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Solid grounding is addopted for voltages below
100 V
200 V
400 V
660 V
660 V
Solid grounding (or solidly earthed neutral) is typically preferred for low-voltage power distribution systems, specifically those below 660 V, to ensure safety and reliable operation of protective devices. By keeping the neutral at zero potential, the system prevents dangerous voltage build-up relative to the ground and allows fault currents to be high enough for circuit breakers to trip immediately.
Solid grounding (or solidly earthed neutral) is typically preferred for low-voltage power distribution systems, specifically those below 660 V, to ensure safety and reliable operation of protective devices. By keeping the neutral at zero potential, the system prevents dangerous voltage build-up relative to the ground and allows fault currents to be high enough for circuit breakers to trip immediately.
IfтАЛ=ZsтАЛVphтАЛтАЛ тАФ represents the fault current in a solidly grounded system where ZsтАЛ is the low source impedance
VlineтАЛ=3тАЛVphaseтАЛ тАФ relates line and phase voltages in the star-connected system typically employed
In a solidly grounded system, the neutral point of the transformer secondary is connected directly to the earth. During a line-to-ground fault, this creates a low-impedance path for fault current (IfтАЛ=ZsтАЛVphтАЛтАЛ), which triggers overcurrent protection devices (fuses or circuit breakers) to isolate the faulty section rapidly. This minimizes fire hazards and prevents insulation damage from overvoltage conditions.
Solid grounding is the simplest form of earthing.
It is standard for distribution systems operating at 415 V or 660 V.
The primary goal is the rapid clearing of earth faults.
Used extensively in industrial and domestic LV installations.
Simple and economical implementation.
Effective in limiting overvoltages during faults.
Provides a clear path for fault current detection.
Higher fault current levels can cause damage to equipment.
No continuity of supply during an earth fault (system trips immediately).
Requires robust overcurrent protection.
Domestic power distribution systems.
LV industrial motors and lighting circuits.
Low-voltage power transformers.
In systems above 660 V, impedance grounding (reactance or resistance) is often preferred to limit fault currents.
The 660 V threshold follows traditional international standards (such as IEC) for low-voltage equipment.
D is correct тАФ Solid grounding is generally standard for low-voltage distribution systems operating at or below 660 V.
Always remember that solid grounding is for low voltage (LV) to ensure safety, whereas high voltage (HV) systems use resistance/reactance grounding to control fault current severity.