Neutral Grounding Resistor (NGR) Sizing – Step-by-Step Practical Guide

Writer： admin Time：2026-06-24 18:33:46 Browse：93℃

What is a Neutral Grounding Resistor (NGR)?

An NGR is connected between the transformer or generator neutral point and earth to:
✅ Limit earth fault current
✅ Control transient over-voltages
✅ Enable reliable earth fault detection
✅ Reduce mechanical & thermal stress on equipment

Inputs Required for NGR Sizing 
Before starting calculations, ensure the following inputs are available:
➡️ Single Line Diagram (SLD)
➡️ Transformer / Generator Datasheet
➡️ Earthing Philosophy Document
▪️ Determines grounding type
▪️ Defines allowable earth fault current
➡️ Protection Philosophy / Relay Setting Philosophy
▪️ Earth fault current must be detectable by relays
➡️ Short Circuit Study Report
NGR sizing must always align with protection and earthing philosophy.

Selection of Resistance & Limited Earth Fault Current
The NGR resistance value shall be selected such that:
✔️ Earth fault current is high enough for selective and reliable relay operation
✔️ Earth fault current is low enough to minimize damage during fault conditions
 Important Design Thumb Rule:
The limited earth fault current shall not exceed 50% of transformer/generator full load current

Step-by-Step Approach for NGR Sizing Calculations 

Step-1: Calculate Phase-to-Neutral Voltage (V_ph)
V_ph=V_LL / (√3)
 V_LL = Line-to-Line System Voltage

Step-2: Decide Earth Fault Current (Ief)
✔️ Based on relay sensitivity
✔️ Coordinated with protection philosophy
 Typical values range from 100 A to 1000 A depending on system voltage and protection scheme
(Example: 400 A)

Step-3: Calculate NGR Resistance (R_NGR)
Example:
System Voltage = 11 kV
Selected Earth Fault Current = 400 A
V_ph=11000/√3=6350 V
R_ngr=6350/400= 15.875 ohm

Step-4: Thermal Rating of NGR 
NGR must withstand thermal stress during earth fault duration.

➡️ Power Dissipation (P)
P (Power Dissipation)=I ²×R
Energy=P×t
Select NGR time rating (e.g. 10 s / 30 s / 1 min) based on:
✔️ Relay operating time
✔️ Backup protection clearing time

Key Design Takeaways 
✔️ NGR sizing is not only a calculation exercise, but a protection-driven design decision
✔️ Always align earthing philosophy + relay sensitivity + thermal limits
✔️ Incorrect NGR selection can cause:
❌ Relay mal-operation
❌ Excessive equipment damage
❌ Safety hazards

Good engineering = Balanced fault current + Reliable protection + Thermal safety