The sixth webinar of «Grounding and lightning protection: issues and problems arising in the design series»
Webinar text. Page 3
Now let's talk about grounding. How to build ground protection in a high-rise building? High-rise buildings typically have a fairly large area. Reinforced concrete buildings will surely have a well-developed foundation. And this foundation is set deep enough into the ground. And the foundation will be usually used as a grounding loop, which on the one hand should divert the lightning current, and on the other hand, there is a grounding device on a substation, which feeds the building. And then it turns out that fairly strict requirements should be set for this ground loop, and they are not related to the lightning protection. The requirements are set from the standpoint of electrical safety. For example, if a substation with an isolated neutral has a voltage of 320 to 380 V, you will likely be required to provide a grounding resistance of at least 4 Ohms. You will use it both for the substation and the connection of down conductors. The question is what options do you have?
High-rise buildings grounding
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Заземление высотного сооружения - High-rise structure grounding
- как правило… - As a rule, this is a reinforced concrete foundation of the building and its grounding resistance should meet electric safety requirements
Квадратный фундамент - Square foundation
Длина стороны фундамента, м - Foundation side length, m
Проблемы возникают при ρ>1000 Ом - Problems arise at ρ>1000 Ohm
Can you see this slide? There are two foundations there. One has piles driven to a depth of 10 meters, and the other to a depth of 20 meters. And the building area is 50 * 50 meters. This is a typical high-rise building. And now look what happens. I use a five-meter or ten-meter foundation. Grounding resistance divided by the soil resistivity will depend on the side of the square. For example, I have 4 Ohm. In this case, I'll get 4 ohms divided by p. If I have a soil resistivity of 1000 ohm m, then the foundation area will be huge. It appears that I needed a foundation area somewhere 90 * 90 m, and this is not the case for a high-rise building. And the following thing happens. It turns out that if you have a soil resistivity more than 1000 Ohm*m and you need to decrease it below this point, then you'll not be able to reach a resistance of 4 ohms at all. That is, the characteristics of the foundation, even a foundation 20 meters deep, are generally very limited. And if you build a tall building in a place where the soil resistivity is high, then your choice is very limited. You'll have to deploy more electrodes near the building. And you cannot drive them deeper, as piles 20 m deep are already there. You will have to distribute these grounding electrodes horizontally. And this becomes a very serious problem. The problem becomes even more serious the higher is the resistivity of the soil in comparison with a conventional benchmark value of 1000 ohm m. You do not have other choices here. The only other solution is to replace the soil.
The second questions and answers session
— No questions again?
— There are questions related to the first part of the webinar, about down conductors and mesh cages.
— Please go on with them.
— The question is: “I have a building 60 meters high with the profile size on the roof of 12*12 meters. What cell dimensions should I choose for the mesh cage?” As far as I understand, the question is about the mesh cage cells.
— You already know we have two standards in Russia. We have a cell of 6 * 6 meters. This cell is specified by RD 34 for the second category of lightning protection. And there are cells of 12 * 12 meters, which are mandatory for the third category of lightning protection. But here's one thing. Have you made of a cell of 6 * 6 meters? Or a cell of 12 * 12 meters? The question is what do these cells protect? Let's start from the following notion. If the roof has any facilities on it, for example, a café, as I’ve just been already asked about, then no protection for people sitting at tables in the café will be provided by the mesh cage. The mesh cage will be below the tables and below the people. And the mesh cage will not protect them from any lightning. It will not protect them at all. Now here's the second question. Will it protect the roof? There are two options here. The first option is roofing commonly used in Russia. We make roofing from concrete slabs. And we put a heat- and waterproofing layer on this reinforced concrete slab. And after that, the mesh cage is laid on the layer. I am confident, and I've already said this during one of our previous webinars, that such a mesh cage does not intercept the lightning at all. The lightning strike can accidentally get into the mesh cage but the vast majority of them will get through the waterproofing and strike the rebars of reinforced concrete slabs. It will chip out a piece of concrete at the strike point, then the current will spread through the steel rebars of concrete slabs and then go into the foundation. In fact, this mesh cage protects nobody. The second option is not quite uncharacteristic for Russia. Roofs are made from some dielectric structures. And a metal mesh cage is laid on these structures. And under the roof, at a distance of several meters, there are offices or living premises, which are separated by a dielectric material of this roof. Then the mesh works as a catenary wire. And then this mesh cage really protects. But for Russia, this situation is absolutely uncharacteristic. We have almost no such roofs in use. Is it possible to embed the mesh cage in the concrete structure? Our regulations do not specify such a solution. The mesh cage is placed on top of or under the waterproofing layer if it is not combustible.
