Everyone wanted to know if it was true that after a lightning strike, a Chinese citizen crashed to the ground, quickly jumped up, shook himself off and wanted to move on, but a second lightning knocked him down again and again without fatal outcome. There are many similar stories. Popular books and magazines will tell you about the massive defeat of football players at the stadium, passengers at the bus stop, and almost an entire herd of cows in the pasture. The stories are creepy. A dozen people are in the hospital. But in the hospital, not in the cemetery. Could the danger of lightning be greatly exaggerated if a person is able to withstand its direct impact? But who said that the impact is direct? Most often this is not the case.
A lightning discharge is accompanied by a strong electric current. Even for an average lightning strike it is close to 30,000 A, and for the most powerful it is almost an order of magnitude more. Ultimately, this current spreads in the soil throughout the entire volume of the Earth. Any lightning rod must be grounded. To do this, a grounding conductor is installed at the lightning rod. It is formed by one or more underground grounding electrodes, vertical or horizontal. From the metal electrodes, the current flows into the ground, where, as in any conductor, Ohm's law applies. The product of current and resistance gives voltage, in this case the voltage across the ground electrode:
The expression seems to be familiar, but still not quite, because we are talking about voltage in the ground, which is considered to be zero. After all, that’s why they are grounded, so as not to get under voltage. And here it turns out upside down, and not in a figurative sense, but in a very literal sense. Tension acts on a person through his feet, which are normally and firmly planted on the ground. This requires explanation. And we need to start with the simplest. How good a conductor is soil? The answer seems obvious - certainly a good one, if electricians and safety professionals always talk about grounding. Science and technology are accustomed to specific assessments. The words a lot, a little, good and bad do not explain the essence of the matter. The quality of conductors is assessed by their resistivity. For good soil it is close to 100 Ohm*m - a billion times more than for black steel! The comparison is more than convincing. A very large volume through which the lightning current spreads in the ground helps out.
I don’t want the reader to catch me giving a qualitative description, so I’ll immediately move on to quantitative assessments. To do this, instead of the usual voltage, it is useful to use another parameter from school physics. We will talk about the electric field strength. This is the name given to the magnitude of the voltage drop in some medium per unit length, for example, the voltage drop in the ground over a length of 1 m. By the way, a length of 1 m is the approximate step length of an adult. Remember, voltage is measured in volts per meter. If the electric field in the ground E gr is equal to 1 V/m, a voltage will act between a person’s legs at a length l = 1 m
Time to evaluate the electric field of lightning current in the ground. Let's imagine that it struck a lightning rod, the grounding rod of which is made in the form of a hemisphere with a diameter of d = 0.5 m (a medium-sized saucepan or cauldron for pilaf) and buried in the ground, as shown in Fig. 1. Lightning current I M will flow symmetrically from the surface of the metal hemisphere, where its density will be
For an average lightning strike with a current of 30,000 A, in our case it turns out j M ≈ 7.6 × 10 4 A/m 2. The following is a complete analogy with Ohm's law. To obtain the ground tension E gr, it is necessary to multiply the current density by the soil resistivity ρ.
Even if we focus on highly conductive soil (ρ ≈ 100 Ohm*m), we get a very impressive value of 7,600,000 V/m. The voltage at a step length of 1 m here will be almost eight million volts. It is difficult to imagine that a Chinese television person would be able to endure this without harm to his health. Most likely, a second zipper would not be required.
The value obtained here is called by specialists step voltage (they also say - step tension). It is important to understand how it changes in the vicinity of the lightning strike. If the soil is the same everywhere, everything is determined by the lightning current density. As you move away from the hemispherical ground electrode, the surface through which the current flows due to symmetry will remain hemispherical. and its radius r will continuously increase. Along with it, the area of the hemispherical surface “filled” with current will increase, and its density will correspondingly decrease.
The electric field strength will also begin to decrease rapidly
At a distance of r = 10 m from the initial millions in our example, a little less than 5,000 V/m will remain. This is also sensitive, but, as a rule, not fatal, because the duration of the high voltage, like the duration of the lightning current, is hardly more than 0.1 milliseconds. A high-voltage step can easily knock you off your feet, but a person most likely has enough strength to get up.
If the reader is not tired of the numbers and has reached this line, then it will be easy for him to understand where the old recommendation not to hide from a thunderstorm under large trees came from. Due to the significant height, a lightning strike is most likely to occur in them. When struck, current will flow through the root system of the tree as through a ground electrode. Close to the roots, the electric field is especially strong. It is clear that standing here is not recommended, sitting and especially lying down too, because the length of a person is twice the length of his step.
If we return to the numbers again, we must admit that they are not at all overestimated. A lightning current of even 100,000 A is not particularly rare, and the soil resistivity can be tens of times greater than that used in the estimates. For this reason, life-threatening step voltage can be kept at a sufficiently large distance from the point of lightning strike. Finally, the shape of the ground electrode must be taken into account. All estimates above were made for a hemispherical ground electrode. Its electric field, as can be seen from the above formulas, decreases very quickly - inversely proportional to the square of the distance. More often, grounding conductors are mounted from long busbars or rods that bear little resemblance to a hemisphere. Their electric field decreases much more slowly. As a result, the radius of dangerous exposure to lightning increases very noticeably, sometimes up to many tens of meters. This explains the mass casualties of people on the beach or on the football field.
