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Unit step responses (USRs) of a USR-measuring system recommended by the International Electrotechnical Commission have been computed using the finite-difference time-domain (FDTD) method for various conditions of grounding and shield ring. The USR measuring system subject to analysis has a resistor divider of height 3.3 m and resistance 9 kΩ. It follows from FDTD-computed results that USR parameters are not much influenced by either the laboratory-floor conductivity σ or its relative permittivity εr if σ is higher than or equal to 10 mS/m, and they are close to the USR parameters for σ = ∞. When σ is lower than about 0.1 mS/m, USR parameters are more influenced by σ and εr. Variations in the partial response time Tα, the experimental response time Tn, the settling time Ts, and the overshoot β , due to the differences of σ and εr, are 8 ns, 14 ns, 150 ns, and 50%, respectively. Even if the width of a horizontal metal sheet is extended from wg = 2 to 3 m, these variations are not much reduced. The extension of the width of a vertical metal sheet from wv = 1 to 2 m does not influence the USR parameters much, either. The installing height of a shield ring h influences the USR parameters: with decreasing h, Tα and Tn decrease while β and Ts increase. Note that the FDTD-computed USR for σ = 0.1 S/m and εr = 10 agrees well with the corresponding measured USR.