Unit step responses (USRs) of a USR-measuring system recommended by the International Electro-technical Commission (IEC) have been calculated, 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 3.3m and resistance 9kΩ. The conductivity and relative permittivity of the laboratory floor, on which the USR measuring system is placed, are set to a range from σ = 0.001mS/m to ∞ and a range from
εr = 1 to 10, respectively. It turns out that USR parameters are not much influenced by either σ or
εr if σ is higher than or equal to 10mS/m, and they are close to the USR parameters for σ = ∞. When σ is lower than about 0.1mS/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 8ns, 14ns, 150ns, and 50%, respectively. Even if the width of horizontal ground metal sheet is extended from
wg = 2 to 3m, these variations are not much reduced. The extension of the width of vertical metal sheet from
wv = 1 to 2m does not much influence the USR parameters, either. Both the installing height of shield ring
h and its diameter
d influence the USR parameters:
Tα decreases with decreasing
h and/or
d. Note that the FDTD-calculated USR for σ = 0.1mS/m and
εr = 10 agrees reasonably well with the corresponding measured USR:
Tα = 19ns,
Tn = 6ns,
Ts = 310ns, and
β = 53% vs.
Tα = 16ns,
Tn = 10ns,
Ts = 280ns, and
β = 44%.
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