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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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When earth fault occurs, healthy phase voltage increased to
times
times
1.73 times
times
times
Quick Summary: In an ungrounded or isolated neutral system, when a single line-to-ground (SLG) fault occurs on one phase, the voltage of the healthy phases with respect to the ground increases from phase voltage ($V_{ph}$) to line voltage ($V_L$). Since the line voltage is $\sqrt{3}$ times the phase voltage, the potential of the healthy phases relative to ground rises by approximately 1.732 times.
In an ungrounded or isolated neutral system, when a single line-to-ground (SLG) fault occurs on one phase, the voltage of the healthy phases with respect to the ground increases from phase voltage (Vph) to line voltage (VL). Since the line voltage is 3 times the phase voltage, the potential of the healthy phases relative to ground rises by approximately 1.732 times.
VL=3×Vph — Relationship between line and phase voltage
Vhealthy=3×Vph — Healthy phase voltage under SLG fault in ungrounded system
During normal operation, the voltage of each phase relative to the ground is Vph=3VLine. When one phase is shorted to the ground, the fault phase potential becomes 0V relative to ground. Because the source neutral remains floating, the system neutral shifts, forcing the healthy phases to maintain their potential difference relative to each other (which is VL). Consequently, the healthy phase voltage vectors relative to ground rotate such that their magnitude equals the line-to-line voltage.
The 1.73 times increase is specific to isolated or ungrounded neutral systems.
The fault phase voltage drops to nearly zero potential relative to the earth.
This phenomenon causes increased stress on the insulation of the healthy phases.
System operators must use proper surge arresters or grounding transformers to mitigate this rise.
Allows continued operation of the system during a single earth fault
Provides high reliability for sensitive industrial loads
Increases insulation stress on healthy phases
Potential for arcing ground faults if not cleared
Isolated neutral distribution systems
Delta-connected medium voltage networks
In effectively grounded systems, the voltage rise on healthy phases is significantly lower (typically 0.8 to 1.4 p.u. depending on the grounding coefficient).
Option 2 times, 3 times, and 1.5 times are incorrect as they do not represent the standard trigonometric relationship derived from the phasor geometry of a 3-phase system.
C is correct — The healthy phase voltage rises to 1.73 times its normal value because it effectively takes on the line-to-line potential relative to the grounded faulted phase.
Always remember that in an isolated neutral system, the vector sum of voltages is zero. If one phase is grounded, the remaining two form a line-to-line triangle with the ground, making the voltage relative to ground equal to the line voltage.