Here are the results of calculating the step voltage for a typical grounding device, which is recommended by the domestic lightning protection standards. It consists of a horizontal bus 10 m long and three vertical rods 5 m each - two at the edges of the bus and one at the middle. Soil resistivity 1000 Ohm*m (unmoistened sand), lightning current 100 kA. This is powerful lightning - 98% of lightning discharges have less current. The numbers on the graph are impressive - hundreds of kilovolts directly at the ground electrode, over 70 kV at a distance of 15 m and at least 10 kV at a distance of 40 m.
When the Cathedral of Christ the Savior was being restored in Moscow, the designers took into account that given its considerable height, one should expect an almost annual lightning strike. It is possible that this blow will occur on a holiday, with a large crowd of people on the porch. To guarantee the safety of parishioners, it was necessary to ensure that the lightning current spreads through a very extensive system of underground busbars, thereby minimizing step voltages.
A strong electric field in the ground brings another nuisance. When the field strength rises to 1 MV/m, ionization begins in the ground. Under certain conditions, this leads to the growth of a plasma channel, which slides along the surface of the soil, slightly burrowing into it. Channels (and there may be several of them, as in this photograph taken in the laboratory) can move from the point where the lightning current is introduced
tens of meters. In fact, they should be considered as a continuation of lightning, only not in the air, but along the surface of the earth. It must be said that this does not make them any less dangerous, because the current in the channel is tens of percent of the lightning current, and the temperature is obviously higher than 6000 0. I hope the reader does not need much imagination to imagine the consequences of such a channel coming into contact with a fuel leak area on an oil loading rack or with an underground cable, for example, a telephone cable or one that controls a microelectronic system.
In the dry year of 2010, central television broadcast a report from a village in the Omsk region that was completely burned down in a thunderstorm. A Moscow correspondent asked the village grandmothers: “Why didn’t they extinguish it?” They answered in unison; “It was scary - fiery arrows were crawling along the ground.” Take another look at the photo. Does it really look like it? The grandmothers were not in vain to fear. The electric field at spark channels is not much different from the field at metal busbars. Getting close to them can easily end in death.
What is presented is enough to convince oneself of the ingenuity of lightning. You have installed reliable protection from above with the help of lightning rods, and it breaks through to you with a roundabout maneuver, making its way along the surface of the earth. That is why almost all popular articles end with an appeal not to forget about the professionals. It is risky to joke with menacing natural phenomena and it is unacceptable to treat them lightly.
E. M. Bazelyan, Doctor of Technical Sciences, Professor
Energy Institute named after G.M. Krzhizhanovsky, Moscow
Krzhizhanovsky. Lightning strikes into the tower were photographed simultaneously from several buildings in the vicinity of Ostankino.
The currently accepted theory of lightning formation is that particle collisions in clouds lead to the appearance of large areas of positive and negative charges. When large oppositely charged areas come close enough to each other, some electrons and ions, running between them, create a channel through which the rest of the charged particles rush after them - a lightning discharge occurs. The air heats up to 30 thousand degrees - five times more than the surface temperature of the Sun.
The hot medium expands explosively and causes a shock wave, perceived as thunder. Interestingly, lightning is observed not only on Earth, but also in the atmospheres of Venus, Jupiter and Saturn. About 2,000 lightning storms occur on Earth at the same time. More than 100 lightning strikes the Earth's surface every second.
Probably many people notice that lightning flickers. It turns out that one lightning usually consists of several discharges, each of which lasts only a few tens of millionths of a second. There are two types of lightning between a cloud and the ground: positive and negative. Positive discharges occur only in 5% of cases, but they are stronger.
It is believed that it is positive discharges that lead to forest fires.
It is common practice these days to avoid bringing theology into the explanation of lightning. However, it should be noted that lightning was considered messages from the gods in many cultures. The most famous lord of lightning is probably the ancient Greek god Zeus. In ancient Athens, it was believed that the place where lightning struck was sanctified by Zeus. Another famous master of thunder and lightning is the Scandinavian god Thor.The ancient Romans believed that a person killed by lightning had done something wrong before the god Jupiter, and no burial ceremony was performed for him. Many peoples made medicines from stones that were struck by lightning. The Romans, Hindus and Mayans believed that mushrooms grew in places where lightning struck the ground.
Can a person survive a lightning strike?Yes. A person has a significant chance of surviving a lightning strike. Firstly, although the temperature during a discharge is very high, it usually does not last long and does not always lead to serious burns. Secondly, the main lightning current often passes along the surface of the body. This is why most people struck by lightning do not die. According to various estimates, from 5% to 30% of those affected die. Your chances of survival increase significantly if you have a person nearby who knows how to perform artificial respiration and cardiac massage.
Often victims of lightning strikes appear dead, but in fact they have suffered cardiac arrest. Immediate use of artificial respiration and cardiac massage can bring them back to life.An American named Roy Sullivan, a forester by profession, was included in the Guinness Book of Records because he survived seven lightning strikes that he experienced between 1942 and 1977. Twice the hair on his head caught fire, he received several burns on his body, but survived! He is a true professional. Don't try to repeat this.
How safe is it to be on a plane during a thunderstorm?
According to statistics, lightning strikes airplanes on average three times a year, but these days it rarely leads to serious consequences. The worst aviation accident caused by lightning occurred on December 8, 1963 over Ecton in Maryland, USA. Then the lightning that struck the plane penetrated the reserve fuel tank, which led to the ignition of the entire plane. As a result of this disaster, 82 people died. Since this tragedy, a number of changes have been made to aircraft design, and modern airliners are now fairly well protected from lightning strikes. However, thunderstorms still pose a significant risk to aircraft due to the presence of strong updrafts and downdrafts.Will a car save you from lightning?
