What kind of lightning protection to orient to?

5. What kind of lightning protection to orient to?

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

Before to start talking about height and number of lightning rods, we should stop on their conception. A construction site raises and that is why a constant re-installation of lightning rods is inevitable. Stationary devices on welding are little applicable here. The already mentioned paragraph 1.11 of the instruction AD 34.21.122-87 indicates the possibility of using bolt connections for the installation of temporary lightning protection. This should be the primary orientation, simplifying the frequent replacement of lightning rods to a higher level. Closed wire lightning rods recommended by the IS 153-34.21.122-2003 (p. 3.3.2.5) are worth attention. Tey guarantee high protection reliability at a comparatively small elevation over the protected territory, and the required exceeding of ∆h doesn't have to be inceased in the process of construction when the construction walls are raising.

The efficiency of protection of a construction site 20x20 m with a closed wire lightning rod at different exceedings of ∆h. It was supposed to move the wire outside the protected territory to 1,5 m

Fig. 6. The efficiency of protection of a construction site 20x20 m with a closed wire lightning rod at different exceedings of ∆h. It was supposed to move the wire outside the protected territory to 1,5 m.

Вероятность прорыва – breakthrough probability; Высота стройплощадки, м – construction site height, m

The III protection level with the reliability of 0,95 (probability of a lightning breakthrough 0,05) can be achieved at ∆h = 4 m at the height of the construction site up to 50 m. But it is worth to remember the height of people and equipment at the site and consider them!

It is desired to orientate to some natural lightning rods, but probably only a tower crane can act in this role. There is usually nothing more appropriate at the construction site. This perspective is far from being promising. On fig. 7 the calculated probability of a construction site protection of not the highest dimensional building with the size of 40 x40m. The height of the crane is 70 m, its jib is placed strictly along the axis of the construction site. We can see, that a crane acting as a lightning rod can provide protection reliability of not less than 0,99 only up to 40 m. When the construction site raises to 55 m, the reliability will drom to 0,9 and will hardly satisfy the customer.

Reduction of protection reliability with the incease of height of the protected structure can be considered an overall tendency in lightning protection. We cannot avoid it. It is practically impossible to provide reliability at the level of 0,99 at a construction site 150 m high and higher. It would be necessary to mount a too high lightning rod for this purpose. Even a closed wire lightning rod doesn't change the situation a lot. In any case, for the protection of the site 20 m in radius at the height of 150 m, it should be placed 40 m higher than the protected level and increase the radius of coverage to at least 2 m. It is not occasional that lightning rod zones more than 150 m are excluded from the authorizing document IS-153-34.21.122-2003.