What lightning rods are required at the construction site?

3. What lightning rods are required at the construction site?

The third part of the article "Lightning protection at a construction site"

Instruction on lightning protection AD 34.21.122-87 gives an unambigous answer to this question (p.1.11). Starting from the height 20 m, all construction sites with the working personnel should be protected from direct lightning strikes by lightning rods, providing zone B. This prescription reveals a certain contradiction. Because in the same regulation, the buildings already built should be provided with lightning protection only at the height more than 30 m, and only in case of their removal from other buildings to the distance more than 400 m. There is nobody to explain this variant reading. In the newer normative document IS-153-34.21.122-2003, the queston is not discussed at all. It is necessary to start from actual reliability B. In the officially approved text of the Instruction AD 34.21.122-87, the reliability of protection zones is not specified. This most important parameter of the lightning rod is only mentioned in the textbook (is not a regulatory document!), where the reliability of 0,95 is prescribed to zone B without any ground. Computer check on the base of the statistical method showed that the real reliability of zone B is significantly lower and in practice is equal to 0,84. So, the use of a lightning rod with zones B is able to provide only IV protection level of construction sites from direct lightning strikes (see tab. 2.2 from the IS 153-34.21.122-2003).

Fig. 4. The estimation of the real reliability of Zone B from the Instruction AD 34.21.122-87

Высота, м – height, m

Зона б – zone B

Радиус защиты, м –protection radius, m

Imagine a shooter with a blindfolded. Shooting blindfolded, he will shoot beside the mark with the probability 0,999. Is the risk to stand instead of the target great? It is clear that everything depends on the number of allowed shots. One is ok, but if 1000?! The situation with lightning is similar. For a construction 30 m high in middle latitudes of the RF, where the number of strikes into the protected object will hardly exceed 1 in 13-15 years of use, the reliability of lightning protection on the level of 0,9 is more than enough, that is why 1 lightning will breakthrough to the protectd territory in 130-150 years. It is not reasonbale to count on such delayed construction. But if the built building is higher than 200 m and is located in subtropics, where the density of lightning strikes into the ground is not 3, but 10 per 1 km² per year? Then the frequency of dangerous impacts will grow in approximately 140 times ([200/30]2*10/3) and in the direct lightning strike will hit the construction site almost annualy. As you see, the difference is crucial.

E. M. Bazelyan, DEA, professor
Energy Institute named after G.M. Krzyzanowski, Moscow

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