Space under the sphere trace is considered to be protected

Зона защиты - Protection zone

The method has been a subject of detailed analysis during the webinars and in special articles. However, we have to repeat major considerations once again trying to persuade the designers to stop rolling an easily rolling sphere. Today, it is related to very active attempts to integrate the calculation method to the Russian lightning protection practice within GOST R copying the respective section of standard IEC 62305.

The rolling sphere method is very simple, that's why it's attractive. To determine the protection zone, you have to roll a sphere (in your mind) of a specified radius around the lightning rod. The space under the sphere trace is considered to be protected.

From formula it follows that standardized radii Rst correspond to rather weak lightning currents, i.e. ~3, 5, 10, 15 kA for protection levels I, II, III, and IV. Such choice has been made on the basis of the unreasoned belief that only weak lightnings break through to the protected facility, so, when you provide a protection against them, you can be certain in their absolute protection against stronger discharges. We can only be sorry that there are no reasons for this.

Read more details in the article by Prof. E. M. Bazelyan How Does A Lightning Orient and Can We Control This?.

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