— There was a question whether it can be laid into a layer of heat insulation?
— Can the mesh cage be laid inside a layer of heat insulation?
— I've already answered this. And I emphasize that RD 34 says the mesh cage can be placed under the heat insulation layer or between the layers if this insulation is not flammable. If it is combustible, the mesh cage should always be laid on top of the heat insulation. At the same time, the authors of the standard assume that such a mesh cage will protect the combustible insulation from contact with the lightning channel. It is nonsense! There will be no protection in this case. And this recommendation, in fact, is incorrect. You cannot remove the mesh cage as Gostekhnadzor will not allow you doing this. But keep in mind that it will not protect the heat insulation. More questions?
- Yes, of course, there are still questions. First of all, about the building having a height of 60 meters. Our listener asks what to do with that? How to protect the building, what air terminal height should be selected or is it necessary to use some isolated system?
— I will come back to that later because first I want to show some slides. ОK? Please be patient with this question, I'll get back to it.
— All right, there is still another question about buildings made from cast in-situ reinforced concrete. Can I use rebars in such buildings as down conductors?
— Today, according to our standards, if the rebars in the buildings are connected with the rebar steel using tie-in or overlapping connection, then yes, you can. If at least 50% of rebars are connected that way, then these elements can be used as natural down conductors. We can say 'yes' because it is considered that this condition is satisfied almost every time. For example, you can use reinforced concrete columns to drain the lightning current. And it will happen anyway, you know, like it or not, and reason is as follows. At the top, the rebars of reinforced concrete slabs are connected to the rebars of columns, so you will get the lightning current into them, like it or not. You can either say this kind of grounding is allowed or not allowed, but this will not change the fact. So, when the drainage of lightning current is calculated for a high-rise building, then you can use reinforced concrete columns for that purpose. I've provided an example with double glazing, not with concrete columns, simply because the example was very intuitive. Do you understand me? And some high-rise buildings, as I know, have a design with a distance between columns of approximately 6 meters. And when the designers calculated the lightning current drainage system using the metal structures of the columns, they got almost the same effect as I've shown you. So the answer is yes, you can use it.
— There is another question on the ground electrode system. How do you carry out a grounding system if the high-rise building has a small foundation? Our listener asks how to get 4 ohms if the foundation has a small length?
— You know what? I cannot imagine a high-rise building with a small area of foundation except for an antenna of a tower. Yes indeed, there may be a small area and what should you do in these cases? Use a comprehensive system of horizontal buses in addition to the foundation. What can be done in this case? You can do the following. Here's your foundation, can you imagine it? For example, it has the form of a circle. You are, of course, using it as a grounding element, but this grounding is emitting radial rays. And these radial rays can have coverage of tens of meters. The system of such calculation really exists. Moreover, we hope that the calculation system will be presented by ZANDZ.com project in the cloud after a while. It can be done. And if you can't do anything at all, then there's only one option - filling the upper layer of soil as I've told you during the previous webinar. It is very-very expensive, there's no other alternative. In the case of the Cathedral of Christ the Savior in Moscow, the problem was even more complicated by the fact that there were underground floors in the building, and there is a porch there. A lot of people gather on the porch, so there was a need to dramatically reduce the grounding resistance. And they chose the option I've just told you about. They've introduced outward rays to drain a significant proportion of lightning current away from the people.
— And we have a question: can you recommend a soil resistivity value when a foundation is used as grounding.
— No. No one can give you any recommendation in this case and the reason is as follows. You need to be able to calculate the grounding resistance of the foundation. If you feel an urgent need for this and there will be enough interested people, then I think we can arrange for a special webinar for this case. And how do you calculate the foundation? I am ready for such a seminar. And if you, for example, write you need such a webinar, then perhaps, the owners of ZANDZ.com project will organize it. I have the necessary content for it.
— One more question not related to the topic. It is related to the calculations of protection zones or the protective angle method, or a protective sphere method. Do you want to answer the question now? Then I will read the question aloud.
— You know what? I really do not want to answer that question, for the following reason. Methods you are citing are specified in the IEC 62-305 standard by the International Electrotechnical Commission, which is not approved in Russia. The calculation method used in Russia uses neither the method of rolling sphere nor the method protective angle for a simple reason: both methods give tremendous deviations in a number of cases. We've talked about these cases during our first seminar. I will probably not repeat them once again but I can say the following thing. As for the rolling sphere method, we have the following thing. Imagine that you have a conventional lightning rod with a height greater than the radius of the sphere, and there are 4 such spheres. The smallest one is 20 meter for the first level of protection. If your lightning rod has a height greater than 20 meters, then a further increase in its height will produce no effect. This is nonsense not supported by practical experience in any country. Nevertheless, this stuff is a legal standard approved by the International Electrotechnical Commission. As for me, if you try to implement this standard in Russia, then I will make every effort to ensure that this standard is not approved in Russia. I must say that I'll get enough supporters for that. So let's go ahead with your questions.
