltage appears on him. Voltage will increase by increasing peak walue of current wave. If voltage exceeds the value of 100 kV, then something like "electric arc" will appear on homan body. So we can say that we got one one more paralel resistance. The electric arc resistance is much smaller than thesum of body resistance and contact resistance (Rt + 2Rk). The current that passes through human body has value of few ampers. However, the number of people who survived direct hit is small inspite the low value of current.
INDIRECT LIGHTNING HIT
Indirect lightning strike is a situation when a man isn't hit with whole lightning current but only with some part of it. The danger for man is huge if he is in circle within 100 meters from place of lightning hit. From the lightning hit place current is spreadding without control. If the current has value bigger than 100 A it can kill a man inspite of indirect lightning strike. Body parts through which the current enters or leaves have important role. A danger for man is bigger if the distance between limbs touching the ground is bigger (by walking, swimming, lying etc.).
DANGER FROM MALFUNCTIONED LIGHTNING ROD INSTALATION
Grounded metalrod creates protected area
from lightning strike arount itself. But this shouldn't deceive us. If the grounded
metal rod isn't set in the right way, the ressistance of groundning is too big
and it could be dangerous for man. The voltage between appliance for holding
and the certain place on earth (about 1-meter distance) is called "touch
voltage". Danger for man can appear when current passes through metal rod
into the ground, because his body has much smaller resistance than ground. The
lightning current passing through ground creates voltage drop which distribution
is exponentialy: near the appliance for holding voltage valuse is at the maximum
and is dropping with bigger distance from it. If a man wit his own body touches
two different points then the certain voltage will have effect on him. Voltage
between two points on earth (about 1-meter distance) is called "step voltage".
Danger from touch voltage and step voltage will be smaller if the lightning
rod instalation is properly installed, ground resistance must be smaller as
possible.
PROTECTION FROM INDIRECT LIGHTNING STRIKE
When a person is out side of area which has good lightning rod instalation, it is hard to recommend and prescribe conficiently protective measures from lightning strike. General recommendation is to take any cover, and for this activity stays between 5 and 10 mins after we hear first weak thunder. The best covers in that situation are cars, railroad wagons or abandoned houses. In many cases we don't have option to find safety in closed places. In that case is recommended to "lower" our own height. It is good to find cover in some ditch or under a tree.
PROTECTION FROM DIRECT LIGHTNING STRIKE
People can suffer damages from lightning strike if they are near some objects that have malfunction lightning rod instalation or don't have lightning rod instalation at all. If we are inside an object that doesn't have lightning rod instalation, it is recommended that we stand in the middle of object, far away from walls as possible.
For protection of people near the lightning rod instalation, sometime we use metal panel because it prevents appearance of touch and step voltage. During setting metal plate we need to have in mind that it must be connected with metal rod. It is also important that both, metal plate and metal rod have much smaller resistance than human body so the current doesn't passes through it.
PROTECTION
FOR ELECTRIC POWER PLANTS
Atmospheric overvoltages in electric-power plant can be developed in two ways: during the direct lightning hit in some parts of a plant and the travelling wave from overground that enters into that plant.
PROTECTION FROM DIRECT HIT is made of lightning
rod sticks or protection ropes, which we described with some specific qualities.
Usualy, ends of lightning-rod sticks are grounded. We need to have in mind that
the lightning current creates overvoltage that appears on casing of grounded
devices in electic plants (transformators, generators, etc.) That overvoltage
should not be so high to indanger the insulation of devices. The only way to
influence that overvoltage is to reduce ground resistance as low as we can.
Transformer is a device that needs to be most protected. Often the casing of
transformer is grounded about 15 meters from lightning rod groundning. If that
solution is not possible, then we protect the transformer with extra overvoltage
drains.
PROTECTION FROM TRAVELLING WAVES is managed
with overvoltage drains. Overvoltage drains are devices, which operate like
this:
On overvoltage drain could appear a voltage smaller than voltage which goes
through drain. In that case, it behaves like insulator and it will not couduct
current. When voltage in drain exceeds the voltage that goes through drain,
it starts conduct current. The current through drain is variable, but the voltage
in the overvoltage is same as overvoltage that goes through drain.
