A simple formula of grounding grid resistance in vertical two-Layer soil

Large ground grids usually were located in vertical two-layer soil. A study was done in this paper to get a simple and accurate grounding resistance expression of it. At first, some simulations were done about a same grid buried in different vertical two-layer soil to get resistances of it. An analysis was given about the relationship of grounding resistances when the grid buried in homogeneous medium or vertical two-layer medium. Then, the curve of the relationship was fitted and a formula was gotten. By discuss the mathematical and physical significance of the formula; the coefficients of it were determined. At last, an error analysis of the formula was done and an example was given to show how to calculate grounding resistance of a grid locate in vertical two-layer soil using this formula.