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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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During faults in a power system severe over voltages are produced with neutral
Solidly earthed
Isolated
Earthed through an inductive coil
Earthed through low resistance
Isolated
In an isolated (ungrounded) neutral system, an earth fault causes the voltage of the healthy phases to rise to the line-to-line voltage level relative to the ground. This phenomenon, known as arcing ground, leads to severe overvoltages due to the repeated charging and discharging of the line capacitance through the fault arc.
In an isolated (ungrounded) neutral system, an earth fault causes the voltage of the healthy phases to rise to the line-to-line voltage level relative to the ground. This phenomenon, known as arcing ground, leads to severe overvoltages due to the repeated charging and discharging of the line capacitance through the fault arc.
Vfault=3Vph — Phase voltage rise during SLG fault
Ic=3ωCVph — Total capacitive fault current in isolated system
When a single-line-to-ground fault occurs in an isolated system, the capacitance of the healthy phases to ground is charged to the full line-to-line voltage VLL instead of the phase voltage Vph. The fault current is purely capacitive and leads the voltage by 90°. As the arc extinguishes and restrikes, the potential oscillates, causing high-frequency transients that can reach several times the rated voltage, stressing the insulation.
Isolated neutral systems suffer from 'arcing ground' transients.
The fault current is exclusively capacitive in nature.
Healthy phase voltage rise is 1.732 times the normal phase voltage.
Requires high insulation levels compared to grounded systems.
System remains operational during a single-line-to-ground fault.
No large fault currents need to be interrupted by breakers immediately.
High magnitude of overvoltages during faults.
Potential for destructive arcing and insulation failure.
Difficult to detect and locate high-impedance earth faults.
Small distribution networks where continuity of service is critical.
Industrial systems with limited spatial extent.
| Feature | Fixed Potential | Floating Potential |
|---|---|---|
Neutral Status | Solidly Earthed | Isolated (Ungrounded) |
Arcing grounds were a major design concern in early power system evolution.
Neutral grounding (e.g., Petersen Coil) is often used to mitigate these effects by neutralizing the capacitive current.
B is correct — Isolated neutral systems are prone to high-frequency voltage transients and arcing grounds during earth faults.
If an MCQ mentions 'Arcing Grounds', always associate it with ungrounded/isolated systems; if it mentions 'Resonant Grounding', associate it with Petersen Coils.