---
title: "Neutral earthing and insulation level"
subject: "Transmission and Distribution System Design"
chapter: 3
key: "semester_8/transmission_and_distribution_system_design:neutral-earthing-insulation-level"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/neutral-earthing-insulation-level
site: BCT Engineering
---

# Neutral earthing and insulation level

Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design

- Asked in: 2077 Chaitra, 10 marks; 2073 Magh, 6 marks; 2070 Bhadra, 8 marks (asked 3 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/neutral-earthing-insulation-level
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-3

## Question

Discuss the role of system (neutral) earthing in the selection of overhead line insulation level.

## Short answer

Effective (solid) earthing limits healthy-phase voltage during earth faults to about 80% of line voltage, allowing lower-rated surge arresters and a reduced BIL. This permits fewer insulator discs, smaller clearances, and cost savings of 15–20% for lines 132 kV and above.

## Answer

**System (neutral) earthing** is the connection of the star point of generators/transformers to earth, either solidly or through a resistance/reactance. It decides how high the voltage of the healthy phases rises during an earth fault, and so it decides the rating of surge arresters and the insulation level (BIL) of the line and equipment.

#### Earthing coefficient
$$
C_e = \frac{\text{highest rms phase-to-earth voltage of healthy phase during earth fault}}{\text{rms line-to-line voltage}}
$$
- **Isolated (ungrounded) neutral:** healthy phases rise to the full line voltage, $C_e \approx 1.0$ (100%). Arcing grounds can produce transients as high as 5–6 pu.
- **Effectively (solidly) earthed:** $X_0/X_1 \le 3$ and $R_0/X_1 \le 1$; the healthy-phase voltage rises to only about 0.8 of the line voltage, $C_e \le 0.8$ (80%).
- **Resistance/reactance earthed:** in between.

#### Effect on surge arrester rating
Arrester rated voltage $= C_e \times V_{max}$ (times a small margin):
- 220 kV, $V_{max} = 245$ kV: effectively earthed → $0.8 \times 245 = 196$ kV arrester (≈"80% arrester"); isolated neutral → 245 kV arrester ("100% arrester").
- A lower-rated arrester has a lower protective (residual) level.

#### Effect on insulation level (BIL)
- BIL $= $ protection ratio × arrester protective level (protection ratio about 1.2–1.4).
- With effective earthing the protective level is about 20% lower, so a **reduced BIL** can be chosen. Example for 220 kV: full insulation 1050 kV, reduced 950 or 900 kV (850 kV in some practice).
- Power-frequency withstand voltages and switching over-voltages (which also depend on the phase-to-earth voltage) are lower too.

#### Effect on overhead line insulation
- Fewer discs per string, shorter cross-arms, smaller clearances, shorter towers and a narrower right-of-way. The saving can be 15–20% of insulation cost at 132 kV and above.
- With an isolated neutral, the insulation must be designed for full line voltage to earth and for arcing-ground surges; this is acceptable only at low voltages (≤ 33 kV), where insulation is cheap.

| Earthing | $C_e$ | Arrester | Insulation level |
|---|---|---|---|
| Isolated | ≈1.0 | 100% | Full |
| Resistance/reactance | 0.8–1.0 | 85–100% | Full |
| Effective (solid) | ≤0.8 | 80% | Reduced |

#### Practice
- 132 kV and above: neutrals are effectively (solidly) earthed so that reduced insulation can be used; this is standard in Nepal and India.
- 11 kV and 33 kV distribution: often resistance or solid earthing; insulation is cheap, so full insulation is common.
- Effective earthing also gives fast, selective earth-fault relaying and avoids arcing grounds, but it causes high earth-fault current and possible interference with communication lines.

Thus neutral earthing directly lowers or raises the voltage stress on insulation and is a key input to insulation coordination and to the number of discs chosen for an overhead line.

## Check yourself

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

1. What is the earthing coefficient Ce for an effectively earthed system?
   A. 1.0 or more
   B. About 1.5
   C. Less than or equal to 0.8
   D. Zero
   Answer: C. With X0/X1 <= 3 and R0/X1 <= 1, healthy phases rise to at most 80% of line voltage. (Key point: Earthing coefficient)

2. For a 220 kV system (Vmax 245 kV), what arrester rating applies if effectively earthed?
   A. 245 kV
   B. About 196 kV (0.8 x 245)
   C. 110 kV
   D. 300 kV
   Answer: B. Arrester rated voltage = Ce x Vmax = 0.8 x 245 = 196 kV (80% arrester). (Key point: Arrester rating)

3. How does effective earthing allow a reduced BIL?
   A. A lower rated arrester has a lower protective level, and BIL = protection ratio x protective level
   B. It raises the arrester residual voltage
   C. It increases the number of discs
   D. It removes surges
   Answer: A. Lower protective level (about 20% less) gives a lower BIL, e.g. 900 kV instead of 1050 kV at 220 kV. (Key point: BIL effect)

4. Why is isolated neutral acceptable only at low voltage (up to 33 kV)?
   A. Insulation must be for full line voltage and arcing-ground surges, which is only cheap at low voltage
   B. Isolated neutral gives lower BIL
   C. It gives high fault current
   D. It needs no insulation
   Answer: A. Full insulation and arcing-ground transients are affordable only where insulation is cheap. (Key point: Isolated neutral)

## Related in Transmission and Distribution System Design

- [T/F: impulse withstand and neutral earthing](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-impulse-withstand-neutral-earthing) — Transmission and Distribution System Design, chapter 3
- [Criteria for distribution line route selection](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Adistribution-route-selection-criteria) — Transmission and Distribution System Design, chapter 7, asked 3×
- [Conductor selection, 220 kV line for 200 MW](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aconductor-selection-220kv-200mw-120km) — Transmission and Distribution System Design, chapter 4
- [Single vs double earth wire selection](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Asingle-double-earth-wire-criteria) — Transmission and Distribution System Design, chapter 3
- [Underground vs overhead distribution systems](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aunderground-vs-overhead-distribution) — Transmission and Distribution System Design, chapter 5, asked 4×

## Related in other subjects

- [Fault at load end of overhead line](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Afault-load-end-33kv-line) — Power System Analysis II, chapter 3
- [Line currents with isolated and grounded neutral](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Astar-impedance-unbalanced-voltages-neutral) — Power System Analysis II, chapter 4
- [Voltage rise at cable–overhead line junction](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asurge-cable-overhead-junction) — High Voltage Engineering, chapter 3, asked 2×
- [Star system with three loads, neutral current](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Astar-three-loads-400v-neutral-current) — Basic Electrical Engineering, chapter 8