---
title: "Why measure earth resistivity"
subject: "High Voltage Engineering"
chapter: 2
key: "semester_8/high_voltage_engineering:earth-resistivity-necessity"
source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/earth-resistivity-necessity
site: BCT Engineering
---

# Why measure earth resistivity

High Voltage Engineering · Chapter 2: Electric Shocks

- Asked in: 2072 Magh, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/earth-resistivity-necessity
- Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-2

## Question

Why is it necessary to measure earth resistivity?

## Short answer

Earth resistivity must be measured because earthing design, electrode resistance, step and touch potentials, tower footing performance and corrosion assessment all depend on it. It varies widely with soil type, moisture and temperature, so the Wenner method gives the value needed to choose safe and effective earthing, preferably in dry season for worst case.

## Answer

Earth resistivity $\rho$ (Ω-m) is the resistance of a 1 m cube of soil. It must be measured because every earthing design depends on it, and it changes widely (from about 10 Ω-m in wet clay to over 1000 Ω-m in rock) with soil type, moisture, salts and temperature.

#### Reasons for measuring earth resistivity
1. **Design of earth electrodes and substation grids:** the earth resistance is directly proportional to $\rho$ (e.g. rod $R = \frac{\rho}{2\pi L}\ln\frac{2L}{a}$). Without $\rho$ the number, size and depth of electrodes cannot be chosen.
2. **Safety of personnel:** step and touch potentials depend on $\rho$ of the soil and surface layer; IEEE 80 design needs $\rho$ to check safe limits.
3. **Tower footing resistance:** lightning performance and back-flashover rate of lines depend on footing resistance, which depends on $\rho$.
4. **Choice of site and best location** for earth pits, and the best depth (layered soil), since resistivity changes with depth.
5. **Protection operation:** a low earth resistance gives enough fault current for relays to operate quickly.
6. **Corrosion assessment:** low-resistivity soil is more corrosive for buried metal and pipelines; also needed for cathodic protection design.
7. **Geophysical surveys:** locating water, ore bodies and rock layers.

It is measured mainly by the Wenner four-electrode method ($\rho = 2\pi aR$), preferably in the dry season to get the worst-case value.

## Check yourself

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

1. Why is rho needed for electrode design?
   A. Earth resistance is directly proportional to rho
   B. Resistance is independent of rho
   C. Only for colour
   D. To measure frequency
   Answer: A. For a rod R = rho/(2*pi*L) ln(2L/a). (Key point: Electrode design)

2. How does rho relate to personnel safety?
   A. Not related
   B. Step and touch potentials depend on rho of soil and surface layer
   C. Only tower height matters
   D. Only relay settings
   Answer: B. IEEE 80 design needs rho to check limits. (Key point: Safety)

3. Why is measurement preferably done in the dry season?
   A. Soil gives worst-case (highest) value
   B. Soil is easier to dig
   C. Tester works only in dry
   D. Resistivity is lowest
   Answer: A. Design for the worst case. (Key point: Dry season)

4. Which soil is more corrosive for buried metal?
   A. High resistivity rock
   B. Low-resistivity soil
   C. Dry sand
   D. Frozen soil
   Answer: B. Low resistivity indicates conductive, corrosive soil; also needed for cathodic protection design. (Key point: Corrosion)

## Related in High Voltage Engineering

- [Wenner's method of earth resistivity measurement](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Awenner-earth-resistivity) — High Voltage Engineering, chapter 2, asked 3×
- [Methods of earth resistivity measurement](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aearth-resistivity-methods) — High Voltage Engineering, 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
- [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×
- [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×
- [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