---
title: "Grounded versus ungrounded systems"
subject: "Power Plant Design"
chapter: 4
key: "semester_8/power_plant_design:grounded-vs-ungrounded"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/grounded-vs-ungrounded
site: BCT Engineering
---

# Grounded versus ungrounded systems

Power Plant Design · Chapter 4: Electric System Design of a Power Plant

- Asked in: 2079 Chaitra, 6 marks
- Video: [Grounded and ungrounded system](https://www.youtube.com/watch?v=iEebdqSHS1A) — Vinay Kolur
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/grounded-vs-ungrounded
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4

## Question

Compare grounded and ungrounded power system.

## Short answer

A grounded system holds neutral near earth, healthy phases stay near phase voltage, suppresses arcing overvoltages, provides selective relaying with large fault current, but trips on first fault. An ungrounded system has no intentional earth connection, healthy phases rise to line voltage, suffers transient overvoltages up to6 pu, fault detection difficult, but can continue with a single line-to-ground fault.

## Answer

A **grounded system** has its neutral connected to earth (solidly or through an impedance). An **ungrounded (isolated neutral) system** has no intended connection to earth; it is only coupled to earth through the line-to-ground capacitances.

#### Comparison

| Point | Grounded system | Ungrounded system |
|---|---|---|
| Neutral potential | Held near earth | Floats; can shift |
| Healthy-phase voltage during L-G fault | Near phase voltage (solid) | Rises to line voltage ($\sqrt{3}$ times) |
| Arcing-ground overvoltages | Suppressed | Can reach 5–6 pu, damaging insulation |
| Earth-fault current | Large enough to trip relays | Small capacitive current ($3I_{C0}$) |
| Fault detection | Easy, selective earth-fault relays | Difficult; only alarm via voltage relays |
| Insulation level | Graded insulation; cheaper equipment | Full line-voltage insulation needed |
| Continuity of supply | Faulted section trips at once | Can run with one earth fault for a while |
| Safety | Better; touch voltage cleared fast | Second fault can be dangerous |
| Lightning arresters | Lower-rated arresters can be used | Higher rating needed |
| Interference | Earth-fault current may disturb telecom | Little |
| Typical use | Modern systems, generators, above 33 kV | Old small systems, some industrial LV |

#### Capacitive current in an ungrounded system

When one phase is earthed, the other two line capacitances charge to line voltage. The fault current is

$$
I_f = 3\,\omega C_0\,V_{ph}
$$

At fault currents above about 4–5 A this capacitive current causes **arcing grounds**: the arc repeatedly strikes and extinguishes, building up voltage on the healthy phases.

#### Conclusion for practice

Almost all modern power systems are grounded. The method depends on voltage level: solid for LV and EHV, resistance/reactance for MV, and high-resistance for unit-connected generators.

## Check yourself

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

1. In an ungrounded system with a line-to-ground fault, what happens to healthy-phase voltage?
   A. Stays at zero
   B. Unchanged
   C. Rises to line voltage (sqrt3 times)
   D. Falls
   Answer: C. The neutral shifts, so healthy phases see line voltage to earth. (Key point: Healthy-phase voltage)

2. What is the earth-fault current in an ungrounded system?
   A. Small capacitive current 3 w C0 Vph
   B. Zero always
   C. Equal to load current
   D. Very large
   Answer: A. The current returns through line-to-ground capacitances. (Key point: Fault current)

3. Which system allows graded, cheaper insulation?
   A. Neither
   B. Grounded
   C. Ungrounded
   D. Both equally
   Answer: B. In grounded systems phase-to-earth voltage stays near phase voltage. (Key point: Insulation)

4. What advantage does an ungrounded system have?
   A. Easier fault detection
   B. Can keep running with one earth fault for a while
   C. No overvoltages
   D. Lower arrester rating
   Answer: B. Small fault current permits temporary operation, though a second fault is dangerous. (Key point: Continuity)

## Related in Power Plant Design

- [Advantages of solidly grounded generators](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asolidly-grounded-advantages) — Power Plant Design, chapter 4
- [Thermal versus nuclear power plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Athermal-vs-nuclear) — Power Plant Design, chapter 1, asked 2×
- [Tidal versus wind power plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Atidal-vs-wind) — Power Plant Design, chapter 1
- [Concept of power system security](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apower-system-security-concept) — Power Plant Design, chapter 2, asked 3×
- [Power system planning and grid integration](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aplanning-deregulated-grid-integration) — Power Plant Design, chapter 2
- [Power system expansion planning for a utility](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aexpansion-planning-national-utility) — Power Plant Design, chapter 2

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