# Design, Modeling, and Analysis of IEEE Std 80 Earth Grid Design Refinement Methods Using ETAP
## 4 Refinement Methods
### Increasing the Tolerable Safety Limits of Touch and Step Voltage  
The second method tested was the variation in the fault current duration to assess its impact on the tolerable safety limits of touch and step voltage. In power systems protection, prompt fault clearance is one of the fundamental requirements for the protection of people and equipment, preventing total system failure, ensuring the reliability of unaffected areas, and limiting damage to equipment. In earth grid design, fault duration plays a significant role concerning the touch and step voltages as it determines the level of severity when a  
Figure 10: Imported fault currents.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{6}{c}{**Variation in Current Limitation**} \\ \hline \multicolumn{2}{c}{**Fault Current Limit (A)**} & \multicolumn{2}{c}{**Group Potential Rise (V)**} & \multicolumn{2}{c}{**Touch Voltage (V)**} & \multicolumn{2}{c}{**Step Voltage (V)**} \\ \hline
**NER Application?** & **Fault Current (kA)** & & **Calculated** & **Tolerable** & **Calculated** & **Tolerable** \\ \hline No NER used & 22.759 & 7562.2 & 1122.5 & 1144.3 & 503.1 & 4026.9 \\ Yes—1000 A & 1.093 & 349.7 & 51.9 & 1144.3 & 23.3 & 4026.9 \\ Yes—800 A & 0.876 & 280.2 & 41.6 & 1144.3 & 18.6 & 4026.9 \\ Yes—500 A & 0.549 & 175.6 & 26.1 & 1144.3 & 11.7 & 4026.9 \\ Yes—400 A & 0.439 & 140.4 & 20.8 & 1144.3 & 9.3 & 4026.9 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Application of the current limitation and its impact on the safety limits.  
person and/or equipment are exposed to elevated voltages: the longer the time of exposure, the more severe the damage.  
For this study, the durations of the same fault current of \(I_{f}=22.579\) kA were varied between 300 ms and 500 ms to assess the impact of the small-time margins that exist between these times and how critical it is for power systems to have reliable and highly effective protection schemes to ensure that a fault is cleared promptly at all times. The initial input fault current duration settings of the substation earth grid were varied from 300 ms to 500 ms in incremental scales of 50 ms as shown in Figure 11.  
The results of the simulations are tabulated in Table 4, which shows the impact of the fault current duration on the performance of the safety limits of touch voltage, step voltage, and the ground potential rise.  
From the simulation results tabulated in Table 4, it is observed that, by varying the fault current duration, the safety limits of the earth grid are greatly influenced. The results show that, the sooner the fault is cleared, the higher the tolerable threshold of the safety limits. At a fault duration of 0.3 s, the touch and step voltage tolerable voltages are 1321.3 V and 4649.9 V, respectively, with calculated values of 1136.2 V and 509.2 V. When the fault duration is gradually increased, it is observed that the tolerable thresholds decrease; for instance, at a fault duration of 5 s, the touch and step voltage tolerable voltages are 1023.5 V and 3601.8 V, with calculated values of 1114.3 V and 499.4 V. This variation of 2 s results in a tolerable voltage difference of 297.8 V and 1048.1 V for the touch and step voltage, respectively.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Variation in Current Limitation**} \\ \hline
**Fault Duration (A)** & **Group Potential Rise (V)** & **Touch Voltage (V)** & \multicolumn{2}{c}{**Step Voltage (V)**} \\ \hline  & & **Calculated** & **Tolerable** & **Calculated** & **Tolerable** \\ \hline
0.3 & 7654 & 1136.2 & 1321.3 & 509.2 & 4649.9 \\
0.35 & 7601.7 & 1128.4 & 1223.3 & 505.7 & 4305 \\
0.4 & 7562.2 & 1122.5 & 1144.3 & 503.1 & 4026.9 \\
0.45 & 7531.3 & 1118 & 1078.8 & 501 & 3796.6 \\
0.5 & 7506.6 & 1114.3 & 1023.5 & 499.4 & 3601.8 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Application of the fault duration and its impact on the safety limits.  
Figure 11: ETAP earth grid current duration settings.