Step and touch voltage
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Напряжение шага и прикосновения - Step and touch voltage
Расстояние от фундамента, м - Distance from the foundation, m
Если ρ=100 Ohm m и Im=…. - Если ρ=100 Ohm m and Im=100 kA, then Ustep=20 kV near the 80x80 m building
При частоте ударов более 10 за год это небезопасно - With the frequency of lightning strikes more than 10 per year, this is unsafe
Let's go ahead, and there's another important question. A high-rise building. We have several dozen lightning strikes into this high-rise building a year. And then the following thing must be considered. People may be present near the high-rise building. The question is can the step and touch voltage affect these people? What is it like? Is it substantial or not? And I've looked what numbers we can talk about for ordinary soil types. It turns out that you are using the foundation as a ground electrode system, and you have to do it as there's no other option. And if you have a lousy soil with a resistivity somewhere at the level of 1000 Ohm m, then the calculated curves shown here illustrate how the step voltage changes depending on how far is a person stands from the building wall. So we have the following thing. Even if you have a huge building, for example, 80 * 80 meters, then with a current of 100 kA, you step voltage would reach approximately 20 kV. Is this acceptable? I cannot tell you this for the following reason. Neither Russian nor international step voltage norms apply to the pulse impact. Personally, I have been exposed to the step voltage of 20kV once. And I can tell you one thing. I do not want to experience this again. But I survived. Do you remember the recent case with the Chinese guy who has been shown on the TV, do you remember this? He is falling on the pavement because of the nearby lightning strikes, as he is certainly exposed to the step voltage from high-rise buildings. The situation will be something like this. No one is fighting against such situations, neither in Russia nor in Europe, for unknown reasons. And there is another point. If you have low-voltage underground communication lines, for example, control or automation circuits in the vicinity of this building, then these communication lines will be severely affected by such exposures. So you need to protect these communications by SPDs, of course.
Transmitting high potential from high-rise buildings
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Транспортировка высокого потенциала от высотного сооружения - Transmitting high potential from high-rise buildings
U/Iм, Oм - U/Im, Ohm
Oм - Ohm
Импульс тока 10/350 мкс - Current pulse 10/350 microseconds
Сопротивление заземления 1 Ом - Grounding resistance 1 Ohm
Время, мкс - Time, microseconds
There is another point we need to discuss. Let's assume a utility line enters the high-rise building. A utility line, for example, a water pipeline, exits the high-rise building. These utility lines are a path to transmit high potential. And we talk not about transmitting the high potential into the high-rise building but on the contrary, transmitting it from the building. I mean that the pipes that connect the building with the neighboring lower buildings can transport a high voltage. Look at the diagram. I have the lightning pulse which acts on the high-rise building. The lightning current is 100 kA and the pulse is 10/350 microseconds - a calculated lightning pulse value, which we always use. The high-rise building's grounding resistance is perfect, i.e. 1 ohm for the ground loop. And now I look at those pulses that run through the pipe with a diameter of 10 centimeters at a distance of 200 meters. There are 200 ohms m in the soil, so here is 500 Ohm m. Now let's see what happens. It turns out that the voltage divided by the current is about 0.7. What does this mean? This means that a current pulse of 1 kA produces 0.7 kV of voltage. And if the pulse is 100 kA, then 70 kV of voltage will come to the adjacent building. That is the transmission of the high voltage from the high-rise building to adjacent buildings through underground communications is a very plausible thing. And we should fight against that. And the first means of protection is to connect all communication to the ground loop of the low-story building, which is located near the high-rise building. This is a traditional measure, but the measure is very effective.
Technical facilities on the roof
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Технические объекты на кровле - Technical facilities on the roof
Подавляющая часть поражений вызвана восходящими молниями - The vast majority of strikes is caused by upward lightning
Предпочтительнее мультиэлектродная…. - Multielectrode system is preferable, and if an air terminal installed on the facility, then insulated lightning protection is preferred
Finally, let's discuss the roofs. And this is a necessary thing to talk about. Let me explain that. The latest construction trend is to place something on the roof. What do you have to do? Look at the picture on the right: this is the roof of a building in Bavaria. I've taken that photo myself. The part of the roof painted in light color is a place where climate control equipment is installed. Now look how they protect it. The protection is as follows. They don't have any mesh cage on the roof. They have a stockade of lightning rods around all the climate control units. These lightning rods have a very small height, about 3 meters each. And they are installed in increments of about 5 meters to surround the entire protected area. And the approach that provides for the installation of a large number of low-height air terminals is very effective and may well be recommended. But here, in Russia, I would prefer a little bit different design.
