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A single line to ground fault occurs on a three-phase isolated neutral system with a line to neutral voltage of V kV. The potentials on the healthy phases rise to a value equal to
2тАЛV kV
3тАЛV kV
3V kV
(3тАЛ1тАЛ)V kV
3тАЛV kV
In a three-phase isolated neutral system (ungrounded system), if one phase experiences a line-to-ground fault, the potential of the faulted phase drops to the potential of the ground (0 V). Consequently, the voltage of the two healthy phases relative to the ground rises from the line-to-neutral voltage (VLNтАЛ=V) to the line-to-line voltage, which is equal to 3тАЛV.
In a three-phase isolated neutral system (ungrounded system), if one phase experiences a line-to-ground fault, the potential of the faulted phase drops to the potential of the ground (0 V). Consequently, the voltage of the two healthy phases relative to the ground rises from the line-to-neutral voltage (VLNтАЛ=V) to the line-to-line voltage, which is equal to 3тАЛV.
VhealthyтАЛ=3тАЛVLNтАЛ тАФ Healthy phase voltage during a single line-to-ground fault in an isolated system
Under normal operating conditions in a star-connected system, the phase voltage to ground is V. When a ground fault occurs on phase R, its potential becomes 0. Since the system is isolated, the voltage across the healthy phases (Y and B) now effectively behaves as the line-to-line voltage of the system. In a balanced system, the line-to-line voltage is 3тАЛ times the line-to-neutral voltage, causing the healthy phases to sustain 3тАЛV relative to ground.
The fault current in an isolated neutral system is primarily capacitive due to the line-to-ground capacitance of the healthy phases.
The potential rise on healthy phases imposes higher voltage stress on the insulation of healthy lines.
This phenomenon is a primary reason why ungrounded systems are susceptible to arcing ground faults.
System can continue operation during a single line-to-ground fault.
Lower fault currents compared to solidly grounded systems.
Increased voltage stress on healthy phases during faults.
Risk of transient overvoltages due to arcing grounds.
Used in critical industrial processes where continuous operation is required.
Common in low-voltage distribution systems where safety against touch potential is prioritized.
In a solidly grounded system, the healthy phase voltage remains at V during an L-G fault, as the neutral is clamped to ground potential.
Option C (3V) is incorrect as it typically relates to zero-sequence calculations in specific transformer configurations, not healthy phase voltage.
B is correct тАФ In an isolated neutral system, a single line-to-ground fault causes the healthy phases to rise to the line-to-line voltage, which is 3тАЛ times the original phase-to-neutral voltage.
Remember that in isolated systems, the capacitive current leads the voltage by 90┬░, and the fault current is 3├ЧICтАЛ, where ICтАЛ is the normal charging current per phase.