From the series of articles Complete Information on the Protection Zones of the Lightning Arreste".
We have to state clearly that the protection zones used in the Russian regulatory documents are computer models. Two provisions above were used as source data in the calculation algorithm: a typical protection zone for the lightning rod with a form of a round cone, with a cone apex located in the lightning arrester axis, and the higher the required protection reliability, the lower it is located relative to the apex. To draw a zone, the so-called probabilistic technique developed by Krzhizhanovsky ENIN was used. Its application in the lightning protection design is currently recommended in Technical Bulletin No. 25/2009 of the "Roselectromontazh" Association approved by Gostekhnadzor. The software feature is that it directly counts the expected number of lightning strikes into the system of on-ground objects (a full set of lightning arresters and protected objects) and the number of breakthroughs to a particular protected object (or its structural piece). The number of on-ground objects in the system as well as their shapes and dimensions may be any. The ratio of the determined number of breakthroughs to the full amount of the lightning strikes is used to define the breakthrough probability. The software was tested using the actual data on lightning strikes in the high-voltage air power lines with the ground wires and according to the observations of storms from the Ostankino TV Tower.
To draw protection zones provided in Instruction SO-153-34.21.122-2003 using the software, a large series of calculations for lightning rods and wires with the height of up to 150 m was performed, wherein the dimensions were chosen such that to obtain the protection reliability at the specified level (0.9, 0.99, or 0.999). Then, based on the obtained results, empirical formulas were chosen, which were introduced into the regulatory document (Section 3.3.2 in SO-153-34.21.122-2003).
Unfortunately, such work could be performed for single and double lightning rods as well as for single and double ground wires only. Pretty accurate approximate calculations for more complex lightning arrester systems appeared to be a much more complicated task.
We need to repeat once again that the use of the protection zone provides a guarantee for the designer that the reliability of the object protection against direct lightning strikes will not be less than that used for the zone generation. However, the particular reliability value is unknown.
Read further "4. Why Are the Protection Zones So Wrong?"