In the current standard IS-153-34.21.122-2003 there is no specific, certain information on this matter. In the previous document AD 34.21.122-87 buildings related to the type of residential and public buildings have the III (last) category of lightning protection, which is not always needed. So what should be the reliability of lightning protection of low-rise buildings and high-rise buildings?
In the current norm of IS-153-34.21.122-2003 for this segment, it is prescribed to operate lightning protection of residential and public buildings having the reliability of Φ=0.9 or more. This means that lightning will pass by the protective device and will fall into the structure Npr = NM (1-Ф) number of times. This indicator will correspond to the number of years during which lightning strikes the building once.
That is, this parameter determines the time period for which one strike into the house is expected. This period depends on the height of the structure.
Degree of reliability for low-rise buildings
Putting the data into the formula, we determine that 1 lightning strike into an ordinary cottage of a medium size 1 will hapen for a period of Tмол=50 years of use. When installing protective devices having a lightning protection rating of 0.9, this period increases in theory up to 500 years. In practice, it will be slightly less, because the lightning rod, even the smallest, increases the height of the structure and, accordingly, reduces this period.
Nevertheless, this period is quite long, even for English mansions, and the lightning strike, compared to the frequency of other natural disasters, will not take a leading position. Any homeowner will be satisfied with this state of affairs, therefore a non-complex device will fit well at the reliability of a lightning protection 0,9. It is enough to install 2 small lightning rods or a cable that exceeds the roof edge by 1.5-2 meters.
Level of reliability for high-rise buildings
In case of installing lightning protection on buildings of several floors, calculating the period of operation of a house with one alleged lightning strike may not be so pleasing. For example, calculating the period a of a lightning strike for a 10-storey building, we get a period of 6 years. That corresponds to an average of 60 years with a lightning protection reliability of 0.9. Add the height of a lightning rod and we get 50 years. However, there is no point in strengthening lightning protection from the recommended position of 0.9 in order to make the roof more protected. Modern skyscrapers, built from a monolith or reinforced concrete slabs, will not be destroyed by lightning. The concrete roof will successfully withstand electrical aggression.
Only equipment and various installations on the roof can be seriusly damaged. But their service life is much less than half a century, and therefore lightning protection of residential and public buildings can be set in the amount indicated by the standards.
The material was created on the basis of an article by Professor Eduard Meerovich Bazelyan "What reliability of lightning protection to focus on?".
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