Webinar «Features of high-rise buildings lightning protection», page 1

High-rise construction has become fashionable in our country, perhaps unjustifiably fashionable with our poorly covered territories. However, high-rise buildings have become a reality, almost daily routine. Height of the object leads to the increase of the number of lightning strikes into it. Correct to say - to the increase of frequency of being struck by lightning, because lightning strikes not only hit high-rise buildings, they are also initiated on their tops, moving toward the charged cell of a thundercloud. If the building is higher than 200 m,  it gets hit by downstream lightning 2 - 3 per year and if the building is higher, the number of strikes doesn't increase. Only the number of reverse lightning grows (from the building to the cloud - upstream). The behavior of the downstream and upstream lightnings is fundamentally different. The trajectory of the first ones is poorly predictable, they can hit the side surface of a high-rise building far below its top. Any high-altitude lightning rods are ineffective for protection against downstream lightnings. Upstream lightnings usually start from the highest component of the structure, especially if its top is of a small radius.

The seminar offers to consider the prospects of protection of high-rise buildings from direct lightning strikes, and special attention will be paid to the equipment installed on the roof. In addition, we will study the question about electromagnetic impact of lightning current on the internal electrical circuits of high-rise buildings; and we will analyze the peculiarities of such buildings' grounding device.

Finally, it will be important to track the influence of high-rise buildings to the stormy conditions in its immediate surroundings. Here two points are most important - consequences of attraction of lightning, in the result of what,the intensity of storm activity increases locally and the excitation of high voltages at the spreading of lightning current in the ground becomes more frequent, which increases the risk of bringing the high potential into neighboring buildings and life hazard of humans and animals by step voltage.

Webinar text. Page 1

Peculiarities of lightning protection of high-rise buildings

Good morning, good afternoon, depending on where my students are, I don't know. But that gives me great pleasure to congratulate you with Happy New Year, because New Year in Russia is the kind of thing that lasts for a month. So Happy New Year, dear colleagues, wish you new happiness and let everything be will be well with you! I am again with Nadezhda at the camera. My assistant will deal with your questions and I will try to answer them as it was last time. That is I will give some block of information, and then I'll try to answer your questions and then a new information block. OK let's begin. You know, we've already talked about protection of individual buildings. Our organizers say that now we have to talk about high-rise buildings. It should be marked to what extent the lightning protection of these high-rise buildings differs from, let's say, protection of single -family houses or five-storey building. Frankly speaking, I really don't understand we still build high-rise buildings having such vast territories. I understand when these buildings are industrial and when this height is justified by the technological purpose, but we  build high-rise residential buildings even where there is a lot of free space. Nevertheless, they are being built and the designers need to protect them. And I will think what actually needs to be done to protect these high-rise buildings? What is different between the lightning protection of what we are doing.

The objective of the seminar

We will study the following questions. But, first of all, it is necessary to see what is done to the building constructions, are they facing anything dangerous or not? Secondly, the inner filling of the building. Thirdly, its electrical circuit. And finally - the environment. And what will happen around this high-rise building? And what news could be here? I should say that the first paragraph, concerning construction of building structures, was most likely invented in order to seem important. Lightning can do nothing to any building structures of modern buildings. Let me give you an example. There was a flagpolen the Ostankino tower , on which there was the USSR flag. After 10 years of being blown by the wind it became shabby and patted climbers climbed to replace the flag cloth. We persuaded them to remove the crown of the flagpole, which was struck by at least several hundreds of lightnings for these 10 years. And what did we see? We saw traces of lightning strikes on this steel bar, that were similar to pockmarks in the portrait of a person who had smallpox. This tiny holes not more than 1 mm deep and with a diameter of at most - about 10 mm, but mostly a few millimeters. And that's why it is naive to talk about building structures and that these constructions may be affected by lightning. So I am not coming back to this question.

Normative base

Now about the normative base. It is always necessary to start with the instructions for lightning protection of buildings, industrial communications structures - industry standard 153. If you open that instruction, you will see there is nothing special in the buildings. The only thing there is - is protection of lightning rods. But lightning rod protection zone reach height of 150 meters, industry standard 153 doesn't work further than 100. Today, 150 m can hardly be considered as a very tall building. In any case - it is not a skyscraper. If you see AD 34, right here, then you will really find is high-altitude lightning rod protection zones. And these zones of high-altitude lightning rods extend to 600 meters. Let's have a look at these areas for a moment. First of all, I have to say that these zones are largely imagined and computer designed, but they are practically not justified by operating experience. Because we have such an operating experience. First of all - this is the power line, they are maximally  60 m high. And secondly, some holes, like on the Ostankino television tower. Unfortunately, there is no great experience of operation of high-rise buildings in the range of 200 - 500 meters in Russia. But Europe has no experience like that either. Therefore, these zones should be studied, but we should remember, that these zones are to a great extend fabricated.

What changes with the height of a ground facility?

