This paper presents an equivalent circuit synthesis for grounding impedance of foundations on the basis of FDTD (Finite Difference Time Domain) analysis results. The circuit model is expressed by a Foster second type circuit and each parameter is given as a function of the foundation size and soil resistivity. The model can express a grounding foundation whose width is from 2 m to 16 m, depth is 2 m to 8 m, and the soil resistivity around the foundation is from 10 ･m to 1000 ･m. By using the proposed model, the grounding impedance of a foundation including its high frequency characteristics can be calculated without a time-consuming 3-dimensional numerical electromagnetic analysis, and a transient response of a structure against lightning current can be simulated by any circuit analysis program.

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