--- title: "Equipment earthing objective and earth resistance factors" subject: "Industrial Power Distribution and Illumination" chapter: 2 key: "semester_6/industrial_power_distribution_and_illumination:earthing-objective-factors" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/earthing-objective-factors site: BCT Engineering --- # Equipment earthing objective and earth resistance factors Industrial Power Distribution and Illumination · Chapter 2: Earthing - Asked in: 2071 Magh, 4 marks; 2068 Bhadra (old course), 8 marks (asked 2 times) - 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/earthing-objective-factors - 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? Explain in brief the various factors affecting the earth resistance. ## Short answer Equipment earthing connects non-current-carrying metal parts to earth for safety, fast protection operation, surge discharge and static dissipation. Earth resistance depends on soil type, moisture, salt content, temperature, electrode size and depth of burial, number and spacing of parallel electrodes, backfill contact, and corrosion condition. ## Answer #### Main objective of equipment earthing Equipment earthing connects the non-current-carrying metal parts of equipment (frames, enclosures, conduits) to earth through a low-resistance conductor. Its objectives are: 1. **Protect persons from electric shock:** when insulation fails, the metal body would become live. Earthing keeps its potential near zero, and the fault current flows to earth, not through a person. 2. **Quick operation of protective devices:** the low-resistance earth path lets a large fault current flow, so fuses, MCBs or earth-fault relays trip quickly. 3. **Protect equipment and buildings from fire** by preventing long-lasting leakage and arcing. 4. **Safely discharge static electricity and lightning/switching surges.** #### Factors affecting earth resistance 1. **Type of soil:** clay, loam and garden soil have low resistivity; sand, gravel and rock have very high resistivity. 2. **Moisture content:** resistivity falls sharply as moisture rises. Dry soil (below about 15% moisture) has very high resistivity. This is why earth pits are watered and resistance is higher in the dry season. 3. **Salt (chemical) content:** dissolved salts make the soil water conductive. Adding salt, charcoal or bentonite lowers resistance. 4. **Temperature:** resistivity rises as temperature falls, very sharply when soil water freezes. 5. **Size and shape of electrode:** a larger plate or longer rod has more contact area and lower resistance. Rod length matters more than diameter. 6. **Depth of burial:** deeper electrodes reach moist, stable soil and have lower and steadier resistance. 7. **Number and spacing of electrodes:** several electrodes in parallel lower resistance, if spaced at least about twice their length apart. 8. **Contact between electrode and soil:** loose or dry backfill raises resistance; well-compacted, treated backfill lowers it. 9. **Condition of electrode and connections:** corrosion of the electrode or loose joints raises resistance over time. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a main objective of equipment earthing? A. To protect persons from shock by keeping metal parts near zero potential B. To raise the body voltage C. To reduce the supply frequency D. To increase fault impedance Answer: A. Earthing diverts fault current to earth and keeps exposed metal near earth potential. (Key point: Objective of earthing) 2. How does soil moisture affect earth resistance? A. Resistance rises with moisture B. Moisture has no effect C. Only frozen soil matters D. Resistance falls as moisture rises; dry soil is very high Answer: D. That is why earth pits are watered and resistance is higher in the dry season. (Key point: Moisture) 3. How can earth resistance be lowered? A. Use dry sand as backfill B. Use loose dry backfill C. Add salt, charcoal or bentonite to the soil around the electrode D. Reduce the electrode size Answer: C. Dissolved salts make the soil water conductive and lower resistivity. (Key point: Treatment of soil) 4. How should several parallel electrodes be spaced? A. Touching each other B. At least about twice their length apart C. As close as possible D. Exactly 10 cm Answer: B. Close spacing makes the resistance zones overlap and reduces the benefit. 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