Overvoltage drains are installed on the entrance into the electric plant. One
side is connected on phase conductor and the other side is grounded. If the
travelling wave passes through overground transmission line, it will come across
overvoltage drain on the entrance into plant, which starts to work and loses
only smaller overvoltages. That overvoltage is same like voltage that goes through
drain and it is not dangerous for devices in plant. However, if lightning hits
overground transmittion line near by plant, than overvoltage drain can' t conduct
away all overvoltages and could be destroyed. To avoid that, the overground
transmittion lines near electric plants are higher protected and that eliminates
possibility of direct lightning hit near plant. It is called "protected
approach".
Travelling wave originated far avay from plants is not dangerous because of
conductor resistance influence and phenomena called "corona". Such
wave through drain isn't dangerous for drain and plant any more.
PROTECTION FOR OVERHEAD TRANSMISSION LINES
Because of its form, the overhead electric-power
lines are sort of objects that are pretty well exhibited to risk of lightning
hits. Lightning that hit the overhead electric-power lines could make a higher
voltage than insulators on overhead transmission lines may stand and then will
come to skipping and short circuit. Current bow made on that way will not turn
off for a long time because the voltage of phase conductor is enough for its
maintaining.
When the lightning hits transmission line, travelling waves could appear, which
will be taken to the plant or consumers and cause large damage. It could also
cause a protection to turn on and shut down the line. For this reason, it is
very important to obstruct those hits.
One of the fundamental ways for protection
of overhead transmission line is putting the protecting rope, which is connected
with transmission-line pylon and each pylon is good grounded. Protection rope
should be installed over the conductor so they can be inside the protected area.
As permeable distance depends on peak lightning current and overvoltages on
conductors too. Question is what is the minimum value of current we should protect
the conductor from? The protected area is always calculated for critical current
while the lightning with smaller current than critical could still hit the conductor.
Criterion is that the critical current should not make the overvoltages, which
will be larger than voltages that an insulation of conductor can hold out. We
protect the transmission lines from that critical current.
Calculations for determining the value of that current are complicated, but
foundations are visible in next animation.
In praxis, with protecting rope are covered
all transmission lines over 30 kV. Mostly, there are two protecting ropes. When
lightning hits the protecting rope there could come to large drop of voltage
while passing through pylon and grounded conductor. In that case phase conductors,
which are on nominal voltage, would have a voltage much smaller than part of
pylon they are hooked on. The insulation between pylon and conductor wouldn't
stand that, and "recurrent skip" would appear. Such skipping could
permanently damage the insulation and throw the line out of operation for a
long time.
The resistance of grounded pylon should be smaller to prevent this. It should
not be larger than 15 ohms.
There is one more device for decreasing that overvoltage named protecting sparklet. It is set parallel to insulator chain where are phase conductors and it's insulating the phase conductors from transmission-line pylon. The voltage of skipping on sparklet is smaller than insulation of transmission line could stand. The main function is to move the arch originated from recurrent ship on insulator chain and that way it protects the insulator from damage. The sparklets are usually installed on the transmission lines over 110 kV but sometimes on the nets of lower voltages, on especially critical places. Its function is also to make the more favourably distribution of electrical field around insulators chain and on that way it protects from different kinds of discharge (corona).
FOR THOSE WHO WANT TO KNOW MOREWhen lightning strikes the transmission lines, a travelling wave appear. It means that lightning current divides into two separate currents and each of these currents goes to one part (side) of transmission line. Those currents are shaped as a wave, they quickly achieve their peak value but their decreasing lasts much longer.
Travelling waves spread along the transmission lines with speed of light (300000 km/s). Because of the resistance of the lines, their peak value of current decreases as they move. Also, steep of their current decreases with time. That fact is explained with enhancement of electrical resistance for very high voltages and currents. A part of the wave with small value of current and voltage moves with speed of light (all electromagnetic waves travel with speed of light). Another part of wave with high values of current and voltage is slows down due to appearance of corona. That way corona lowers the steep of the current.
