# Grounding Transformers (Zigzag ZN) and Neutral Earthing Resistors (NGR): Sizing & Fault Duty

> Creating artificial neutral points in ungrounded delta systems, zero-sequence impedance (Z0) calculation, 10-second thermal rating of NGRs, and sensitive earth fault protection.

- **Category:** Earthing & Grounding
- **Author:** Voltformer Power System Protection Desk
- **Publication Date:** 2026-07-30
- **Reading Time:** 8 min read
- **Key Tags:** #Grounding Transformer, #Zigzag Winding, #NGR, #Zero Sequence, #IEEE 142
- **Canonical URL:** https://voltformer.com/articles/grounding-transformers-zigzag-zn-and-neutral-earthing-resistors-ngr
- **Markdown Source:** https://voltformer.com/articles/grounding-transformers-zigzag-zn-and-neutral-earthing-resistors-ngr.md

### 1. Artificial Neutral Grounding in Delta Networks

Ungrounded medium-voltage systems (delta windings) experience severe transient overvoltages during arcing ground faults. Grounding transformers with **Zigzag ($ZN$)** or **Wye-Delta ($Yd$)** windings establish an artificial neutral point.

### 2. Neutral Earthing Regime Comparison

| Grounding Method | Ground Fault Current ($I_g$) | Transient Overvoltage ($V_{ph-g}$) | Protection Selectivity |
|---|---|---|---|
| **Ungrounded Delta** | $< 10\text{ A}$ (capacitive) | High ($> 2.73 \times V_{ph}$ arcing ground) | Hard to locate faulty feeder |
| **Solid Grounded** | High ($> 10\text{ kA}$) | Low ($1.0 \times V_{ph}$) | Immediate trip, severe equipment damage |
| **Low-Resistance Grounding (NGR)** | **Controlled ($100\text{ A} - 1000\text{ A}$)** | **Low ($\le 1.73 \times V_{ph}$)** | **Fast selective relay trip, minimal damage** |
| **High-Resistance Grounding (HRG)** | $5\text{ A} - 10\text{ A}$ | Low ($\le 1.73 \times V_{ph}$) | Alarm only, system remains in service |

### 3. Neutral Earthing Resistor (NGR) Resistance Formulation

For a desired ground fault current $I_{fault}$ (typically $200\text{ A} - 400\text{ A}$):

$$ R_{NGR} = \frac{U_{LL} / \sqrt{3}}{I_{fault}} $$

#### Example for $33\text{ kV}$ Grid, $400\text{ A}$ Fault Limit:
$$ R_{NGR} = \frac{33000 / \sqrt{3}}{400} = 47.63\text{ }\Omega $$

### 4. 10-Second Thermal Withstand Rating

NGRs are rated for **10 seconds** full current duty per **IEEE 32**:

$$ E_{thermal} = I_{fault}^2 \cdot R_{NGR} \cdot t_{fault} \quad [\text{Joules}] $$

Stainless steel grid elements withstand temperature rises up to **$760^\circ\text{C}$** without deformation.

*Reference: IEEE Std 142-2007 (Green Book); IEEE Std 32-2020; IEC 60076-6.*

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