An N-port equivalent circuit approach is adopted to model a multipoint grounding system, N being the number of injection points. A methodology is proposed to determine the parameters of the N-port equivalent circuit from either numerical simulations or experimental measurements. For the case of a symmetrical grounding system, analytical closed-form expressions are derived for the equivalent parameters of the N-port circuit. We show that the concept of the equivalent input impedance (harmonic impedance) could be used in the case of a symmetrical multipoint grounding system. We show also that the experimental evaluation of the impedance of a grounding system using a single injection point is a conservative engineering practice, since the obtained impedance might be overestimated at high frequencies.