It is quite safe to be inside a car during lightning if the body and roof are made of metal. The interior of a car, made of rubber and plastic, serves as a good insulator, and the main lightning current usually passes through the outer metal body of the car. One day, strong lightning struck a car driving along a highway in Iowa, USA. The broken down car stopped, but the driver remained safe and sound and was only very frightened. The car's electrical system had completely failed, there were many small holes in the metal body, and the tires had melted. A small crater about ten centimeters deep formed around the car. But the most significant consequence for the driver, whose name was Rod, was that after this incident his acquaintances began, jokingly, to call him Rod-Lightning.
First of all, lightning is a very beautiful phenomenon in itself.
Secondly, lightning regulates the amount of nitrogen in the air that is consumed by factories. But sometimes lightning works wonders. For example, according to an article published in Scientific American in 1856, an intense lightning strike that struck the ground in the city of Kensington, New Hampshire in the United States created a well about 30 centimeters wide and 3 meters deep, which soon filled with clean water. Another amazing case occurred with a man, an electrician by profession, from the city of Greenwood in North Carolina. After a direct lightning strike that struck him 31 years ago, he survived, but after that he completely stopped feeling the cold. Now he can spend hours outside in summer clothes at sub-zero temperatures without feeling any discomfort. There are stories about how some blind people regained their sight after being struck by lightning. There is published evidence that being struck by lightning has led to an improvement in a person's intellectual abilities, which has been confirmed by psychological tests. One gentleman claimed that after being struck by lightning he became "oversexed" because no one could satisfy him anymore.
Security measuresIn general, being in a house during lightning is quite safe. During a thunderstorm, you should not talk on the phone (excluding wireless and cellular), hold on to metal pipes, or repair electrical wiring.
However, in rare cases, lightning can also get inside the house.
This happened, for example, with one house in Denmark. Lightning penetrated through the chimney, knocked off the plaster on the walls of the living room, tore the curtains to shreds and smashed the wall clock to smithereens, leaving unharmed a canary sitting in a cage next to the clock... then the lightning, breaking 60 window frames and all the mirrors, passed through the door into the backyard, killing a cat and a pig there.Do only thunderstorms produce lightning?
Lightning usually appears during a thunderstorm, most often in summer or spring. It is rare, but it happens that lightning strikes in winter during heavy snowfalls and snowstorms. Winter lightning is very strong and produces very loud and long thunderclaps. In some cases, lightning has also been observed inside giant smoke clouds above active volcanoes. For example, lightning strikes and even miniature whirlwinds of smoke, reminiscent of a tornado, accompanied the spectacular birth of a volcano on the island of Setsi near Iceland. Lightning is also known to appear in the giant plumes of smoke produced by forest fires.Where on Earth is there the most lightning?
There is a myth that lightning can only strike when it rains. In fact, lightning can travel up to ten kilometers from an area where it is raining.
Apparently, this is where the expression “bolt from the blue” arose. Recent studies of deaths due to lightning strikes show that most accidents occur after a thunderstorm. During a thunderstorm, people usually hide from the rain, but as soon as it passes, they come out of their shelters. However, the danger of a lightning strike remains for about ten or even more minutes after the rain stops. Remember that if you hear thunder, you are still dangerously close to the thunderstorm.
According to studies, lightning strikes oak trees more often than other tree species. As for people, statistics say that lightning strikes men much more often than women. In the UK over a two-decade period, 85% of those killed by lightning were men. A recent study of lightning deaths in Florida, USA, shows that 87% of those killed were men.
An amazing story happened with the husbands of the Bulgarian woman Marta Maikia. In 1935, American tourist Randolph Eastman asked to wait out the storm in her house during a thunderstorm.
A week later they got married, but 2 months later the man was killed by lightning. Martha Maikia later remarried, this time to a Frenchman named Charles Morteau. And while traveling in Spain, the second husband was also struck by lightning.Summarizing a large amount of evidence made it possible to draw up an average “portrait” of ball lightning. Most often it has the shape of a ball, but they also talk about pear-shaped, oval and jellyfish-shaped lightning. Its size in most cases ranges from 5 to 30 centimeters, the “lifetime” is usually about 10 seconds, but sometimes more than a minute;
it moves at a speed of 0.5-1 meter per second. Color is usually red, orange or yellow, much less often blue, white or dark blue. Ball lightning can enter a room not only through an open window or door. Sometimes, it becomes deformed and seeps into narrow cracks or even passes through glass without leaving any traces in it.
The behavior of ball lightning is unpredictable. Sometimes it simply disappears, and in other cases it explodes, sometimes causing significant damage. There is a hypothesis that ball lightning occurs as a consequence of a linear lightning discharge. However, in 20% of cases, ball lightning was observed in clear weather.
A mysterious and tragic incident occurred in 1978 with a group of climbers in the mountains of the Western Caucasus. Ball lightning in the form of a bright yellow tennis ball penetrated into the tent in which five people were lying at night. At first, the ball moved slowly at a height of one meter above the floor, and then began to attack sleeping climbers, burning through their sleeping bags. At the hospital, the victims were found to have severe wounds. But these were not burns - in some places pieces of muscle were torn out literally down to the bone. The ball killed one climber. International master of sports in mountaineering V. Kavunenko said something strange: “It was not ball lightning that was operating here... The fiery beast mocked us for a long time and persistently...”
Today, there are more than a hundred hypotheses that claim to explain the physical essence of ball lightning.
