--- title: "Factors influencing earth resistance" subject: "Industrial Power Distribution and Illumination" chapter: 2 key: "semester_6/industrial_power_distribution_and_illumination:earth-resistance-factors" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/earth-resistance-factors site: BCT Engineering --- # Factors influencing earth resistance Industrial Power Distribution and Illumination · Chapter 2: Earthing - Asked in: 2072 Magh, 4 marks - Video: [Factors affecting earth resistance](https://www.youtube.com/watch?v=_Tt4qfTtgE4) — Chetan Sawant - 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/earth-resistance-factors - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-2 ## Question Explain various factors influencing the earth resistance. ## Short answer Earth resistance depends on soil type and moisture (lower in wet clay, higher in dry sand), temperature (freezing increases it), dissolved salts, electrode size and depth (deeper rods reach moist stable soil), and electrode spacing. Good compacted backfill with charcoal/salt and parallel electrodes reduce resistance, while corrosion and loose connections increase it. ## Answer **Earth resistance** is the resistance between the earth electrode and the general mass of earth. It must be low so that fault and surge currents flow easily to earth. The main factors influencing it are: 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 has very high resistivity, so earth resistance is highest in the dry season. 3. **Dissolved salts (chemical content):** salts make soil water conductive. Adding salt, charcoal or bentonite lowers the resistance. 4. **Temperature:** resistivity rises as the soil cools, very sharply when soil water freezes. 5. **Size and shape of electrode:** a larger plate or longer rod has more contact with soil and lower resistance. Rod length matters much more than diameter. 6. **Depth of electrode:** deeper electrodes reach moist, stable soil, so resistance is lower and changes less with season. 7. **Number and spacing of electrodes:** electrodes in parallel lower the resistance, provided they are spaced at least about twice their length apart. 8. **Contact with soil:** loose, dry backfill increases resistance; compacted, treated backfill reduces it. 9. **Condition of electrode and joints:** corrosion and loose connections increase resistance with time. **Example:** a pipe electrode giving 4 Ω in the monsoon may give 10 Ω or more in the dry winter months, because the soil loses moisture. Watering the pit and treating it with charcoal and salt keeps the value low. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which soil has the lowest resistivity? A. Rock B. Dry sand C. Gravel D. Clay or loam Answer: D. Clay, loam and garden soil have low resistivity, while sand, gravel and rock have very high resistivity. (Key point: Soil type) 2. Why are salt, charcoal or bentonite added around an electrode? A. They make the soil water conductive and hold moisture, lowering resistance B. They insulate the electrode C. They raise soil resistivity D. They prevent any current flow Answer: A. Dissolved salts increase conductivity of soil water, lowering the earth resistance. (Key point: Dissolved salts) 3. A pipe electrode gives 4 ohm in the monsoon. Why may it give 10 ohm or more in dry winter? A. The pipe shrinks B. The soil loses moisture, raising its resistivity C. The monsoon adds salts permanently D. Resistance is always constant Answer: B. Drying soil has higher resistivity; watering and treatment keep the value low. 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