- 2079 Chaitra · 6 marks
Grounded versus ungrounded systems
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.
Compare grounded and ungrounded power system.
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 ( times) |
| Arcing-ground overvoltages | Suppressed | Can reach 5–6 pu, damaging insulation |
| Earth-fault current | Large enough to trip relays | Small capacitive current () |
| 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
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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