Steps of grounding mat design

A grounding (earthing) mat is a buried grid of horizontal conductors with vertical rods under a powerhouse and switchyard. It must carry fault current to earth safely and keep step and touch voltages within tolerable limits. Design normally follows IEEE Std 80.

Layout

  +----+----+----+----+   Horizontal grid
  |    |    |    |    |   (Cu or GI conductor
  +----+----+----+----+    at 0.5-0.8 m depth)
  |    |    |    |    |
  +----+----+----+----+   o = earth rods
  o    |    |    |    o
  +----+----+----+----+

Steps

1. Site data: area of switchyard/powerhouse; measure soil resistivity $\rho$ by the Wenner four-pin method; resistivity of surface layer (crushed rock) $\rho_s$.
2. Fault current and time: find maximum earth fault current $3I_0$, split factor $S_f$ and decrement factor $D_f$:

$$I_G = D_f\, S_f\, 3I_0$$

Fault clearing time $t_f$ and shock duration $t_s$ (e.g. 0.5–1 s). 3. Conductor size: from IEEE 80 (Onderdonk-type) formula, with $I$ in kA

$$A_{mm^2} = \frac{I}{\sqrt{\dfrac{TCAP\times 10^{-4}}{t_c\,\alpha_r\,\rho_r}\ln\dfrac{K_0+T_m}{K_0+T_a}}}$$

then add corrosion allowance and mechanical minimum. 4. Tolerable step and touch voltages (50 kg body):

$$\begin{aligned} E_{step} &= (1000 + 6C_s\rho_s)\frac{0.116}{\sqrt{t_s}} \\ E_{touch} &= (1000 + 1.5C_s\rho_s)\frac{0.116}{\sqrt{t_s}} \end{aligned}$$

5. Initial design: choose grid spacing $D$, depth $h$, total conductor length $L_T$ and number of rods, covering the whole area plus about 1 m beyond the fence.
6. Grid resistance (Sverak):

$$R_g = \rho\left[\frac{1}{L_T} + \frac{1}{\sqrt{20A}}\left(1 + \frac{1}{1 + h\sqrt{20/A}}\right)\right]$$

7. Ground potential rise: $GPR = I_G R_g$. If $GPR < E_{touch}$, the design is safe; go to step 10.
8. Mesh and step voltages:

$$E_m = \frac{\rho K_m K_i I_G}{L_M},\qquad E_s = \frac{\rho K_s K_i I_G}{L_S}$$

9. Check and modify: require $E_m < E_{touch}$ and $E_s < E_{step}$. If not, reduce spacing, add conductors or rods, add crushed rock, or extend the grid, and repeat.
10. Detailed design: connect all equipment frames, neutrals, fences, lightning arresters and cable sheaths; check transferred potentials; target resistance typically below 1 Ω for large stations.