I would suggest the following thing. Let's go back to that question with the building irrespective of its height. It doesn't matter if its 20 meters, 150 meters or 200 meters. There's a cafe on the roof. That would I do? The authorities will force me to install a mesh cage on this roof anyway, and there's nothing I can do with that. So I'll take this mesh cage, rip it off the roof and raise over the cafe area. And I will keep the distances specified in the PUE standard. The distance is about 5 meters above a person. So what will I have above the visitors? I will get a raised mesh cage, and I can decorate it in any manner I want. For example, I can seed buttercups - a vining plant that will climb over the roof. Or I can install some advertising boards in the worst case scenario. The mesh cage will not be annoying to anyone, but this raised mesh will reliably protect visitors who come there to drink coffee or eat cakes.
Look at the left picture, and you'll see another option. I must make a confession: I've taken this option from an advertising leaflet printed by a Bavarian firm Dehn+Sone. I know them very well, so they gave me a permission to use the picture. There is a cellular antenna on the roof. The antenna is very low, just 10 meters. But as you remember, it's more than enough to make upward lightning strikes fly from the antennas. And protection offered by the firm is quite appropriate. They do the following thing. They install an air terminal directly on this antenna. But the air terminal is insulated from the antenna, and the insulation withstands the voltage that can be produced by the electromagnetic interference. And the lightning current is drained from the air terminal by a special insulated cable to the first level of equipotential bonding. And this first level of equipotential bonding is the mesh cage that is laid on the roof. And this solution guarantees the protection of the antenna electronic units. And as I recall, the units cost 30,000 dollars apiece, which is very expensive. So in this case, the units remain well-protected. This is quite a typical project. So what's the point? - We are talking about this. If you really do not want to leave your roof vacant and you want to build something on this roof, then you have to protect it against lightning. Install a local lightning protection system. You can either use the isolated lightning rod or a series of lightning rods that cover the area. Or, I still highly recommend you to tear off the mesh cage and raise it over the area to be protected. This protection option for high-rise buildings is acceptable.
And now I will tell you a couple of words about American skyscrapers. When I arrived in America, I particularly asked my hosts to show me a skyscraper. You know, the roof is vacant there. They do not want to mess with any lightning protection of the roof, so there is absolutely nothing on it - no antenna systems, no cafes, and even no viewing platforms on the roof. All these things are hidden under the roofing. The roof itself is empty and not protected. And its metallic coating is able to accept lightning strikes without any negative effect. And deep in my heart, I like this approach more. I know it's very difficult to do it in Russia because recently built high-rise buildings have roofs made from double-glazed windows that pass light to the winter gardens located on the top floor. You have no other options except for the option I've told you about. I am ready to answer questions if any.
The third questions and answers session
- There are some questions from colleagues who have asked questions before and want to hear some more specific answers. For example, regarding the heat insulation and the mesh cage. You've said that Russian regulations allow their use and the question is: at what depth the mesh cage is allowed to be laid in the insulation?
-I will say it again. Details you want to know are not specified in any normative documents. Standards tell you only the following. If the insulation is combustible, then it is not an option. It is allowed to put the mesh cage on top of the heat insulation only. Once again, on this insulation cannot be protected this way. If the insulation is non-combustible, you can lay the mesh cage as you like, including laying it under the insulation and in between the layers. The insulation will not stop the lightning, you know. You can lay the mesh cage under any insulation if it is non-flammable, and at any thickness.
- And our listener wants a piece of advice about the protection of a building 60 meters high. Can I lay a mesh cage with an area 5 * 5 meters and use a system of lightning rods and down conductors isolated from the building?
- First of all, I'd still like to say the following. We still need to respect Russian regulatory framework. And the regulations specify the mesh cage of 6 * 6 meters, not 5 * 5 meters. I do not know why you want to use the cage of 5 * 5 meters, but if you want ...
- The instruction manuals say...
