---
title: "Earth resistance factors and lightning arrester earthing"
subject: "Industrial Power Distribution and Illumination"
chapter: 2
key: "semester_6/industrial_power_distribution_and_illumination:earth-resistance-lightning-arrester"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/earth-resistance-lightning-arrester
site: BCT Engineering
---

# Earth resistance factors and lightning arrester earthing

Industrial Power Distribution and Illumination · Chapter 2: Earthing

- Asked in: 2079 Chaitra, 8 marks
- Video: [Basics of Lightning Protection and Earthing/Grounding | IEC 62305](https://www.youtube.com/watch?v=obkUNBH1xnY) — Axis Electricals
- 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-lightning-arrester
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-2

## Question

What are the factors affecting the earth resistance? Explain briefly the earthing process for lightning arrester.

## Short answer

Soil type, moisture, dissolved salts, temperature, electrode size/depth, number/spacing, and contact condition affect earth resistance. For lightning arresters, a short straight copper/GI earth lead connects to a separate low-resistance (≈1–10 Ω) earth pit near the arrester, bonded to the station earth grid to equalize potential during surges.

## Answer

#### Factors affecting earth resistance
1. **Soil type:** clay and loam have low resistivity; sand, gravel and rock have high resistivity.
2. **Moisture content:** more moisture means much lower resistivity. Dry soil can have very high resistance, so resistance is worst in the dry season.
3. **Dissolved salts:** salts in the soil water make it conductive. Salt, charcoal or bentonite added around the electrode lower resistance.
4. **Temperature:** resistivity rises as temperature falls, very sharply below freezing.
5. **Electrode size and shape:** larger plates and longer rods have lower resistance; length is more effective than diameter.
6. **Depth of electrode:** deeper electrodes reach moist and stable soil.
7. **Number and spacing of electrodes:** parallel electrodes reduce resistance if spaced well apart (≥ about twice their length).
8. **Contact with soil and condition of electrode:** poor compaction, corrosion and loose joints raise resistance.

#### Earthing of a lightning arrester
A lightning arrester (surge arrester) must send a very high, very fast surge current (several kA) to earth with the smallest possible voltage rise. Its earthing is therefore done as follows:

```text
 Line ──┬───────────── to transformer
        |
      [LA]
        |  short, straight earth lead
        |  (copper / GI strip)
        └───┐
  ground ───┼──────────────
           [E] separate LA earth pit
            |   (pipe or rod, low R)
            └── bonded to station earth
                  grid / transformer body
```

1. **Short and straight earth lead:** the lead from the arrester's earth terminal to the electrode is as short and straight as possible, without sharp bends or loops. The inductance of a long lead adds an L·di/dt voltage to the arrester's residual voltage and reduces protection.
2. **Separate earth electrode:** each arrester (or set of three) has its own earth pit (pipe/rod electrode) near the arrester, separate from the transformer neutral and body earth pits. Some utilities provide two LA earth pits at a distribution transformer.
3. **Low earth resistance:** the LA earth resistance is kept low (in the order of 1–10 Ω, lower in substations), using chemical treatment or several electrodes where needed.
4. **Interconnection (bonding):** the arrester earth is bonded to the transformer tank and the station earth grid. This keeps the equipment and the arrester at the same potential during a surge, so the insulation sees only the arrester's residual voltage.
5. **Conductor size:** copper or GI strip/wire of adequate cross-section (for example 25 × 3 mm GI strip or 8 SWG GI wire, as per practice) to carry the surge and fault current, protected from mechanical damage.
6. **Placement:** the arrester is mounted as close as possible to the equipment it protects (transformer bushings).
7. **Testing:** the earth resistance of LA pits is checked periodically, especially before the monsoon (lightning season).

## Check yourself

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

1. Why is earth resistance usually highest in the dry season?
   A. Dry soil has very high resistivity because it has less moisture
   B. The electrode grows longer when dry
   C. Salts in the soil increase
   D. Temperature drops sharply
   Answer: A. Resistivity falls sharply as moisture rises, so dry soil gives high earth resistance. (Key point: Moisture effect)

2. Why must a lightning arrester's earth lead be short and straight?
   A. To save copper only
   B. To make the lead look neat
   C. The lead's inductance adds L*di/dt voltage to the residual voltage, reducing protection
   D. A long lead would lower the arrester voltage rating
   Answer: C. A fast surge current in a long or bent lead creates an inductive voltage drop that adds to the voltage across the protected equipment. (Key point: Short straight earth lead)

3. Why is the arrester earth bonded to the transformer tank and station earth grid?
   A. So the equipment and arrester stay at the same potential during a surge
   B. To raise the earth resistance
   C. To avoid the need for any earth pit
   D. To disconnect the transformer during a surge
   Answer: A. Common potential means the insulation sees only the arrester's residual voltage. (Key point: Bonding of arrester earth)

4. How does soil temperature affect earth resistance?
   A. Lower temperature lowers resistivity
   B. Resistivity rises as temperature falls, sharply below freezing
   C. Temperature has no effect
   D. Only high temperature raises resistivity
   Answer: B. Cold soil has higher resistivity, and it rises very sharply when soil water freezes. (Key point: Temperature effect)

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