When travelling wave comes to any crossing, it divides itself into several smaller waves and each of them proceeds to other branch. Value of current in each branch depends on total resistance in that branch. If the resistance is smaller, the current is higher and vice versa. That fact is good for us when travelling wave spreads through the protecting rope. The rope is grounded on every post and a part of travelling wave is conducted to ground.
The main danger for transmission lines is appearance of high voltage between conductor and the grounded post or its construction. That voltage can cause a spark and discharge from the conductor to grounded construction of the post. That danger increases with the speed of current change (di/dt), or with higher value of steep of current of the travelling wave that is the same thing. The most inconvenient circumstance is that that way started current will not stop flowing after travelling wave is gone. Nominal voltage of the transmission lines is high enough to keep it flowing. If that situation last long, the insulators that separate conductors from the construction could be damaged or destroyed and whole transmission line shut down for long time.
This is an occasion to explain all of fundamental
elements of lightning-rod installations and their functions for protection of
buildings and other objects against lightning strike.
The first element is appliance for holding. It is the most emphasised part of
lightning-rod and it's function is to pull closer and take over the lightning
strike and in that way to protect objects below it. It appears in two primary
shapes: like a rod or like a rope.
The second function of lightning-rod installation is to bring away the taken
current of lightning from appliance for holding to earth with certainty. In
this purpose, here we are putting one or more drains. They must resist the warming
up caused by current of lightning passing through them.
The third function is to bring away the taken current of lightning as good as
possible to the earth. In this purpose are subservient grounded conductor, which
are dug in earth and connected to drain. Their resistance should be as small
as possible so that drop of voltage could be smaller. That drop of voltage appears
on the drain of lightning-rod and if it isn't small enough there could appear
skipping across drains toward the other objects. It is especially relate to
objects that are grounded on other way (for example, plumbing or gas-installations).
For these reasons there is tendency to make the grounded resistance and the drop of voltage smaller. In the same time, the measure named equalisation of potential is executed. It means that the grounded conductor should be connected with other metal parts from surrounding.
That way the skipping across drains toward
the other objects can be barred.
Important part of lightning-rod installation is grounded conductor. It must
conduct the current of lightning to the earth well, that is, it's resistance
should be small as possible. That resistance is depending on earth feature and
shape of grounded conductor. The earth feature important for construction of
good-grounded conductor is "specific resistance of soil". It is defined
as an electrical resistance of one cube (size 1m3) of homogeneous soil. If the
specific resistance is bigger then dimensions of grounded conductor should also
be bigger. The references of specific resistance of soil are visible in this
table:
| type of material | specific resistance [Ohm*m] |
| sea water | 0.5 |
| water of rivers and lakes | 0.5 |
| swamp ground | 90-150 |
| plowed ground | 90-150 |
| humid small-grained sand | 90-150 |
| humid big-grained sand | 200-400 |
| dry small-grained sand | 500 |
| dry big-grained sand | 1000-2000 |
| rocky ground | 1000-3000 |
| limestone | 500-1000 |
| concrete | 150-500 |
The most usable constructions of grounded
conductor are:
-lace - formed like metal lace which is covered with earth. Most often, the
lace is mode of galvanised steel and less often it is made of copper.
-stick - formed like metal stick or tube which is covered with earth vertically
-fundamental - metal conductors which are installed in foundations of object
and over the large area of concrete they are in contact with surrounding soil.
While current is passing through the grounded conductor
and its wide-spread through the earth, there is distribution of potential.
Most often, it is in form of potential funnel. It means that the potential is
the largest near the grounded conductor and with increase of distance from grounded
conductor its value is falling off. It is logic because current makes the largest
drop of voltage while it spread on a small area near grounded conductor. Farther
away from the grounded conductor, current has larger area and smaller resistance
so the drops of voltage are smaller.