Results of an extensive research, conducted using a technique developed to calculate the grounding resistance of a complex grounding system consisting of a foundation grounding system with an external grid, buried in two-layer soil, are presented and discussed. It is shown that connecting a foundation grounding system to the already existing external grid provides only a small reduction of the overall grounding system resistance compared to the resistance of the external grid alone. A number of tables related to the most frequent cases of soil-concrete structures are given for practical assessment of the grounding resistance of such complex grounding systems.
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