What is NGR (Neutral Grounding Resistor)?
An NGR (Neutral Grounding Resistor) is used in electrical power systems to limit the current that flows during a single line-to-ground fault.
Instead of allowing a very high fault current that can damage transformers, generators, motors, switchgear, and cables, the resistor safely restricts the current to a predetermined value.
This improves equipment protection, reduces arc flash hazards, and allows protective relays to detect and isolate faults effectively.
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How Does an NGR Work?
The NGR is connected between the transformer or generator neutral point and earth. During normal operation, almost no current flows through it.
When a phase conductor accidentally touches ground, the NGR provides a controlled path for the fault current, preventing excessive electrical stress on the system.
Advantages of using an NGR
- Limits ground fault current to a safe level.
- Protects transformers, generators, motors, and cables from damage.
- Reduces arc flash energy and improves personnel safety.
- Minimizes voltage stress on healthy phases.
- Enables reliable operation of earth fault protection relays.
- Reduces repair costs and improves system reliability.
Applications of NGR
NGRs are commonly installed in industrial plants, power generation stations, substations, mining operations, and petrochemical facilities, particularly in medium-voltage systems such as 3.3 kV, 6.6 kV, 11 kV and 33 kV networks where controlled neutral grounding is required.