However, none of them can be confirmed with a sufficient degree of reliability. The exotic behavior of ball lightning gives scope for the most unbridled fantasies. Often in descriptions of eyewitnesses there is an attitude towards lightning as a living creature. There is an opinion that lightning is an analogue of a UFO or a creature from a parallel world with an incomprehensible mind and logic.
15. Overvoltage of direct lightning strike Experts call overvoltage any short-term increase in voltage in the electrical network above its nominal level. Here we will consider overvoltages that are caused by lightning current at the site of the strike. The simplest situation is when lightning is absorbed by a specially installed lightning rod. Her current I through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance R voltage is released U Experts call overvoltage any short-term increase in voltage in the electrical network above its nominal level. Here we will consider overvoltages that are caused by lightning current at the site of the strike. The simplest situation is when lightning is absorbed by a specially installed lightning rod. Her current R= through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance they say Experts call overvoltage any short-term increase in voltage in the electrical network above its nominal level. Here we will consider overvoltages that are caused by lightning current at the site of the strike. The simplest situation is when lightning is absorbed by a specially installed lightning rod. Her current h. This is a lot of stress. For example, when through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance mol = 100 kA and voltage is released z = 10 Ohm it turns out
R = 1000 kV. Approximately the same potential will be in the immediate vicinity of the lightning rod. A nearby underground cable will take up almost the same potential and, unless special measures are taken, will transmit it along the cable inside the protected building, causing damage to the insulation, which was not designed for such a high voltage. voltage is released Let's reproduce another practically significant situation, assuming that the metal lightning rod mast simultaneously performs the function of a lighting mast and therefore the insulators of the overhead line feeding the lamps are attached to it. The potential of the mast at the point where the lamp insulators are attached is noticeably higher than R, because the voltage drop across the inductance of the mast (or the down conductor busbars that are laid along it, if the mast itself is non-conducting) is added to the voltage drop across the ground electrode. Voltage amplitude across inductance L voltage is released equal to R, because the voltage drop across the inductance of the mast (or the down conductor busbars that are laid along it, if the mast itself is non-conducting) is added to the voltage drop across the ground electrode. Voltage amplitude across inductance(L=/di dt )max, where the expression in brackets determines the rate of current growth at the pulse front. In assessing the average duration of the pulse front of the first lightning component T L=/di f » 5 µs for a current of 100 kA, easy to obtain ( Experts call overvoltage any short-term increase in voltage in the electrical network above its nominal level. Here we will consider overvoltages that are caused by lightning current at the site of the strike. The simplest situation is when lightning is absorbed by a specially installed lightning rod. Her current)max" )max, where the expression in brackets determines the rate of current growth at the pulse front. In assessing the average duration of the pulse front of the first lightning component they say/ R, because the voltage drop across the inductance of the mast (or the down conductor busbars that are laid along it, if the mast itself is non-conducting) is added to the voltage drop across the ground electrode. Voltage amplitude across inductance f = 2´1010 A/s, which is for inductance voltage is released equal to R, because the voltage drop across the inductance of the mast (or the down conductor busbars that are laid along it, if the mast itself is non-conducting) is added to the voltage drop across the ground electrode. Voltage amplitude across inductance(L=/di= 30 µH (mast height ~ 30 m) gives voltage is released)max = 600 kV. Total value voltage is released they say = voltage is released R+ voltage is released supposedly that’s why almost all the tension voltage is released the pier acts on the insulation of the power circuit relative to the ground, ultimately blocking it. This is a typical example of lightning overvoltages, equally dangerous for both low-voltage networks and high-voltage power lines, where a support or lightning protection cable of the line acts as a lightning rod.
16. Induced overvoltages from lightning
This is the most common type of overvoltage, for which the electromagnetic field of lightning is responsible. Here we will consider separately the consequences of a change in the magnetic field of the lightning current and the consequences of a change in the charge carried by its channel approaching the ground. To some extent, this division is a convention, but it is convenient for understanding the essence of the matter.
If an arbitrary circuit is placed in a magnetic field B, an EMF of magnetic induction will be induced in the circuit voltage is released magician" - SA B. Here A B=d B/d t– rate of change of magnetic flux penetrating the contour of the area S. Let, for example, this circuit be created by a twisted pair of wires that are connected to a computer. Then the circuit area is very small, about 10 cm2 (based on a cable several meters long). Let us also assume that the wire runs along the wall of a building at a distance r = 1 m from a down conductor parallel to it, which diverts the lightning current from the lightning rod to the ground. The estimate from above should focus on the extremely high rate of growth of the lightning current A I. Current regulatory documents give the value A I = 2∙1011 A/s. The growth rate of the magnetic field, which corresponds to it, is estimated as
,
where m0 = 4p∙10-7 H/m – magnetic permeability of vacuum. In the example under consideration F B » 4∙104 V/m2 and therefore voltage is released magician = - SF B » 40 V. The obtained value should not be neglected. It is an order of magnitude greater than the operating voltage of a modern microcircuit and will certainly damage it.
An idea of a different scale of overvoltages is given by estimates for an overhead power line with a voltage of 220/380 V. Here, the area of the circuit formed by the phase and neutral wires easily reaches S= 100 m2. Even a distant lightning strike at a distance r= 100 m from the line leads to an average growth rate of the magnetic field of ~ 400 V/m2, which gives an overvoltage of 40 kV, which is certainly dangerous both for the transformer substation and for the consumers it supplies. 
