From a series of articles The Truth About the Lightning Protection Grid.
Concerning the lightning protection, it is very difficult to find such a confusing term as the lightning protection grid. Surprisingly, this is primarily associated not with the regulatory document but rather its peculiar interpretation by the readers, especially by the authorities' representatives. The grid has been used to protect buildings and structures for many years. Anyway, the book titled "Storm Protection of Industrial Structures and Buildings" that was published by the Academy of Sciences of the USSR in 1951, contains a chapter titled "Storm Protection of Buildings with Non-Metal Roofs Using a Grid". The name of the section closely reflects the main aspect: the grid is designed only for the protection of buildings and structures having non-metal roofs. This is a very specific wording. It does not allow for any ambiguous interpretations. A non-metal roof means new synthetic materials, corrugated fibre sheet, wooden laths or even wheat straws but not metal tiles, metal profiles, or reinforced concrete plates filled with the earthed metal bars. If there are no metal elements on the roof, the grid with the rated sizes can protect people and equipment to some extent on the top floor of the building. This is the only purpose it is designed for.
2. The Grid on the Roof of a Modern Building
My first independent place of residence was a small flat in a well-known panel 5-storey building erected in the 1960s. At that time, we had to do almost anything with our own efforts, and we had to visit the roof many times. Its intermediate floor made of reinforced concrete was covered by thin sheets of the waterproofing layer impregnated with bitumen. A lightning protection grid was installed on the sheets, which was made of a 3-mm steel bus. The current collectors went from it along the building walls to the foundation. The builders explained that our apartment house could not be put into operation without the grid. They did not take into account my professional explanations.
It is known that, in order to provide protection against the direct lightning strikes, the lightning arrester should be much higher than the protected structure. The higher it is, the better the protection. The protection zones according to Instruction SO-153-34.21.122-2003 state that even with the protection reliability 0.9, the lightning arrester should be by at least 15% higher than the object, which is about 2 m for a 5-storey building. The question is: why do we need a metal grid that is higher than the earthed metal steel bars of the reinforced concrete intermediate floor of this house by about 1 to 2 cm? And the thin concrete layer of a non-standard quality over the steel bars can hardly be considered as a serious additional insulation.
Рис. 1
Вероятность прорыва молнии - Lightning breakthrough probability
Превышение сетки, м - Grid height, m
Здание 48 х 24 м высотой 15 м - Building 48 x 24 m with the height 15 m
E. M. Bazelyan, Dr. Sc. (Eng.), Professor
Krzhizhanovsky Energy Institute, Moscow
Read further 3. When We Use the Grid According to Its Intended Use.
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