Isolated neutral transmission system is not recommended as the

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Option D: All of above

Explanation & Concepts:
An isolated (ungrounded) neutral system is prone to severe voltage oscillations and arcing ground faults. In such a configuration, a single line-to-ground fault causes the potential of the healthy phases to rise to full line-to-line voltage with respect to earth, increasing the stress on the system insulation significantly.

Working Principle: In an isolated system, the neutral point is held at ground potential only by the system's inherent capacitance to earth. When one phase faults to ground, the healthy phases' voltages relative to ground shift from the phase voltage ($V_{ph}$) to the full line voltage ($V_L$), resulting in a factor of $\sqrt{3}$ increase. This constant over-voltage stresses the insulation and can cause intermittent arcing faults, leading to catastrophic equipment failure.

Key Formulas: $V_{L-G (fault)} = \sqrt{3} 	imes V_{L-G (normal)}$ — Voltage stress on healthy phases during an earth fault; $I_c = j\omega C V_{ph}$ — Capacitive charging current through the faulted phase

Key Points: Isolated systems are difficult to monitor for first earth faults.; Arcing grounds occur due to the repeated charging and discharging of line capacitance.; Insulation failure often progresses from line-to-ground to line-to-line faults.; Protective relaying is significantly complicated by the lack of a ground return path for fault current.

Advantages: Continued operation during first line-to-ground fault (if fault is minor).; Lower magnitude of initial earth fault current.

Disadvantages: High transient over-voltages.; Difficulty in locating earth faults.; Risk of insulation damage to healthy phases.

Applications: Low-voltage distribution systems in mines (with ground monitoring).; Specific industrial processes requiring high continuity.

Additional Info: Option A: Insulation is stressed because the phase-to-ground potential increases by $\sqrt{3}$ times.. Option B: Continuous over-stress degrades dielectrics, making the system prone to flashovers and inter-phase faults.. Option C: Since no path is provided for the earth fault current, protective relays often fail to detect the fault, leaving the system in a vulnerable state.

Answer: D is correct — Isolated neutral systems are avoided because they cause voltage instability, insulation stress, and hinder reliable earth fault detection.

Exam Tip: Always remember that solid grounding is preferred for high-voltage systems to maintain phase-to-ground voltage constant, while high-resistance grounding is used where supply continuity is critical.

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Option A: System insulation is over stress due to over voltages

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Option B: Insulation over stress may lead to its failure resulting in phase to phase fault

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Option C: System is not adequately protected against earth fault

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