Now about the electrical component of induced overvoltages. It is caused by a flow of electric charge, which is induced by the electric field of the lightning channel. The charge of the channel is quite heavy, about 0.5 - 1 mC per meter of length, and the electric field near the ground that it excites is many times greater than the electric field of a thundercloud. Score by field E they say » 200 kV/m will not be too high. Now imagine a conductor with electrical capacitance WITH, placed above the ground at a height h. This could be a horizontal wire (for example, an antenna), a metal casing of some kind of unit, or a building structure. The potential from the lightning channel charge is high h, equal voltage is released el = E R= h will induce a charge on a grounded conductor Q = C.U. email After a lightning strike to the ground, when the charge of its channel is neutralized and the electric field disappears, the induced charge flows from the conductor into the ground through the grounding resistance through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance h. The current from the flowing charge will create a voltage drop across the conductor relative to the ground. This can be quite a decent amount. If, for example, the capacity of an object C = 1000 pF (a wire about 100 m long), and the height of its suspension above the ground is 5 m, then the charge of the lightning channel will create a potential of up to voltage is released el = E R= h= 200´5 = 1000 kV. As a result, the induced charge will be Q = C.U. el = 10-9´106 = 10-3 Cl. When neutralizing the ground part of the lightning channel in time D t» 1 μs current will flow through the grounding resistance of the conductor i»
Q/D t= 10-3/10-6 = 1000 A, which will cause a voltage drop across the ground resistance through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance z = 10 Ohm size voltage is released el = ithrough the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance z = 1000´10 = 10 kV.
17. High potential skid 
This not very euphonious and not entirely accurate phrase in lightning protection refers to the delivery of high voltage to the protected object via its overhead or underground communications. The object itself may not be struck by a direct lightning strike. Let lightning strike a completely different structure, a tree, or even just the ground. Spreading in the ground near the affected structure, the lightning current will create a very high voltage on its ground electrode, voltage is released z = Experts call overvoltage any short-term increase in voltage in the electrical network above its nominal level. Here we will consider overvoltages that are caused by lightning current at the site of the strike. The simplest situation is when lightning is absorbed by a specially installed lightning rod. Her current R= through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance h. (for example 300 kV if through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance h. = 10 Ohm, a Experts call overvoltage any short-term increase in voltage in the electrical network above its nominal level. Here we will consider overvoltages that are caused by lightning current at the site of the strike. The simplest situation is when lightning is absorbed by a specially installed lightning rod. Her current mol = 30 kA). The metal shell of the communication, which is connected to the same ground electrode, will be under the same voltage. A voltage wave can spread over long distances along a communication line, especially if it is ground-based and does not leak electrical charges into the ground. But even underground communications can transport a high voltage wave over a distance of hundreds of meters without noticeable attenuation. The higher the soil resistivity, the more efficient the transportation. In rocky formations, dry sands or permafrost soils, high drift potential is dangerous even over distances of several kilometers.
Of particular note are modern communications made of plastic pipes. Inside they have an electrolyte (in extreme cases, tap water, which is also a good conductor), quite suitable for transmitting high voltage over long distances, and on the outside there is high-quality plastic that reliably isolates the internal environment from contact with the ground. Now leaks into the ground are completely eliminated. It is easy to imagine the consequences of a person touching the metal tap of such a communication. Standing on the ground at zero potential, he will be exposed to the full voltage that is transmitted through the liquid channel.
18. Overvoltage from lightning current propagation through metal shells
The metal shell is reasonably considered an effective electromagnetic shield. However, it does not completely protect against the effects of lightning surges on internal circuits. The cause of overvoltage can be easily understood from the following figure. Lightning current, spreading along a metal shell of length l, creates a voltage drop D across it voltage is released = through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance 0lI, Where through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance 0 – resistance 
units of shell length. The inner wire is connected to the beginning of the shell and therefore accepts its potential at the point of contact. Potential of the other end of the shell due to voltage drop from current Experts call overvoltage any short-term increase in voltage in the electrical network above its nominal level. Here we will consider overvoltages that are caused by lightning current at the site of the strike. The simplest situation is when lightning is absorbed by a specially installed lightning rod. Her current on D voltage is released less. This means that there will be a voltage between the end of the inner conductor and the end of the sheath voltage is released e = D voltage is released = through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance 0lI. The following estimate allows us to understand what values we can talk about here. Let the length of the steel shell l = 100 m, and its cross-sectional area is 100 mm2. Then the linear resistance will be through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance 0 = 0.001 Ohm/m, which is with lightning current Experts call overvoltage any short-term increase in voltage in the electrical network above its nominal level. Here we will consider overvoltages that are caused by lightning current at the site of the strike. The simplest situation is when lightning is absorbed by a specially installed lightning rod. Her current= 100 kA will result in overvoltage voltage is released e = through the lightning rod, and then through the down conductors enters the ground electrode and spreads in the ground. At the same time, at the grounding resistance 0lI = 0.001´100´100 = 10 kV. This is quite enough to damage the insulation of a 220/380 V lighting cable.
A more rigorous analysis shows that the metal sheath does not completely protect against overvoltages in two-wire systems. The fact is that the potential accepted by the internal conductor depends on its internal location. All conductors are equivalent only in a circular shell. If the cross-section of the shell is not circular (for example, it is a rectangular box), the potentials of the conductors will be different and a voltage will appear between them. As a rule, it is orders of magnitude lower than the value just estimated, but this is also enough to damage the microcircuit to which the cable pair is suitable.
19. Protective effect of lightning rods
Since the times of Franklin and Lomonosov, it has been accepted that lightning is directed to the highest structure on the earth's surface. This position can still be accepted today, but with a fundamental caveat: Lightning is most likely to travel towards the tallest structure.
