Neutral Grounding Resistor (NGR) Sizing (A Practical, Protection-Oriented Engineering Guide)

Neutral Grounding Resistor (NGR) Sizing

A Practical, Protection-Oriented Engineering Guide ⚡

Neutral Grounding Resistor (NGR) selection is not just a formula-based calculation — it is a protection-driven engineering decision that directly impacts system safety, equipment life, and fault detection reliability.

---

🔹 What is a Neutral Grounding Resistor (NGR)?

An NGR is installed between the neutral point of a transformer or generator and earth to:

✅ Limit single line-to-ground fault current
✅ Control transient over-voltages
✅ Ensure reliable earth fault detection
✅ Reduce mechanical & thermal stress on windings
✅ Minimize arc flash energy during ground faults

It is widely used in medium voltage (MV) systems such as 6.6 kV, 11 kV, and 33 kV networks.

---

📑 Essential Inputs Required Before Sizing

Accurate NGR sizing requires coordination across multiple engineering studies:

➡️ Single Line Diagram (SLD)
➡️ Transformer / Generator Datasheet
➡️ Earthing Philosophy Document
➡️ Protection & Relay Setting Philosophy
➡️ Short Circuit Study Report
➡️ Fault Clearing Time (Primary + Backup Protection)

📌 NGR sizing must always align with protection coordination and earthing philosophy.

---

🔹 Selecting the Earth Fault Current

The selected earth fault current must satisfy two conditions:

✔️ High enough for sensitive and selective relay operation
✔️ Low enough to limit thermal and mechanical damage

📌 Common Industry Practice:

Typical MV system values range between:

• 100 A (sensitive systems, generators)

• 200–400 A (industrial MV systems)

• 800–1000 A (where faster detection is required)

🔎 Practical Thumb Rule:

The limited earth fault current should not exceed 50% of transformer/generator full load current.

---

🧮 Step-by-Step NGR Sizing Calculation

Let’s take a practical example:

🔹 System Voltage (VLL) = 11 kV
🔹 Selected Earth Fault Current (Ief) = 400 A

---

Step 1️⃣: Calculate Phase-to-Neutral Voltage

$$V_{ph} = \frac{V_{LL}}{\sqrt{3}}$$ $$V_{ph} = \frac{11000}{\sqrt{3}} = 6350 \, V$$

---

Step 2️⃣: Calculate NGR Resistance

$$R_{NGR} = \frac{V_{ph}}{I_{ef}}$$ $$R_{NGR} = \frac{6350}{400} = 15.875 \, \Omega$$

✔ Required NGR Resistance ≈ 15.9 Ω

---

Step 3️⃣: Determine Thermal Rating 🔥

The NGR must withstand the thermal stress during fault duration.

Power Dissipation:

$$P = I^2 \times R$$ $$P = 400^2 \times 15.875$$ $$P = 2.54 \, MW$$

Energy Dissipation:

$$Energy = P \times t$$

Where:

• $t$ = fault clearing time (e.g., 10 sec / 30 sec / 60 sec)

📌 Select NGR time rating based on:

✔ Primary relay operating time
✔ Backup relay clearing time
✔ Utility requirements

Common time ratings: 10 sec / 30 sec / 60 sec

---

🔥 Critical Design Considerations

✔ Ensure relay pickup setting < selected earth fault current
✔ Verify coordination with REF / SEF protection
✔ Consider system charging current (especially in long cable networks)
✔ Check insulation coordination
✔ Provide NGR monitoring (temperature / broken resistor detection)
✔ Ensure proper NGR enclosure IP & ventilation

---

⚠️ Risks of Incorrect NGR Selection

❌ Relay mal-operation or non-detection
❌ Excessive winding stress
❌ Insulation damage
❌ Overheating of resistor
❌ Safety hazards

---

🎯 Engineering Takeaway

NGR sizing is not just mathematics —
it is a balance between:

✔ Protection sensitivity
✔ Equipment protection
✔ Thermal capability
✔ System stability

Good engineering = Controlled fault current + Reliable detection + Thermal safety

---

💬 In your projects, what earth fault current do you typically adopt — 100 A, 400 A, or 1000 A?
What factors influence your decision most: relay sensitivity, cable length, or utility requirement?

https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg RRB JE, SSC AE/JE UPSSSC JE, SSC JE, CIVIL ENGINEERING MCQs, ELECTICAL ENGINEERING MCQs, preavious year quesion papers, dmrc, lmrc, drdo,rrb ntpc, ntpc, pgcil, dsssb, states board, GATE IES EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA