Some people say impedance and some resistance when talking about grounding?
The lightning surge event is characterized by having very fast changes in current and voltage, sometimes called the dv/dt and di/dt. In essence it is a high frequency event and as such the ground system is better considered as an ac impedance rather than dc resistance. The subject is complicated and requires knowledge of transmission line theory and special techniques to measure the effective impedance of the grounding system under impulse condition. Enough said!