The probability of a lesser defeat is also non-zero. From the most general considerations it is clear that this probability decreases with increasing height difference. This means that for reliable protection, the height of the lightning rod must be greater than the height of the protected object. The greater the required reliability, the higher the lightning rod must be.
Lightning rods are often selected based on their protection zones. It is assumed that the reliability of protection will not be lower than the specified value if the object is entirely located inside the protection zone. For a rod lightning rod, the protection zone is represented in the form of a cone, the apex of which lies on the vertical axis of the rod. From the above it follows that the top of the zone should be located below the top of the lightning rod if the guaranteed protection reliability is greater than 0.5. To verify this, it is enough to assume two closely spaced grounded rods of equal height, considering one of them a lightning rod and the other an object. It is clear that over a long observation period the rods will absorb an equal number of lightning strikes (50% protection reliability). To ensure a reliability of 0.9 or 0.99, the rod designated as a lightning rod must necessarily become taller in order to absorb most of the lightning. The above is equally true for cable lightning rods.
Even with a very large difference in height, a lightning rod cannot provide ideal protection. In the photo presented here, lightning missed the top of the Ostankino TV tower by 202 m. This case is not unique.
In practice, they operate with a protection reliability of 0.9 or 0.99 (one lightning strike out of 10 or out of 100 breaks through to the protected object), rarely - 0.999. For a single rod lightning rod with a height h£
The 30 m radius of the protection zone with a reliability of 0.9 at ground level is approximately r 0 = 1,5h. and with a reliability of 0.99 r 0 = 0,95h. The use of a system of many lightning rods significantly expands the protection zone. With a reasonable location, the protected volume can be several times larger than the sum of the protection zones of each lightning rod individually. This is widely used by specialists.
If you correctly calculate and install a lightning rod on the roof of your house or near it, you can hardly worry about roof burns. Even with a protection reliability of 0.9, less than one lightning bolt will break through to a house of relatively small height in 100 years. Unfortunately, such a lightning rod will have almost no effect on the electromagnetic effects of lightning. It is these impacts that become the main cause of emergency situations.
20. Protection from electromagnetic effects of lightning
For modern technology this is the most important problem. Firms with a staff of thousands of people develop and produce equipment to protect power electrical circuits, telephone lines, television channels, and even means of protecting your home from unwanted “guests” from electromagnetic influences.
Protective devices, regardless of their design, are often called surge suppressors. Imagine some kind of two-wire electrical circuit that enters your home. Let it be, for example, a 220 V network. You will not have problems if the magnitude of lightning overvoltages in the network is limited to a level that is safe for insulating internal wiring and equipment connected to the network (for example, a TV, microwave oven or computer). At an operating voltage of 220 V, the insulation will briefly withstand an increase in voltage by 3 to 5 times, hardly more. This means that at the entrance to the house it is necessary to install a device that will prevent the overvoltage from rising higher.
The mechanical system is unsuitable here due to its inertia. Any mechanical relay operates in units to tens of milliseconds, and lightning overvoltage caused by lightning current grows approximately 100 times faster. The required speed is provided only by semiconductor or gas-discharge devices. Today both of them are successfully used.
The basic idea is this. At the point where the overhead network enters the house, a washer sintered from zinc oxide is installed parallel to the wires. Its thickness is chosen so that at a voltage of 220 V it practically does not allow current to pass through and behaves as a perfect insulator without affecting the electrical circuit. However, when a lightning overvoltage occurs, the conductivity of the washer increases very quickly. In fractions of a microsecond, it approaches the conductivity of a metal conductor. The short circuit thus created does not allow the overvoltage to pass to the equipment inside the building and it remains undamaged. When the lightning current dies out and the overvoltage disappears, the zinc oxide washer returns to a non-conducting state in the same fractions of a microsecond. During such a short time of its operation, the circuit breakers and fuses do not have time to operate and the power supply to the house is not disrupted.
Other semiconductor devices, varistors, work in approximately the same way. Only their operating voltage changes (it can be very low to protect microprocessor technology), but the principle of operation remains unchanged). Due to their simplicity of design, semiconductor surge suppressors (SVRs) are widely used. They can be mounted in a small-sized case, approximately the same as household machines, and can be easily mounted on a line of conventional switching equipment. However, today specialists are increasingly turning to old and long-known gas-discharge devices. In them, the protected circuit is closed not by a semiconductor washer, but after the breakdown of a special short-length spark gap.
Gas-filled spark gaps are a more complex device than a semiconductor limiter. It must include a device for breaking the arc with short circuit current in the electrical network. This arc cannot go out by itself; it is extinguished by a special blast. But the spark gap is more reliable, and most importantly, it does not suffer at all from a random, not very strong, but long-term increase in voltage in the electrical network, say, when, due to phase imbalance, 270 - 300 V is maintained instead of the normal 220 V. From such an overvoltage, oxide - the zinc washer opens slightly, begins to pass current, overheats and fails. Nothing like that threatens the spark gap.
21. Why lightning is at odds with amateurs
The chapters you read give an idea of the versatile weapons of lightning. After all, one of her weapons might work. It is not easier for a person if, having managed to protect his structure from a direct lightning strike, he suffers from high potential drift, lightning overvoltages in the electrical network or failures of electronic equipment that sent a false command. Lightning protection must be comprehensive and necessarily compatible with the technological purpose of the facility.
Half measures are not suitable here. Moreover, it is possible that a short-sighted decision could aggravate the dangerous effects of lightning. That is why a lightning protection project must be prepared by a specialist. He must carefully evaluate the danger of all possible effects of the high-temperature channel, current and electromagnetic field of lightning. Not only the design features of the protected object must be taken into account, but also its surroundings on the surface of the earth and even underground communications. An amateur cannot do this.
