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Which of the following neutral system will require the lightning arrester of least voltage rating
Insulated
Solidly earthed
Resistance earthed
Reactance earthed
Solidly earthed
A solidly earthed system maintains the neutral potential at ground potential (0V) under normal conditions. This ensures that the line-to-ground voltage under fault conditions does not rise significantly, allowing the lightning arrester (LA) to be rated for a lower voltage closer to the phase-to-neutral voltage (VphтАЛ), rather than the phase-to-phase voltage (VLтАЛ).
A solidly earthed system maintains the neutral potential at ground potential (0V) under normal conditions. This ensures that the line-to-ground voltage under fault conditions does not rise significantly, allowing the lightning arrester (LA) to be rated for a lower voltage closer to the phase-to-neutral voltage (VphтАЛ), rather than the phase-to-phase voltage (VLтАЛ).
VLAтАЛтЙе1.414├ЧVphтАЛ
VfaultтАЛ=VLтАЛ (for isolated neutral)
In an unearthed or high-impedance earthed system, a single line-to-ground fault causes the potential of the healthy phases to rise to the full line-to-line voltage with respect to earth. Lightning arresters must be rated to withstand this overvoltage to prevent failure. Solidly grounding the system limits this neutral shift during faults, enabling the use of arresters with lower Maximum Continuous Operating Voltage (MCOV) ratings.
Solid earthing limits the dynamic overvoltage during a line-to-ground fault.
Lightning arrester ratings are selected based on the temporary overvoltage (TOV) capability of the system.
Lower voltage ratings for LAs result in a lower protective level, providing better insulation coordination for equipment.
Isolated systems require LAs rated for full line-to-line voltage to prevent permanent damage.
Reduced cost of insulation for surge arresters
Better protection levels for connected electrical apparatus
High fault current during line-to-ground faults
Increased interference with communication circuits
High voltage (HV) transmission systems (132kV and above)
Sub-transmission networks
| Feature | High Rating Required | Low Rating Required |
|---|---|---|
Neutral State | Isolated/Unearthed | Solidly Earthed |
The lightning arrester rating is strictly tied to the TOV (Temporary Overvoltage) factor of the grounding system.
Option A (Insulated/Isolated) is the worst case for LA ratings as the neutral can shift to full phase potential.
B is correct тАФ Solidly earthed systems exhibit the least neutral displacement during ground faults, allowing for lower lightning arrester voltage ratings.
Always remember that in power systems, 'Solidly Earthed' is synonymous with minimum potential rise, which simplifies protective device selection compared to resonant or isolated systems.