A Neutral Earthing Resistor (NER) is a key part of a power system. It connects between the neutral point of a transformer or generator and the ground.
Its main role is to accurately limit ground fault current, which helps avoid equipment damage or injury.
Compared to direct grounding, an NER uses resistance to create a “soft grounding” effect. This balances safety and continuous operation.
Four Main Functions
Current Limiting Protection: It limits single-phase ground fault current to tens or hundreds of amps. This reduces arc damage.
Protection Triggering: It gives enough current to make the relay work. This helps isolate the fault accurately.
Overvoltage Suppression: The resistor helps reduce resonance overvoltage (by more than 50%).
Equipment Protection: It lowers heat and stress at the fault point. This helps equipment last longer.
Guardian of System Safety
NER is very useful in places needing reliable power, such as factories and data centers:
Personal Safety: It keeps touch voltage below 50V (meets IEC 60479 standard).
Equipment Safety: It stops kilovolt-level overvoltage from breaking insulation.
System Stability: It keeps non-fault phase voltage change within 10%. This prevents short circuits.
Uninterrupted Power: It works with protection systems to keep over 95% of non-fault areas powered.
How It Works
If a ground fault happens on Phase C, the current path is:
Fault point → Ground → NER → Transformer neutral point → Phase C winding
The NER resistor R limits the fault current. The formula is:
I_f = V_ln / (√3 × R)
If R = 10Ω in a 6kV system, the fault current drops from over 1000A to 346A. This makes the system safe and controllable.
Technical Parameters and Design Selection
Key Parameter Table
Parameter
Definition
Typical Value
Rated Resistance (Ω)
Main current-limiting part
5Ω – 100Ω
Rated Current (A)
Max short-term current
400A (10 seconds)
I²t Capacity (kA²s)
Short-term energy withstand
400kA²s (10kA/0.4s)
Insulation Level
Voltage withstand to ground
10kV – 36kV
Golden Rule for Resistance Calculation
Main formula (IEEE Std 32): R = V_ln / (√3 × I_f)
Design Tips:
I_f must meet these conditions:
Larger than system capacitive current I_c (to avoid resonance)
Smaller than equipment limit (e.g. cable thermal limit)
Higher than protection relay current (usually ≥10A)
Thermal Stability Design
Energy Absorption: I²t = I_f² × t (t = protection time)
Example: I_f = 500A, t = 2s → energy = 500 kA²s
Temperature Control: Stainless steel resistor surface ≤350°C to prevent damage
Cooling Methods:
Natural Cooling: For indoor use when I_f < 800A
Forced Air Cooling: Fans increase cooling by 40%, good for hot areas like offshore platforms
Application Scenarios and System Integration
Grounding System Compatibility
System Type
NER Need
Typical Setup
IT System (no grounding)
★★★★☆
High-resistance NER (I_f < 10A)
LRG System (low resistance)
★★★★★
Medium/low resistance NER (50–400A)
Arc Suppression Coil + Resistor
★★★☆☆
Support NER (100–200A)
Applications Across Voltage Levels
Medium Voltage Main Use: 6kV–35kV systems are 75% of global NER usage (McKinsey data), used in chemical plants and mines
Low Voltage Safety: 480V UPS systems in data centers use NER to cut failure rate by 92%
High Voltage Custom Use: Offshore wind power (33kV) uses NER with high protection level (IP56)
Industry Problem-Solving Cases
Oil Platform Explosion Protection: Limit fault energy under 0.25mJ to avoid gas ignition
Subway Power System: With directional zero-sequence protection, fault location time improves to 200ms
Installation and Maintenance Guide
Four Key Installation Points
Ventilation Layout: Leave ≥800mm space at resistor cabinet sides for airflow
Low-Resistance Grounding: Use copper wire ≥120mm², ground resistance <1Ω
Shockproof Design: Offshore installation must pass IEC 60068-3 vibration test
Safety Protection: Protection level ≥IP23 (outdoor) or IP44 (dusty areas)
Maintenance Checklist
Monthly Quick Check:
Use infrared camera to check temperature (hotspot <80°C)
Listen to fan noise (≤65dB)
Yearly Deep Maintenance:
Measure resistance value (error <±5%)
Check terminal torque (per IEC 62454)
Test insulation resistance (>100MΩ @2500V)
Troubleshooting and Lifecycle Management
Diagnostic Toolbox
Current Analysis: Compare actual I_f from fault record to design value
Hotspot Detection: Use infrared to find loose connections (warning if temp difference >15°C)
Material Aging Check: Replace resistor if resistance increases >10% per year
Frequently Asked Questions
What is neutral earthing resistor?
The Neutral Grounding Resistor (NER) connects the neutral point of a transformer or generator to ground via a series resistor.
What is a neutral earthing resistor used for?
Neutral earthing resistors (NERs) serve to ground the neutral points of (medium) voltage power networks. Their purpose is to limit single-phase earth fault currents, mitigate overvoltages, and enhance power system stability.
Why do we need NER?
By limiting fault currents, reducing overvoltages, and improving system stability, the NER safeguards power equipment operations, minimizes power outages, and enhances power supply reliability.