Now, before we study these protection zones, let's talk about this. What can tall objects do with thunderstorm activity in their neighborhood? The first thing that we know well- is the following. The number of strikes increase with the heigh of the building. The experience shows that this increase is proportional to the square of the height in the range which is more or less studied. And if a hundred-meter building is being struck approximately once a year, then the Ostankino TV tower gets 25-30 strikes per year. The dependence is very serious and active. What is the scientific explanation to that? I've never talked about scientific explanations, here I'll have to talk about that. And I must say that all today's ideas about the choise of a lightning strike point are based on the hypothesis of Gold, he was American , and it was formed about 50 years ago. What did Gold say? He said that from the moment of human's existence on the earth, no building,  built by a man,  had never been  struck by lightning. It just never happened. Why? Due to the simple reason that when the lightning started to approach the earth, a plasma channel, which Gold called a counter leader moved towards the lightning from the top of the building. And the meeting of the two channelsm one from a cloud, and the other from the object, whether it be a lightning rod,  or a building, it does not matter - this meeting is a lightning strike to an object. That is, a lightning strike moving towards an object takes the growing channel moving to it upon itself. This hypothesis is accepted by everybody. There is no conflicting fact to this hypothesis. But I must say, there is no serious quantitative study of this hypothesis. Our Russian team made such a research at the time and showed the following.

The main phases of a counter discharge

It showed that the counter discharge of any structure on the ground, whether it is a tree, a lightning rod, a high mast, whatever you want, starts with the fact that a corona discharge develops on its top. Almost the same corona that exists on the power line or in pirate novels, there is a mast on pirate ships called "Lights of Saint Elmo." This corona can exist for a long time without giving rise to any channel. For example, it can exist onan object during the entire thunderstorm, but in order for the channel to rise, which will intercept a lightning, it is necessary that this corona switches to a more powerful formula. It is called a streamer formula - it's a panicle, the branches of which can stretch over to several meters. There is a theory that shows how the corona moves into this form. For that, its current should reach a definite value. This value is quite big for the corona. It's a current at the level of about 10mA. Quite a large current. If this switch happens, then a channel of a counter discharge may appear at the bottom of the streamer flash, which in this case can be organized. For this purpose, the voltage on this outbreak was should be not less than 400 kW. And if this channel appears and turns out to be viable, and this requires specific conductivity, only then a channel that will intercept lightning will be formed.

Streamerless corona in a thunderstorm electric field

Roughly, this theory is written in the literature, it's a Russian elaboration. Let's see - what will happen, if we look at how a channel is formed. Here on this chart I showed you how electric field of a thunderstorm cloud increases in time. It grows for about 10 seconds in this case. And how the current of the corona, which will, for example, be on top of the Ostankino tower be formed? It turns out that this current is not strong enough to fulfil the switch. The current is less than 3 mA, and the critical current is 10 mA. And no matter how much we try, we cannot force the Ostankino tower to screen so strongly that a discharge channel is being formed on it. Thus, however, a lightning strikes the tower.

Additional field source - channel charge of downsteam lightning

How do they hit? The situation is like that. The corona is strengthened and intensively developed by the lightning discharge, which starts from the cloud and begins to approach a high construction. When this channel approaches, it  carries a charge from a storm cloud, brings it to itself and therefore, the corona current starts to grow very intensively and very rapidly. And it is able to exceed a critical value. It means, all the processes from the ground structures provoke not too distant lightnings, which develop from clouds to the ground.

An upstream leader from an object 30 meters in height

As the result of this case, such an event may happen. Here you see the growth rates of the counter discharge towards the lightning from the object of relatively low  height. Here the object is 300 meters high and the distance from the horizontal to the lightning channel is 150 meters. This channel grows in the lightning field, but as soon as the lightning reaches the ground.. Here is what happens. The channel grows, but as soon as he reaches the ground and transfers its discharge to the ground, the growth of this channel stops and the discharging turns out to be incomplete, it does not exist anymore. The lightning hit the ground.

An upstream leader from an object 200 meters in height

But if your object is very high, for example, now I have an object 200 m high, and the lightning is at the distance of 750 m of that channel. Look, the channel is growing, growing, growing, then the lightning hits the ground, it slowed its growth, but then resumed again. Such a discharge occurs in  high-rise buildings.

Experimental verification of the hypothesis about the role of the corona

And here we see this kind of thing. The following thing. It turns out that all the lightning development processes, in between the cloud and the ground, and these processes are powerful enough, because a lightning is a process that goes with the current of tens and even hundreds of kA,  and they have to be connected by the weakest discharge, which exists in the nature with the corona, the current of which is at the level of shares of mA. Is it true or not? Maybe it's agiotage? But it's not agiotage, it is a bare fact which can be confirmed if the following way. We have a building which is 200 meters high. This high-rise building will be placed on the ground during the storm season and it will be hit by a lightning only once or twice per year. Now an experiment. We take a weather rocket, which drags a thin wire and launch it to the skies and this rocket goes 200 meters high in about 3 - 5 seconds. If to launch this rocket run under the stormy cloud, then it is 70% probable that it will be struck by lightning in 3 seconds. What is the difference between these two things? Here we have 200 meters and here 200 meters, what is the difference. There had long been a corona (central image) and the corona charged its charged particles into the atmosphere. And these charged particles, this cloud closed high-rise buildings like a hide-away hat. And here (image on the left) - there was no time for that. Moreover, the rocket flew with the speed of 100 m/s and it took its own ions over, because they move slower. That is why, this connection with the corona is an absolutely real connection which should be considered in any cases, related to high-rise buildings.