--- title: "Low earth resistance and equipment earthing methods" subject: "Industrial Power Distribution and Illumination" chapter: 2 key: "semester_6/industrial_power_distribution_and_illumination:low-earth-resistance-methods" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/low-earth-resistance-methods site: BCT Engineering --- # Low earth resistance and equipment earthing methods Industrial Power Distribution and Illumination · Chapter 2: Earthing - Asked in: 2071 Bhadra, 8 marks - Video: [Earthing, Types Of Earthing with Diagrams |BEE|](https://www.youtube.com/watch?v=gtfl-YTDSU4) — Trouble- Free - 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/low-earth-resistance-methods - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-2 ## Question Why earthing resistance is kept low? State the methods for equipment earthing systems. Explain one of them. ## Short answer Low earth resistance ensures fast protective device operation, low touch voltage, effective lightning protection, stable neutral, and fire prevention. Common earthing methods include plate, pipe, rod, strip, and chemical earthing. Plate earthing uses a buried GI or copper plate (≥3 m deep) surrounded by charcoal and salt layers to achieve low resistance, typically supplemented by a watering pipe and inspection chamber. ## Answer #### Why earth resistance is kept low 1. **Fast operation of protection:** fault current = voltage / (earth loop resistance). A low resistance gives a large fault current, so the fuse/MCB/relay trips quickly. 2. **Low touch and step voltage:** the voltage of a faulty metal body = fault current × earth resistance. A low resistance keeps this voltage safe for people. 3. **Effective lightning protection:** surge arresters and lightning conductors need a low-resistance path so that the voltage rise during a surge is small. 4. **Stable neutral voltage:** keeps the system neutral near earth potential. 5. **Fire prevention:** avoids long-lasting leakage currents and heating at the fault. Typical limits: about 1 Ω for large power stations and substations, a few ohms for small substations and industrial installations. #### Methods of equipment earthing 1. Plate earthing 2. Pipe earthing 3. Rod earthing 4. Strip or wire earthing 5. Earth mat (grid) earthing in substations 6. Chemical (maintenance-free) earthing #### Plate earthing (explained) ```text ground level ══╤═══════════════════╤══ │ funnel+watering │ GI pipe protecting │ pipe (19 mm) │ earth lead │ │ │ charcoal + salt │ │ layers around │ │ ┌───────────┐ │ │ │ plate │◄──┘ earth lead │ │ 60×60 cm │ bolted to plate │ └───────────┘ │ at least 3 m deep ``` 1. **Electrode:** a GI plate of 60 cm × 60 cm × 6.3 mm or a copper plate of 60 cm × 60 cm × 3.15 mm. 2. **Pit:** dug so the top of the plate is at least **3 m below ground**. The plate is placed **vertically**. 3. **Backfill:** the plate is surrounded by alternate layers of charcoal and salt (about 15 cm thick), which keep the soil moist and conductive. 4. **Earth lead:** a GI wire/strip (or copper for a copper plate) is bolted to the plate with nuts and washers, and taken up through a GI pipe for mechanical protection. 5. **Watering pipe:** a 19 mm GI pipe with a funnel and wire mesh is placed near the plate so the pit can be watered in dry weather. 6. **Chamber:** a masonry chamber with a cover is built at the top for inspection. 7. **Testing:** earth resistance is measured with an earth tester; if too high, more plates are added in parallel or the soil is treated further. Plate earthing gives low resistance and is used for substations, transformers and large machines, though it costs more than pipe earthing. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a low earth resistance make protective devices trip quickly? A. Fault current = voltage / earth loop resistance, so low resistance gives a large current B. It reduces the supply voltage C. It increases the loop resistance D. It makes the device slower but safer Answer: A. A larger fault current makes the fuse, MCB or relay operate faster. (Key point: Fault current and trip) 2. What is the voltage of a faulty metal body? A. Supply voltage always B. Fault current x earth resistance C. Fault current / earth resistance D. Zero in all cases Answer: B. Body voltage = I * R_earth, so a low earth resistance keeps it safe. (Key point: Touch voltage) 3. How is a plate earth electrode installed? A. Flat on the surface with no backfill B. Vertically with its top at least 3 m below ground, with charcoal and salt layers C. Horizontally 0.3 m deep in dry sand D. Inside the building wall Answer: B. A 60 x 60 cm plate is placed vertically at least 3 m deep, surrounded by alternate charcoal and salt layers. (Key point: Plate earthing details) 4. What is the 19 mm funnel pipe near the plate for? A. To carry the earth current B. To ventilate the pit C. To hold the plate in position D. To water the pit in dry weather and keep the soil moist Answer: D. Watering keeps the charcoal and salt backfill moist so the resistance stays low. (Key point: Watering pipe purpose) ## Related in Industrial Power Distribution and Illumination - [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× - [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 - [Earth resistance factors and lightning arrester earthing](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aearth-resistance-lightning-arrester) — 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 - [Equipment earthing objective and plate electrode size](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aearthing-objective-plate-electrode) — Industrial Power Distribution and Illumination, chapter 2, asked 3× - [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× ## Related in other subjects - [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 - [Earth compaction equipment with sketches](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aearth-compaction-equipment) — Construction Management, chapter 4, asked 3× - [Behavior of systems at microwave vs low frequency](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-vs-low-frequency-behavior) — RF and Microwave Engineering, chapter 1, asked 3× - [Microwave vs low frequency systems and applications](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-vs-low-frequency-systems) — RF and Microwave Engineering, chapter 1, asked 2×