Webinar «Lightning protection of accessible roof area», page 1

— Today is the thirteen's webinar with Eduard Meerovich Bazelyan, dedicated to protection design. The topic is very interesting - lightning protection of roof, it is a relevant topic which causes many questions, because various systems of climate control are installed on the roofs now, different antennas and even recreation zones and cafes which are very popular during the warm season in the period of thunder activity. Of course, everything should be protected from lightning, so it would be safe to drain lightning current into the ground. And how to do it, if the regulatory documents contain contradictions on this issue and it all causes the problem in the work of designers. Today we will talk to professor Eduard Meerovich Bazelyan and will try to answer all the questions. Please write your questions in the chat and it is preferable to indicate the number of the slide which relates to the question or some phrases of the professor, so he would quickly understand what you are talking about and answer your question in the best way. In order to increase or reduce volume you can point the arrow at the video of the lecturer and adjust the level of volume with the regulator. Also you can zoom in by using green cross magnifier; it is on the toolbar in the bottom under presentation slides. That's all relating to the organizational part. We can begin. Good afternoon, Eduard Meerovich.

— Good afternoon, dear colleagues! It is the thirteenth seminar today. I was a bit nervous about the seminar, not because it is the 13th but because we are going to talk about the roof. And only Carlsson has no problems with the roof. As concerns all the rest people, the situation is not the best. And frankly speaking, the greatest confusions arising today are connected with the roof. I won't be able to answer all questions today, because some of them are simply unsolved on the principal level. And we will have to talk about it and we will need to ask some alternative solutions because you need to design lightning protection today, not in a few years. Though several years ago I had a talk with one very active, young and promising physicist who is seeking the topic of his doctor thesis and I offered him to solve questions with lightning protection of the roof. He was very understanding to that, but was not very enthusiastic because it is really not easy to solve this task. I don't have to explain you what roof is, because roof is really a place which is considered to be a technological space and there is a lot of equipment in this space. But first of all it is traditional machine climate-control and ventilation, practically in all the countries.

AD.34.21.122-87 – Residential and public buildings, the height of which is 25 cm higher than the average height of the surrounding buildings within 400 meters in radius, and also free-standing buildings higher than 30 meters, remote from other buildings not more than to 400 m.

III category of lightning protection in places where the number of thunder hours is >20. (table 1)

— What do requirements of our regulatory documents tell? Here we need to start with the new instructions, because the old one was published in 2003. There is no word about the roof except one single one in IS-153-34.21.122. If to look thoroughly you will find such words there that if the roof has metal coating 0,5 mm thick or more, then this roof can be used as a lightning rod. There is no point from these recommendations, because then there is a condition that this roof should be put on a non-flammable coating.

On buildings and structures with metal roof, the roof itself was used as lightning rods. And non-metal scarcements should be equipped with lightning rods connected to the roof metal.

To what type of roof reinforced concrete slab should be referred?

Вероятность прорыва- breakthrough probability

Здание - building

Сетка - mesh

Превышает кровлю на 20 с – elevates above the roof to 20 cm

Высота объекта, м – object height, m

Сетка не работает на железобетонной плите- Mesh doesn’t work on reinforced concrete slab

— What to do with the mesh? You are forced to put the mesh. You are putting this mesh on reinforced concrete slabs, what for? What can this mesh protect? We returned to this question many times, maybe even at the first seminar we talked about what mesh can do if it is put on reinforced concrete slab. It is supposed that there will be some insulation layer on the reinforced concrete slab and it can be hydro insulation, maybe thermal insulation, but the thickness of this coating will be hardly greater than 1-2 cm, as a rule even less. With such an elevation, this mesh doesn't protect from anything. We made a numerical modelling of such a form: we took a building 40x40 meters in square, took its height from 10 to 60 meters, considered that there is hydro insulation coating of incredible thickness of about 20 cm on the reinforced concrete slabs of this building and looked how many lightning will miss this mesh and hit the reinforced concrete slab. It turned out that 35-40% of lightning will miss the mesh. So there is no protection from mesh at all. What to put it for? I have one answer to this question and it will disappoint you.

Principal difference between the standard IEC 62305 and IS -153-34.21.122-2003

Height of the electrode on the roof 2 m

Уровень защиты  - protection level

Радиус защиты на уровне земли, м- protection radius at the ground level, m

Радиус стягивания молний – lightning contraction radius

Явное недоразумение – obvious mistake

Стержень высотой до 4,5 м на крыше 30 метрового дома как молниеотвод не работает – the rod 3,4 m high on the roof of a 30-meter house doesn’t work as a lightning rod

Не меньшая глупость- also a stupidity

— Now if we seriously talk about lightning protection, then problem begins with the very beginning. There is a picture in front of you, which I took from IEC standard. On this picture, the lightning rod installed on the roof counts its height from the roof level, if to talk about protective action of what is located on the roof. And it counts its height from the ground, if we talk about protection by this lightning rod placed on the ground. IEC interprets lightning protection this way. How do Russian documents interpret lightning protection for a lightning rod installed on the roof? In a completely different way. Wherever you have a lightning rod installed, on whatever roof it stands, at the edge or in the middle of the roof- it doesn't really matter. Its height is counted from the ground level. Neither this, nor that can be considered correct. It is some convention which was accepted this way in one place and in Russia- in another way. What will it bring to? Let's study IEC standard first. I want to protect everything that is placed on the roof by the lightning rod installed on it. Then the height of this lightning rod can be very small, mine is not high, about 2 meters. The protective angle which defines the straight pitch, which limits the protection zone, turns out to be very large. This angle is in the range of 70 to 80 degrees. If you multiply the lightning rod height to the tangent of the pitch angle, then protection zone radius at the ground surface will be of a very large size. See, for the first lightning protection level for a two-meter-high lightning rod it is at the level of 5,5 meters, and for the third protection level, which is more often applied in practice in residential buildings is 8 meters. What do 8 meters of radio protection mean? There is a known parameter: lightning attracts lightning strikes from the radius equal to the three-times height of the lightning rod. It means that for a two-meter lightning rod, the radius of lightning contraction is 6 meters and the protection radius is 8 meters according to IEC, this is a complete absurd. How can it be so? Lightning get constricted only from a smaller distance, and greater distance is protected. It is impossible and absolutely incorrect. And it is clear that it will depend on anything else. What is the size of the roof? Is it big or small? Where does the lightning rod stand? In the middle of the roof or somewhere on the side? It is absolutely unclear too. Russian protection zone brings to another stupidity, you see here is a 10-floor building 30 meters high here. I put a lightning rod on this 10-floor building, but according to the Russian regulations, protection zone starts below the top of the lightning rod and below the top for the reliability of protection of 0,9- is 15%. So if I have a lightning rod 4,5 meters high on my roof, it doesn't protect anything and anybody at all. It is according to the Russian regulations. This is absurd too. This absurd tells that our Russian regulations and IEC standard regulations are made without some serious physical ground.