Many of us encounter the issue of relative requirements of the regulatory documents to the grounding arrangement (GA) design for a vertical lightning rod. We'll try to answer briefly and refer to Guidelines for Arrangement of Lightning Protection for Buildings and Structures RD 34.21.122-87 as a more specific and strict document in this regard.
Note that, first of all, you should use all natural grounding electrodes.
To create the structure of the artificial grounding arrangement, you should choose the lightning protection category for the facility. For Category III facilities, the GA structure is specified in RD 34.21.122-87, item 2.26 at least two vertical grounding electrodes with the length of at least 3 m spaced at the distance of at least 5 m. For Categories I and II facilities, the GA structure is specified in RD 34.21.22-87, item 2.2d at least three vertical grounding electrodes with the length of at least 3 m spaced at the distance of at least 5 m.
The GA resistance for lightning protection is not rated, however, when it is used as a protection system, according to the EIC, item 1.7.55, the requirements to the resistance of the protective grounding are added. In this case, you should calculate the required number of additional grounding electrodes.
If you need assistance in determining the facility lightning arrester category, choosing and calculating the grounding arrangement design, contact the ZANDZ Technical Center.