# Design, Modeling, and Analysis of IEEE Std 80 Earth Grid Design Refinement Methods Using ETAP
## 4 Refinement Methods
### Fault Current Diversion  
In power systems, it is highly advisable to design for the worst-case scenario (N-1 contingency); similarly, in the substation in earthing systems, the norm is adopted. However, even though it is highly advisable to design the earth grid for the worst-case scenario, where the maximum fault current that can be generated by the power system's transformers is dissipated into the earth grid [1], in some cases, it may be necessary to implement the current diversion method to divert a portion of the fault current away from the earth grid to improve the design and safety compliance of the earth grid. In practice, this is computed by applying [PERSON]'s current law to obtain the current division factor between the substation earth grid resistance and the input impedance of each circuit (conductors or overhead lines) [15].  
For this study, the fault current diversion method was applied on an earth grid superimposed on a 20 MVA, 11/6.6 kV power system. The earth grid was initially modeled and simulated as shown in Section 3: Earth Grid Design Criteria to analyze the impact of fault current diversion, where the initial settings of the earth grid's current division factor input data as shown in Figure 12 were varied from 100% to 60% in decremental scales of 10%.  
The simulation results of the fault current division are shown in Table 5, where the impact and influence of the current diversion on the performance of the safety limits of touch voltage, step voltage, and ground potential rise are tabulated.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Variation in Current Limitation**} \\ \hline
**Current Division Factor (A)** & **Group Potential Rise (V)** & **Touch Voltage (V)** & \multicolumn{2}{c}{**Step Voltage (V)**} \\ \hline  & & **Calculated** & **Tolerable** & **Calculated** & **Tolerable** \\ \hline
100 & 7562.2 & 1122.5 & 1144.3 & 503.1 & 4026.9 \\
90 & 6806 & 1010.3 & 1144.3 & 452.8 & 4026.9 \\
80 & 6049.8 & 898 & 1144.3 & 402.5 & 4026.9 \\
70 & 5293.5 & 785.8 & 1144.3 & 352.2 & 4026.9 \\
60 & 4537.3 & 673.5 & 1144.3 & 301.8 & 4026.9 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Application of the current diversion and its impact on the safety limits.  
Figure 12: ETAP earth grid current division settings.  
From the simulation results tabulated in Table 5, it is observed that, by diverting a percentage of the fault current directed to the earth grid, the calculated safety limits are proportionally affected. The results show that, as the percentage of the fault current injected into the earth grid reduces, the calculated touch and step voltages decrease; at a 100% division factor, which is the worst-case scenario, the touch and step voltages are 1122.5 V and 503.1 V, respectively; and when the percentage is gradually reduced to 60%, the touch and step voltages are proportionally reduced to 673.5 V and 301.8 V, respectively, yielding voltage differences of 449 V And 201.3 V for the touch and step voltages, respectively, for the 40% current division difference.