--- title: "Equipment earthing objective and plate electrode size" subject: "Industrial Power Distribution and Illumination" chapter: 2 key: "semester_6/industrial_power_distribution_and_illumination:earthing-objective-plate-electrode" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/earthing-objective-plate-electrode site: BCT Engineering --- # Equipment earthing objective and plate electrode size Industrial Power Distribution and Illumination · Chapter 2: Earthing - Asked in: 2077 Chaitra, 8 marks; 2075 Baisakh, 4+4 marks (asked 2 times) - Video: [Pipe Earthing | Plate Earthing (Basic Electrical Engineering) BE/BTECH 1st year) (in हिन्दी)](https://www.youtube.com/watch?v=CzQcjKlsL7s) — Gautam Varde - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/earthing-objective-plate-electrode - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-2 ## Question What is the main objective of an equipment earthing? Calculate the size of the earth electrode for plate earthing system, if the soil resistivity is 60 Ω and required earth resistance is 8 Ω. ## Short answer The main objective is to protect persons from shock by keeping metal parts at earth potential and enabling fast clearing of fault currents. For a plate electrode, R = (ρ/4)√(π/A). With ρ = 60 Ω-m and R =8 Ω, A = π (60/(4×8))² = 11.04 m², i.e. a square plate of about 3.32 m side. ## Answer #### Main objective of equipment earthing Equipment earthing means connecting the non-current-carrying metal parts (motor frames, panel bodies, conduits, transformer tanks) to the general mass of earth through a low-resistance path. Its objectives are: 1. **Safety from electric shock:** if insulation fails and a live wire touches the body, the fault current flows to earth through the low-resistance earth path, not through a person. The body stays at a low touch voltage. 2. **Fast operation of protection:** the low-resistance path allows a large fault current, so the fuse/MCB/relay trips quickly and isolates the faulty equipment. 3. **Protection of equipment and fire prevention:** prevents sustained arcing and overheating at the fault. 4. **Discharge of static charges and lightning surges** safely to earth. #### Size of plate electrode For a plate electrode (IS 3043): ```text R = (ρ/4) × √(π/A) ``` where R = earth resistance (Ω), ρ = soil resistivity (Ω-m), A = area of one face of the plate (m²). The resistivity is taken as **60 Ω-m** (the unit is printed as Ω). Solving for A: ```text √(π/A) = 4R/ρ A = π × (ρ / 4R)² = π × (60 / (4×8))² = π × (1.875)² = π × 3.5156 = 11.04 m² ``` For a square plate: ```text Side = √11.04 = 3.32 m ``` **Answer: Area of plate ≈ 11.04 m², i.e. a square plate of about 3.32 m × 3.32 m.** Such a large single plate is not practical. In practice a standard plate (for example 60 cm × 60 cm × 6.3 mm GI or 3.15 mm copper) is used with several electrodes in parallel, soil treatment with charcoal and salt (or bentonite) to lower ρ, or a deeper rod electrode, to reach 8 Ω. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does equipment earthing protect a person if insulation fails? A. Fault current flows to earth through the low-resistance path, keeping the body at a low touch voltage B. The body is kept at line voltage C. The current is forced through the person D. The fault is made invisible to protection Answer: A. A low-resistance earth path diverts fault current and keeps the body potential low. (Key point: Objective of earthing) 2. Why does a low earth resistance help protective devices? A. Fault current is limited so nothing trips B. It raises the voltage on the body C. It delays the relay D. A large fault current flows, so the fuse/MCB trips quickly Answer: D. Large fault current operates protection quickly. (Key point: Fast protection) 3. Which formula is used for plate earth resistance? A. R = rho x A / pi B. R = rho / (4A) C. R = (rho/4) x sqrt(pi/A) D. R = 4 rho x sqrt(A/pi) Answer: C. IS 3043 gives R = (rho/4) sqrt(pi/A), with A the area of one face. (Key point: Plate formula) 4. For rho = 60 ohm-m and R = 8 ohm, what is the plate area? A. About 3.32 m^2 B. About 11.04 m^2 C. About 1.1 m^2 D. About 0.35 m^2 Answer: B. A = pi x (60/(4x8))^2 = pi x 3.5156 = 11.04 m^2, a side of about 3.32 m. (Key point: Plate area) ## Close variants set in other exams - [Plate earth electrode size numerical](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aplate-electrode-60-ohm) — Industrial Power Distribution and Illumination, chapter 2, asked 4× ## Related in Industrial Power Distribution and Illumination - [Earth resistance factors and plate electrode size](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aearth-resistance-plate-50ohm-m) — Industrial Power Distribution and Illumination, chapter 2, asked 2× - [Equipment earthing objective and earth resistance factors](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aearthing-objective-factors) — Industrial Power Distribution and Illumination, chapter 2, asked 2× - [Earthing objectives, factors and plate earthing size](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aearthing-objectives-plate-40ohm-m) — Industrial Power Distribution and Illumination, chapter 2 - [Low earth resistance and equipment earthing methods](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alow-earth-resistance-methods) — Industrial Power Distribution and Illumination, chapter 2 - [Earth resistance factors and rod earthing diameter](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aearth-resistance-rod-diameter) — Industrial Power Distribution and Illumination, chapter 2 - [Reducing earth resistance and pipe earthing](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Areduce-earth-resistance-pipe-earthing) — Industrial Power Distribution and Illumination, chapter 2 ## Related in other subjects - [Soil resistivity and reducing earth resistance](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Asoil-resistivity-earth-resistance) — Switchgear and Protection, chapter 5 - [Chemical earthing and earth resistance](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Achemical-earthing-resistance) — Switchgear and Protection, chapter 5 - [Tower footing resistance of 16 Ω electrode design](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atower-footing-16-ohm-electrodes) — High Voltage Engineering, chapter 2, asked 3× - [Tower footing resistance of 20 Ω electrode design](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atower-footing-20-ohm-electrodes) — High Voltage Engineering, chapter 2, asked 3×