From a series of articles The Truth About the Lightning Protection Grid.
Because of a long-term confusion, it is better to repeat that the grid installed on metal tiles or any other roof coating that contains earthed metal structures is almost useless. The lightning channel will not notice the grid without any significant elevation over the roof. In this chapter, we will discuss the efficiency of the grid installed on the dielectric coating of the structure, e.g. corrugated fibre plate or modern elastic fiberglass cloth, of a mobile or temporary object.
Here, it is a good time and place to talk about the rated grid dimensions. The discussion is conducted in the historical context because the structural dimensions of the grid changed several times, and the changes were marked, but always without any justification. In the above-mentioned book of 1951, it was noted that the typical grid dimensions are 5 x 5 m, but in the provided calculation, its size was reduced to 4 m. Document SN 305-69 Guidelines to Design and Install the Lightning Protection of Buildings and Structures rates the cell area rather than linear dimensions of the grid, which should not exceed 36 m2 and 150 m2 for the lightning protection of the second and third categories, respectively (in case of the square grid, it is 6 x 6 m and 12 x 12 m). Similar requirements are also kept in document SN 305-77. But there is a reservation stating that the grid may be installed on a flat roof only.
And finally, Guidelines for the Lightning Protection of Buildings and Structures RD 34.21.122-87 sets the grid cell dimensions of not more than 6 x 6 m to provide lightning protection of the second category and 12 x 12 m for the third category. The grid application is a little bit expanded there. Now, you can use it not only on flat roofs but also on the roofs with the slope of up to 1 : 8.
E. M. Bazelyan, Dr. Sc. (Eng.), Professor
Krzhizhanovsky Energy Institute, Moscow
Read further 4. Calculating the Protection Reliability of the Grid.