It is very important that lightning protection means are not “hung” on an already installed object, but are developed at the project stage. Only then will it be possible to combine lightning protection elements with the structural details of the protected object as much as possible and thereby save a lot of money. It is not uncommon when a completely insignificant change in the design of an object, which does not affect its technological functions, entails a very sharp increase in lightning resistance. Only highly qualified specialists are capable of making such decisions.
After this “News” TV show, even pop stars could not compete with the popularity of high-voltage workers. Everyone wanted to know if it was true that after a lightning strike, a Chinese citizen crashed to the ground, quickly jumped up, shook himself off and wanted to move on, but a second lightning knocked him down again and again without fatal outcome. There are many similar stories. Popular books and magazines will tell you about the massive defeat of football players at the stadium, passengers at the bus stop, and almost an entire herd of cows in the pasture. The stories are creepy. A dozen people are in the hospital. But in the hospital, not in the cemetery. Could the danger of lightning be greatly exaggerated if a person is able to withstand its direct impact? But who said that the impact is direct? Most often this is not the case.
A lightning discharge is accompanied by a strong electric current. Even for an average lightning strike it is close to 30,000 A, and for the most powerful it is almost an order of magnitude more. Ultimately, this current spreads in the soil throughout the entire volume of the Earth. Any lightning rod must be grounded. To do this, a grounding conductor is installed at the lightning rod. It is formed by one or more underground grounding electrodes, vertical or horizontal. From the metal electrodes, the current flows into the ground, where, as in any conductor, Ohm's law applies. The product of current and resistance gives voltage, in this case the voltage across the ground electrode:
The expression seems to be familiar, but still not quite, because we are talking about voltage in the ground, which is considered to be zero. After all, that’s why they are grounded, so as not to get under voltage. And here it turns out upside down, and not in a figurative sense, but in a very literal sense. Tension acts on a person through his feet, which are normally and firmly planted on the ground. This requires explanation. And we need to start with the simplest. How good a conductor is soil? The answer seems obvious - certainly a good one, if electricians and safety professionals always talk about grounding. Science and technology are accustomed to specific assessments. The words a lot, a little, good and bad do not explain the essence of the matter. The quality of conductors is assessed by their resistivity. For good soil it is close to 100 Ohm*m - a billion times more than for black steel! The comparison is more than convincing. A very large volume through which the lightning current spreads in the ground helps out.
I don’t want the reader to catch me giving a qualitative description, so I’ll immediately move on to quantitative assessments. To do this, instead of the usual voltage, it is useful to use another parameter from school physics. We will talk about the electric field strength. This is the name given to the magnitude of the voltage drop in some medium per unit length, for example, the voltage drop in the ground over a length of 1 m. By the way, a length of 1 m is the approximate step length of an adult. Remember, voltage is measured in volts per meter. If the electric field in the ground E gr is equal to 1 V/m, a voltage will act between a person’s legs at a length l = 1 m
Time to evaluate the electric field of lightning current in the ground. Let's imagine that it struck a lightning rod, the grounding rod of which is made in the form of a hemisphere with a diameter of d = 0.5 m (a medium-sized saucepan or cauldron for pilaf) and buried in the ground, as shown in Fig. 1. Lightning current I M will flow symmetrically from the surface of the metal hemisphere, where its density will be
For an average lightning strike with a current of 30,000 A, in our case it turns out j M ≈ 7.6 × 10 4 A/m 2. The following is a complete analogy with Ohm's law. To obtain the ground tension E gr, it is necessary to multiply the current density by the soil resistivity ρ.
Even if we focus on highly conductive soil (ρ ≈ 100 Ohm*m), we get a very impressive value of 7,600,000 V/m. The voltage at a step length of 1 m here will be almost eight million volts. It is difficult to imagine that a Chinese television person would be able to endure this without harm to his health. Most likely, a second zipper would not be required.
The value obtained here is called by specialists step voltage (they also say – step tension). It is important to understand how it changes in the vicinity of the lightning strike. If the soil is the same everywhere, everything is determined by the lightning current density. As you move away from the hemispherical ground electrode, the surface through which the current flows due to symmetry will remain hemispherical. and its radius r will continuously increase. Along with it, the area of the hemispherical surface “filled” with current will increase, and its density will correspondingly decrease.
The electric field strength will also begin to decrease rapidly
At a distance of r = 10 m from the initial millions in our example, a little less than 5,000 V/m will remain. This is also sensitive, but, as a rule, not fatal, because the duration of the high voltage, like the duration of the lightning current, is hardly more than 0.1 milliseconds. A high-voltage step can easily knock you off your feet, but a person most likely has enough strength to get up.
If the reader is not tired of the numbers and has reached this line, then it will be easy for him to understand where the old recommendation not to hide from a thunderstorm under large trees came from. Due to the significant height, a lightning strike is most likely to occur in them. When struck, current will flow through the root system of the tree as through a ground electrode. Close to the roots, the electric field is especially strong. It is clear that standing here is not recommended, sitting and especially lying down too, because the length of a person is twice the length of his step.
