--- title: "Step and touch potential in mat design" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:step-touch-potential" source: https://www.bctengineer.com/q/semester_8/power_plant_design/step-touch-potential site: BCT Engineering --- # Step and touch potential in mat design Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2078 Chaitra, 6 marks - Video: [Extra High Voltage substation grounding|grounding calculation|Touch voltage|Step Voltage|GPR](https://www.youtube.com/watch?v=ibnN0Iwg5kU) — ELECTRICAL TECHNOLOGY and INDUSTRIAL PRACTICE - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/step-touch-potential - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question How are the concepts of STEP POTENTIAL and TOUCH POTENTIAL utilized during grounding mat design of a power project area? Why and how these potentials are appropriately set? ## Short answer Step voltage (between feet) and touch voltage (hand to feet) are limited to prevent ventricular fibrillation. Tolerable limits are set using IEEE Std 80: E_touch = (1000 + 1.5 C_s ρ_s) 0.116 / √t_s. Grid design calculates mesh and step voltages; they must stay below limits. Crushed rock surface and close spacing keep actual voltages low. ## Answer **Step potential** is the voltage between a person's two feet, 1 m apart, when standing on the ground during an earth fault. **Touch potential** is the voltage between the hand touching an earthed metal structure and the feet standing on the ground. Both arise because fault current flowing into the soil makes the ground surface potential uneven. ```text Earthed structure | hand |---o <- touch voltage = V(structure) - V(feet) | /|\ ======|==/=\============== ground surface | feet 1 m apart <- step voltage = V1 - V2 ---- grid conductors (mat) buried ~0.5 m ---- ``` #### How they are used in grid (mat) design (IEEE Std 80) 1. **Tolerable limits are found first** from the body current that the heart can withstand (Dalziel): $I_B = k/\sqrt{t_s}$, with $k = 0.116$ for a 50 kg person. With body resistance 1000 Ω: $$ \begin{aligned} E_{step,50} &= (1000 + 6C_s\rho_s)\,\frac{0.116}{\sqrt{t_s}} \\ E_{touch,50} &= (1000 + 1.5C_s\rho_s)\,\frac{0.116}{\sqrt{t_s}} \end{aligned} $$ where $\rho_s$ is the surface-layer resistivity, $C_s$ the surface layer derating factor and $t_s$ the fault clearing time. 2. **Grid potential rise** $GPR = I_G R_g$ is calculated. If GPR is below the tolerable touch voltage, the design is safe. 3. Otherwise the **actual mesh voltage** $E_m$ (worst touch voltage in a mesh corner) and **actual step voltage** $E_s$ are calculated from the grid geometry (spacing, conductor length, depth, soil resistivity). 4. The design is accepted only if $E_m < E_{touch}$ and $E_s < E_{step}$. If not, the grid is changed and the check repeated. #### Why they are set this way - Ensures that the current through a human body during the worst fault stays below the **ventricular fibrillation threshold** within the fault clearing time. - Touch voltage is usually the critical one, because hand-to-feet current passes near the heart and the feet act in parallel, giving less resistance. #### How they are kept within limits - Close **grid spacing** and more conductors; extra rods at the perimeter. - **Surface layer of crushed rock/gravel** (high $\rho_s$, about 3000 Ω·m), which raises the tolerable limits. - **Faster fault clearing** (smaller $t_s$). - Bonding all metal structures and fences to the grid; grading rings around equipment and at gates. - Lowering grid resistance with deeper rods or treated soil. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is touch potential? A. GPR only B. Voltage across grid C. Voltage between hand on earthed structure and feet D. Voltage between two feet 1 m apart Answer: C. Touch voltage is structure potential minus foot potential. (Key point: Definitions) 2. On what basis are tolerable step and touch voltages found? A. Body current the heart withstands, I = 0.116/sqrt(ts) for 50 kg B. Soil only C. Grid length D. Transformer rating Answer: A. Dalziel's relation sets the fibrillation threshold versus clearing time. (Key point: Tolerable limits) 3. When is a grid design accepted? A. Grid is smallest B. Em < tolerable touch and Es < tolerable step C. GPR is zero D. Rods absent Answer: B. Actual mesh and step voltages must remain below tolerable limits, else redesign. (Key point: Acceptance) 4. Why is a crushed rock surface layer used? A. Lowers grid resistance B. Reduces GPR C. Replaces conductors D. Its high resistivity raises tolerable limits Answer: D. Higher rho_s increases the body-circuit resistance, raising tolerable voltages. (Key point: Gravel) ## Related in Power Plant Design - [Steps of grounding mat design](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agrounding-mat-design-steps) — Power Plant Design, chapter 4, asked 5× - [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 ## Related in other subjects - [Step and touch potentials, shock and safety](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Astep-touch-potential-safety) — High Voltage Engineering, chapter 2 - [Improper earthing and unsafe step/touch potential](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aimproper-earthing-step-touch) — High Voltage Engineering, chapter 2 - [Touch and step potential phenomenon](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atouch-step-potential-figures) — High Voltage Engineering, chapter 2 - [Fundamental concepts of concrete mix design](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amix-design-fundamental-concepts) — Concrete Technology and Masonry Structure, chapter 3