- If you want it then use it! The point is this. The mesh cage size specified by the standard is the maximum allowable size. If you have a mesh cage of 6 * 6 m and you are to use a cage of 0.5 * 0.5 meters, then use it for Heaven’s sake! It is not prohibited. The 6 * 6 size is the limit for the second class lightning protection; 7 * 7 meters are not allowed already, and technical supervision will say: redo it! And if you install a mesh cage of 0.5 * 0.5 meter, then it is allowed, and 5 * 5 is also allowed. Because dimensions specified in RD 34 document is the maximum allowable size. Now, there's the second point. What can you do with the building 60 meters high? This is a typical normal height for a building, which is covered with all the provisions of RD 34 standard without any restrictions. I'm not talking about SO 153 document because it has nothing to do with the case. You will not find anything useful for you there. But if we talk about the RD 34, you can use all its content to arrange the roof protection. Local lightning rods installed on the roof to protect specific facilities can be also used on the top of a building 60 meters high, as this is a building of an ordinary height.
- There's also a question about active lightning protection again. What do you think about using it?
- You know what? I'll tell you one thing. I've told many times what I think about the active lightning protection. Neither I nor my colleagues from other countries throughout the world, including the United States, consider it effective. And by the way, during the last conference on the lightning protection last spring, specialists from the United States were present. Here, at the Russian conference. You see? And we all have a consensus that active air terminals show no efficiency whatsoever. And their performance has not been proven in any way. There were theoretical studies that led to this conclusion. And there were purely practical observations when testers placed active and ordinary lightning rods side by side in comparable conditions. They studied the active protection for three years and found no advantages. Finally, Vasily Kuprienko made a presentation during our last conference. This is a well-known specialist on lightning protection in Russia. I don't know where he has obtained money for his research as this takes a lot of work to study active lightning protection experimentally using a lightning simulator - a 6 million volt pulse voltage generator. But the result in all cases was negative. Therefore, not a single Russian regulatory document permits the use of active lightning rods. You can buy and install it, but if something happens with people, animals or buildings, then you will get a criminal case for wrongly arranged lightning protection. And advertising leaflets by firms that sell active lightning rods will not save you from the criminal responsibility.
- There's a question on the calculation of multiple lightning rods, that is...
- Wait, let me finish.
- The calculation of multiple lightning rods. We gave a software program to ZANDZ.com project. And ZANDZ.com project is readying the program to be provided as an online service. We completed our task, I mean - lightning protection developers. ZANDZ.com project is a responsible company and I believe that they will complete their task, too, and this software will be available soon.
- As this program is not yet available, can you recommend how to calculate the height of multiple lightning rods installed on a high-rise building taking into account that the SO and RD standards require calculating the height of lightning rods starting from the ground level.
- You know what? I believe this is a mistake. The Russian norms have a mistake here. And the mistake is the following. The European regulations require calculating the height of a local lightning rod starting from the roof surface in all cases. It is clear that this is absolutely wrong when the lightning rod is installed close to the roof edge. Russian overcautious regulations always require calculating the height from the ground level. If a lightning rod is installed at the center of the roof then it is an absolutely wrong installation. For this reason, this issue needs to be solved, and not methodologically, as it is methodologically clear what to do next, but not legislatively. And we do not have this legislative revision yet. Today, we all understand that the SB 153 standard is not suitable for practical use. And the only question is who will sponsor the works to make workable documents for designers. I am ready to calculate all these cases but my calculations will not be legalized normative documents. Excuse me, but I can do nothing here.
- Would you calculate from the roof personally?
No, I wouldn’t, as personally, I would do the following. This is the way I would calculate this. Nadezhda is pushing me to tell you how I would calculate this case. I would calculate as follows. If a lightning rod is really close to the roof and the distance to the roof edge is comparable to the height of the lightning rod, I would calculate using the method specified in the Russian regulatory documents. But if the lightning rod is in the middle of the roof and the distance from the lightning rod to the roof edge is about 2 or 3 heights of the lightning rod, then I would assume that the roof is the ground level. And I would calculate all the height starting from the roof level. And I assure you, this result would be plausible. But if you begin calculating such short lightning rods, you know, this will happen almost automatically.
- Dear colleagues, please ask the remaining questions, even if any, as we are finishing as far as I understand.
- We'll wait a bit, as perhaps people are making their questions and entering them now.
- You know, colleagues, if we are really talking about how to calculate foundations, and as far as I understand this question is very important for many of you, then you can write that this question is important in your comments. As far as I understand, this webinar will not be very difficult to organize, as we've elaborated a methodology for such cases very well. We are able to make such calculations ourselves and will share our experiences with you. And because there are no legal restrictions related to this calculation, as this is the case with a lightning rod on the roof, you can freely use the methodology we recommend.
- Okay. There are no more questions. Now I'm publishing a link to the questionnaires in the chat, so you can express your wishes and ask the remaining questions there. We will be very grateful for your feedback. Thank you for your attention!
- I Once again thank all the participants. I was excited working with you.
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