We have to tell frankly that protection zone in the domestic regulatory documents is a result of computer modelling. Two provisions are introduced into the calculation algorithm as the source data - typical protection zone of the rod lightning rod has the shape of a circular cone, and the top of the cone is located on the lightning rod axis lower than its top, the greater the required protection reliability is.
For the construction of the zone, the probability method is used, which was developed by ENIN to the name of G.M. Krzhizhanovsky. Its use for the design of lightning protection is now recommended by the Technical memo № 25/2009 of association "Roselectromontazh" with the approval of Gostekhnadzor. The peculiarity of the program is that it directly counts the expected number of lightning strikes into the system of on-ground structures (totality of lightning rods and protected objects) and the number of breakthroughs to the certain protected object (or its constructive fragment). The number of on-ground structures can be any, like their shape and geometrical sizes. The correlation of the found number of breakthroughs to the total number of lightning strikes defines the probability of the breakthrough. The program was tested according to the actual data about lightning strikes into overhead high-voltage lines with ground wires and according to the observation of thunder activity on Ostankino TV tower.
For the construction of protection zones, given in IS-153-34.21.122-2003, a wide series of calculations for rod and catenary wire lightning rods 150 m high was carried out with the help of a program. The geometric sizes were picked the way in order to get protection reliability on the set level (0,9, 0,99 or 0,999). Then empiric formulas were picked according to the results received and then introduced into the regulatory document (section 3.3.2 of IS-153-34.21.122-2003).
Unfortunately, it was possible to only fulfil this job for single and double rod lightning rods and also for single and double catenary wires. Very precise proximate generations for more complex lightning rod systems turned out to be a monumental task.
We need to repeat that the use of protection zone gives designer the guarantee that the reliability of the object protection from direct lightning strikes will be not less than that taken at the zone generation. However the certain value of the achieved reliability remains unknown.
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