--- title: "Steps of grounding mat design" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:grounding-mat-design-steps" source: https://www.bctengineer.com/q/semester_8/power_plant_design/grounding-mat-design-steps site: BCT Engineering --- # Steps of grounding mat design Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2081 Chaitra, 6 marks; 2081 Shrawan, 8 marks; 2079 Shrawan, 4 marks; 2078 Kartik, 6 marks; 2075 Bhadra, 4 marks (asked 5 times) - Video: [Substation Earthing Design Step by Step | Complete Practical Guide](https://www.youtube.com/watch?v=RWtJ5EjnGgI) — Circuit Masterclass - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/grounding-mat-design-steps - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Describe the major steps that to be followed for design of earthing (grounding) mat in power station and its switchyard. ## Short answer Measure soil resistivity and area; determine grid current from fault current with split and decrement factors; size conductor via IEEE 80 formula; compute tolerable step and touch voltages; design initial grid with conductor spacing and rods; calculate grid resistance and GPR; compute mesh and step voltages; ensure below tolerable limits, else refine; connect all equipment, target resistance below 1 Ω. ## Answer 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 ```text +----+----+----+----+ 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The main purpose of a grounding mat is to: A. Increase fault current B. Carry fault current safely to earth and keep step and touch voltages within tolerable limits C. Raise the ground potential rise as high as possible D. Replace the lightning arrester Answer: B. Safety of people from step and touch voltage during faults is the design goal (IEEE 80). (Key point: Purpose of mat) 2. Which site measurements start the design? A. Wind speed B. Soil resistivity by Wenner four-pin method and surface layer resistivity C. Transformer oil temperature D. River discharge Answer: B. Soil resistivity (and crushed-rock layer resistivity) is the basic input for grid resistance and voltage limits. (Key point: Site data) 3. What must be satisfied for the design to be accepted? A. E_m < E_touch and E_s < E_step B. E_m > E_touch C. GPR must be zero D. Rg must be above 10 ohm Answer: A. Mesh and step voltages must be below tolerable touch and step voltages; otherwise modify the grid. (Key point: Design check) 4. If the mesh voltage exceeds the tolerable touch voltage, you should: A. Increase the clearing time B. Remove rods C. Reduce spacing, add conductors or rods, add crushed rock or extend the grid, then repeat D. Use thinner conductor Answer: C. These steps lower grid resistance and mesh voltage; the check is then repeated. (Key point: Modifying a failed design) ## Related in Power Plant Design - [Step and touch potential in mat design](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Astep-touch-potential) — Power Plant Design, chapter 4 - [Neutral grounding transformer sizing](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aneutral-grounding-transformer) — Power Plant Design, chapter 4, asked 2× - [Types of generator neutral grounding](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-neutral-grounding-types) — Power Plant Design, chapter 4 - [Layout of a medium head Francis station](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Afrancis-medium-head-layout) — Power Plant Design, chapter 3, asked 2× - [DC supply system in a generating station](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adc-supply-system) — Power Plant Design, chapter 4 - [Battery DOD and station service schemes](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abattery-dod-service-station) — Power Plant Design, chapter 4 ## Related in other subjects - [Major components of a power station](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-station-major-components) — Power System Analysis I, chapter 1, asked 2× - [Major components of a power station and functions](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-station-components-functions) — Power System Analysis I, chapter 1 - [Improper earthing and system grounding practice](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aimproper-earthing-system-grounding) — High Voltage Engineering, chapter 2 - [Requirements of steam power station design](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-station-design-requirements) — Power Plant Equipment, chapter 7