Neutral earthing is the practice of connecting the neutral point of a power system (like a transformer or generator) to the earth ground.
Understanding Neutral Earthing
In an electrical power system, the neutral point is typically the common connection point of the star-connected windings of a transformer or generator. Connecting this point to the earth provides a path for fault currents, primarily ground-fault currents, to return to their source. This connection is crucial for system safety and performance.
Why is Neutral Earthing Important?
The primary reasons for neutral earthing are related to fault management and safety.
- Limiting Fault Currents: Neutral earthing helps to control the magnitude of fault currents that occur when a live wire accidentally touches the ground or a grounded object.
- As stated in the reference, "Neutral earthing via an impedance is employed when it is desirable to limit the magnitude of fault current to manageable levels."
- High fault currents can cause severe damage. The reference notes that "High levels of fault current are undesirable as they can lead to irreversible damage to equipment and systems." By earthing the neutral, especially through a controlled impedance, the potential damage during a fault is significantly reduced.
- Ensuring Safety: It helps protect personnel from electric shock by ensuring that faulty equipment surfaces are quickly brought to earth potential, triggering protective devices (like circuit breakers) to isolate the fault.
- System Stability: It can improve system stability and allow for selective tripping of protective relays, ensuring that only the faulty section of the system is isolated, minimizing disruption to other parts.
- Voltage Control: It helps control transient overvoltages that can occur during faults or switching operations.
Common Methods of Neutral Earthing
There are several methods to connect the neutral point to the earth, each offering different advantages and disadvantages regarding fault current levels, cost, and system performance.
Here are a few common types:
- Solid Earthing: The neutral is directly connected to earth with no intentional impedance.
- Pros: Simple, effective for quick fault clearing in low-voltage systems.
- Cons: Results in very high fault currents, potentially causing significant damage and generating large electromagnetic interference.
- Resistance Earthing: An impedance (resistor) is connected between the neutral and earth.
- Purpose (as per reference): Limits fault current to a predefined level, reducing damage compared to solid earthing.
- Types: Low Resistance Earthing (LRE) and High Resistance Earthing (HRE), offering different levels of fault current limitation.
- Reactance Earthing: An impedance (inductor) is connected between the neutral and earth.
- Purpose: Limits fault current while potentially offering benefits for transient overvoltage control.
- Unearthed Neutral: The neutral is not intentionally connected to earth.
- Pros: Allows continued operation during the first ground fault.
- Cons: Difficult to locate the fault, high transient overvoltages can occur, and a second fault can lead to very high fault currents.
Comparison of Selected Earthing Methods
| Feature | Solid Earthing | Resistance Earthing | Unearthed Neutral |
|---|---|---|---|
| Fault Current | Very High | Limited (controlled) | Low (first fault), High (second fault) |
| Transient O/V | Low | Moderate | High |
| Fault Location | Easy (high current) | Varies (easier with HRE) | Difficult |
| Equipment Stress | High | Low | High (due to O/V) |
| Continuity | Low (quick tripping) | Higher (especially HRE) | High (first fault) |
Neutral earthing is a fundamental aspect of electrical power system design, critical for ensuring safety, controlling fault energy, and maintaining reliable operation. The specific method chosen depends on the system voltage, size, and operational requirements.
