# 5. WHY IS A SINGLE LIGHTNING ROD NOT RELIABLE?

From the series of articles "Complete Information on the Protection Zones of the Lightning Arrester".

Read Part 4s "Why Are the Protection Zones So Wrong"

This photo is often shown (Fig. 6) to illustrate low reliability of a single lightning rod. The lightning has passed by the Ostankino TV Tower’s top for 200 m. The lightning arrester, which is the highest in Europe, does not perform its protective functions very well. To some extent, this is true for single lightning rods of any height.

Fig. 6

The reason is that, with a certain position of the lightning channel, the distance from its head to the object’s top gets significantly closer to the distance to the lightning arrester’s top. To make sure, we need to compare the situation for lightnings appeared at the level of the orientation height in points A and B in Fig. 7. The elementary geometrical plots convincingly demonstrate that it is much more difficult for the lightning protection to catch the lightning from point A because these distances are getting closer. The single lightning arrester’s action is somehow unilateral. It is much more correct to circle the protected structure with lightning arresters, thus eliminating the azimuthal dependence of the protection effect.

Fig. 7.

The computer calculations in Fig. 8 show the way how the protection reliability of the tank having the radius of 50 m and the height of 20 m grows with its protection with different numbers of lightning rods of 40 m high. It is supposed that each of N lightning arresters is spaced away from the tank for 10 m and is located with an angle step β = 2π/N.

Fig. 8.

Вероятность прорыва - Breakthrough probability
Высота молниеотводов 40 м - Lightning arrester height 40 m
Объект – резервуар высотой 20 м и радиусом 50 м - Object: tank 20 m high with radius 50 m
Число молниеотводов - Number of lightning arresters

Both single and double lightning rods cannot even provide the protection level 4 against the direct lightning strikes according to SO-153-34.21.122-2003, while the use of the system of four lightning arresters of the same height provides the protection level 1 with an excess. It is clear that we can only design such lightning arresters using a software. The protection zones are absolutely unsuitable for this.

Read further "6. What Can a Lightning Grid Be Used For"

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