If we return to the numbers again, we must admit that they are not at all overestimated. A lightning current of even 100,000 A is not particularly rare, and the soil resistivity can be tens of times greater than that used in the estimates. For this reason, life-threatening step voltage can be kept at a sufficiently large distance from the point of lightning strike. Finally, the shape of the ground electrode must be taken into account. All estimates above were made for a hemispherical ground electrode. Its electric field, as can be seen from the above formulas, decreases very quickly - inversely proportional to the square of the distance. More often, grounding conductors are mounted from long busbars or rods that bear little resemblance to a hemisphere. Their electric field decreases much more slowly. As a result, the radius of dangerous exposure to lightning increases very noticeably, sometimes up to many tens of meters. This explains the mass casualties of people on the beach or on the football field.
Here are the results of calculating the step voltage for a typical grounding device, which is recommended by the domestic lightning protection standards. It consists of a horizontal bus 10 m long and three vertical rods 5 m each - two at the edges of the bus and one at the middle. Soil resistivity 1000 Ohm*m (unmoistened sand), lightning current 100 kA. This is powerful lightning - 98% of lightning discharges have less current. The numbers on the graph are impressive - hundreds of kilovolts directly at the ground electrode, over 70 kV at a distance of 15 m and at least 10 kV at a distance of 40 m.
When the Cathedral of Christ the Savior was being restored in Moscow, the designers took into account that given its considerable height, one should expect an almost annual lightning strike. It is possible that this blow will occur on a holiday, with a large crowd of people on the porch. To guarantee the safety of parishioners, it was necessary to ensure that the lightning current spreads through a very extensive system of underground busbars, thereby minimizing step voltages.
A strong electric field in the ground brings another nuisance. When the field strength rises to 1 MV/m, ionization begins in the ground. Under certain conditions, this leads to the growth of a plasma channel, which slides along the surface of the soil, slightly burrowing into it. Channels (and there may be several of them, as in this photograph taken in the laboratory) can move from the point where the lightning current is introduced
tens of meters. In fact, they should be considered as a continuation of lightning, only not in the air, but along the surface of the earth. It must be said that this does not make them any less dangerous, because the current in the channel is tens of percent of the lightning current, and the temperature is obviously higher than 6000 0. I hope the reader does not need much imagination to imagine the consequences of such a channel coming into contact with a fuel leak area on an oil loading rack or with an underground cable, for example, a telephone cable or one that controls a microelectronic system.
In the dry year of 2010, central television broadcast a report from a village in the Omsk region that was completely burned down in a thunderstorm. A Moscow correspondent asked the village grandmothers: “Why didn’t they extinguish it?” They answered in unison; “It was scary - fiery arrows were crawling along the ground.” Take another look at the photo. Does it really look like it? The grandmothers were not in vain to fear. The electric field at spark channels is not much different from the field at metal busbars. Getting close to them can easily end in death.
What is presented is enough to convince oneself of the ingenuity of lightning. You have installed reliable protection from above with the help of lightning rods, and it breaks through to you with a roundabout maneuver, making its way along the surface of the earth. That is why almost all popular articles end with an appeal not to forget about the professionals. It is risky to joke with menacing natural phenomena and it is unacceptable to treat them lightly.
E. M. Bazelyan, Doctor of Technical Sciences, Professor
Energy Institute named after G.M. Krzhizhanovsky, Moscow
We hope that in the future this site will serve as an elementary textbook on self-defense from lightning. We plan to constantly post here articles about the real dangers of lightning electricity and modern lightning protection means. They are designed to help you understand the essence of the problem and evaluate the ways available to you to solve it.
There is a common stereotype that lightning strikes from top to bottom. This is far from true, because in addition to ground-based lightning, there are also intra-cloud lightning and even lightning that exists only in the ionosphere.
Lightning is a huge electrical discharge, the current in which can reach hundreds of thousands of amperes, and the voltage can reach hundreds of millions of watts. The length of some lightning in the atmosphere can reach tens of kilometers.
The nature of lightning
The physical nature of lightning was first described by the American scientist Benjamin Franklin. In the early 1750s, he conducted an experiment to study atmospheric electricity. Franklin waited for the stormy weather to set in and launched a kite into the sky. The snake was struck by lightning, and Benjamin came to the conclusion about the electrical nature of lightning. The scientist was lucky - at about the same time, the Russian researcher G. Richman, who also studied atmospheric electricity, died from a lightning strike in the apparatus he had designed.
The processes of lightning formation in thunderclouds have been most fully studied. If lightning passes through the cloud itself, it is called intracloud. And if it hits the ground, it is called ground.
Ground lightning
The process of ground lightning formation includes several stages. First, the electric field in the atmosphere reaches its critical values, ionization occurs, and finally a spark discharge is formed, which strikes from the thundercloud into the ground.
Strictly speaking, lightning strikes only partially from top to bottom. First, an initial discharge rushes from the cloud towards the ground. The closer it comes to the earth's surface, the more the electric field strength increases. Because of this, a response charge is ejected from the surface of the Earth towards the approaching lightning. After this, the main lightning discharge is emitted through the ionized channel connecting heaven and earth. He really hits it from top to bottom.
Intracloud lightning
Intracloud lightning is usually much larger than ground lightning. Their length can be up to 150 km. The closer the area is to the equator, the more often intracloud lightning occurs in it. While in northern latitudes the ratio of intra-cloud and ground-based lightning is approximately the same, in the equatorial zone intra-cloud lightning accounts for approximately 90% of all lightning discharges.
Sprites, elves and jets
In addition to the usual thunderstorm lightning, there are such little-studied phenomena as elves, jets and sprites. Sprites are similarities to lightning that appear at altitudes of up to 130 km. Jets form in the lower layers of the ionosphere and appear as blue discharges. Elven discharges also have a cone shape and can reach a diameter of several hundred kilometers. Usually elves appear at an altitude of about 100 km.
