---
title: "Chemical earthing and earth resistance"
subject: "Switchgear and Protection"
chapter: 5
key: "semester_6/switchgear_and_protection:chemical-earthing-resistance"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/chemical-earthing-resistance
site: BCT Engineering
---

# Chemical earthing and earth resistance

Switchgear and Protection · Chapter 5: System Earthing

- Asked in: 2072 Magh, 4 marks
- Video: [Complete process of Chemical Earthing । Making of an earth pit  ।Bol Bulbul ।](https://www.youtube.com/watch?v=64QBjoX2cmw) — BOL BULBUL Industrial Electrical
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/chemical-earthing-resistance
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-5

## Question

What is chemical earthing? Write the factors affecting the earth resistance.

## Short answer

Chemical earthing surrounds the electrode with a moisture-retaining conductive compound (e.g., bentonite-graphite) to achieve low, stable earth resistance with minimal maintenance. Factors affecting earth resistance: soil resistivity, moisture content, salt concentration, temperature, electrode dimensions and depth, number and spacing of electrodes, electrode-soil contact quality, and soil compaction.

## Answer

**Chemical earthing** is an earthing method in which the electrode (GI or copper-bonded rod/pipe) is surrounded by a conductive chemical compound (bentonite, graphite-based or other earth-enhancing compound) instead of the traditional salt and charcoal, to obtain a low and stable earth resistance with little maintenance.

#### Chemical earthing
- A pit or bore is made and the electrode is placed in the centre.
- The space around it is filled with **earth-enhancing compound** (e.g. bentonite clay mixed with graphite/carbon), which absorbs and holds moisture and has very low resistivity.
- Advantages: low resistance even in rocky or dry soil, stable value through seasons, no need for regular watering, longer life with less corrosion, compact.
- Used in telecom towers, substations, data centres and buildings where soil is poor.

#### Factors affecting earth resistance
1. **Soil resistivity:** the main factor; clay and loam have low resistivity, sand, gravel and rock very high.
2. **Moisture content:** resistivity falls sharply as moisture rises (dry soil can be 10 to 100 times worse).
3. **Salt (chemical) content:** dissolved salts increase conductivity.
4. **Temperature:** resistivity rises as temperature falls, and very sharply when soil freezes.
5. **Electrode size and depth:** longer and deeper electrodes reach moist, stable soil layers and have lower resistance.
6. **Number and spacing of electrodes:** electrodes in parallel, spaced at least twice their length apart, lower the combined resistance.
7. **Contact between electrode and soil:** loose backfill, voids or corrosion raise resistance.
8. **Soil compaction and grain size:** compact, fine-grained soil gives better contact.

**Example:** a 3 m copper-bonded rod in dry sandy soil may give 40–50 Ω; with chemical backfill the same rod can give below 5 Ω.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is chemical earthing?
   A. Electrode surrounded by a conductive compound such as bentonite/graphite to give low, stable resistance
   B. Electrode in plain dry soil
   C. Using salt water each day on a bare rod
   D. A neutral reactor
   Answer: A. The compound absorbs and holds moisture and has very low resistivity. (Key point: Definition)

2. How does moisture affect earth resistance?
   A. It raises resistance
   B. Resistivity falls sharply as moisture rises
   C. No effect
   D. Resistance is lowest when dry
   Answer: B. Dry soil can be 10 to 100 times worse than moist soil. (Key point: Moisture effect)

3. What spacing between parallel electrodes lowers the combined resistance?
   A. Very close together
   B. At least twice their length apart
   C. Exactly 10 cm
   D. Spacing has no effect
   Answer: B. Adequate spacing avoids overlapping resistance areas. (Key point: Electrode spacing)

## Related in Switchgear and Protection

- [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
- [Earth fault protection: 20 MVA alternator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aearth-fault-20mva-5-ohm) — Switchgear and Protection, chapter 7

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