Specific conductivity of soil depends on frequency that must be taken into account in calculations of impulse processes in grounding systems under lightning current. The Fourier method currently does not require modification of the standard model of the grounding system, but the scope of its applications is limited by linear problems, and difficulties with the causality principle may arise. We have developed a timediscrete model of a grounding system in soil with arbitrary frequency-dependent complex specific conductivity, suitable for calculations of impulse transient processes using standard methods in time domain. It is shown on numerical examples that overvoltage impulses in grounding systems, which are located in high-resistance soil with frequency-dependent parameters, are significantly smoothed and attenuated under the influence of current impulses with short front and short duration.