# 5. THE REASON FOR A "STRANGE" GRID OPERATION

From a series of articles The Truth About the Lightning Protection Grid.

The basic indicator used in the lightning protection is the lightning collection radius. Its averaged value is usually taken as heights of the surface structure: Ratt = 3h. The area of the lightning collection is restricted by a closed curve spaced from the external boundary of the structure at the distance 3h. For a rectangular building with the sizes d and b

Satt = 6h(d+b) + db +9πh2

For the considered structure 48 x 24 m, the value Satt ≈ 40 000 m2, whereas the roof area S = db ≈ 1000 m2. It means that the overwhelming majority of lightning channels is collected to the building roof side and only S/Satt ≈ of them are formed directly over the roof. Only for these "vertical" lightnings, a grid cell size is important. The paths of other lightnings are only pre-defined by the buses located directly along the external perimeter of the roof since they are met first in the path of the lightning channel attracting on the side. The size of the "internal" grid cells is not important at all. It affects the protection reliability of the object only with the large roof area when it is comparable to the area of the lightning attraction. This is illustrated by the results of computer calculation for the structure 120 x 120 m with the height of 30 m, wherein the ratio S/Satt ≈ 17%. Here, the grid protection reliability is actually much more than the one achieved by the closed wire lightning arrester with the same elevations over the protected building elements. The more the height difference value, the better the difference can be seen (Fig. 4).

Fig. 4.

Замкнутый трос - Closed wire
Сетка 12 х 12 м - Grid 12 x 12 m
Вероятность прорыва молнии - Lightning breakthrough probability
Превышение сетки, м - Grid height, m

Two features following from the calculation should not go unnoticed. First, the grid protection reliability is not very high. To obtain the reliability level 0.99 (the lightning breakthrough probability is 0.01) in the considered example, the grid should have an elevation about 4 m. This is not so low, and it is not always provided for the equipment on the top floor of the building by default. Another condition is related to the rational use of the closed wire lightning arrester. It turns out that you should not rush off to replace it with the typical lightning protection grid.

Fig. 5.

Замкнутый трос с перекрестием - Closed wire with a crossing
Сетка 12 х 12 м - Grid 12 x 12 m
Вероятность прорыва молнии - Lightning breakthrough probability
Превышение сетки, м - Grid height, m

E. M. Bazelyan, Dr. Sc. (Eng.), Professor
Krzhizhanovsky Energy Institute, Moscow

Read further 6. Recommendations for the Designers.

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