Electrical Technology, people

06/01/2026

17/11/2025

Follow the GULF JOBS Daily Updates channel on WhatsApp: https://whatsapp.com/channel/0029VbBX7DJHrDZXVP61PT1k

16/11/2025

✅ Transformer Megger Test – Full Details (Step by Step)

This test checks the insulation condition of transformer windings.

---

📌 1. Understanding the Diagram

A, B, C → HV terminals

a, b, c → LV terminals

n → neutral

E → transformer body (earth)

Red Megger Lead → goes to the winding under test

Black Megger Lead → goes to E (earth/body)

---

📌 2. Types of Insulation Resistance Tests

1️⃣ HV to Earth (HV – E)

Red lead → A, then B, then C (one by one)

Black lead → E (body)

Purpose: Check insulation of HV winding to ground

2️⃣ LV to Earth (LV – E)

Red lead → a, then b, then c, then n

Black lead → E

Purpose: Check insulation of LV winding to ground

3️⃣ HV to LV (HV – LV)

Red lead → HV terminals (A/B/C together if required)

Black lead → LV terminals (a/b/c/n together)

Purpose: Check insulation between HV and LV windings

---

📌 3. Megger Voltage Selection

Transformer Voltage Megger Test Voltage

415V – 3.3kV 500V – 1000V
6.6kV – 11kV 2500V
33kV – 66kV 5000V (5kV)
132kV – 220kV 10kV

For most power transformers → 5kV Megger is used.

---

📌 4. Step-by-Step Procedure

Step 1: Safety

Ensure transformer is fully isolated from system

Remove all jumpers

Disconnect neutral

Remove all temporary earths except one

Step 2: Megger Setup

Connect red and black leads correctly

Select the correct test voltage (5kV for HV)

Step 3: Perform the Test

Press Start/Test

Take 1-minute reading

For PI: take 1-min and 10-min readings

Step 4: Record Results

HV–E

LV–E

HV–LV

---

📌 5. Expected / Acceptable IR Values

Transformer Size Minimum IR Value

Small (500 kVA) 200 MΩ – 1 GΩ
Medium (1–10 MVA) 500 MΩ – 2 GΩ
Large (10–50 MVA) 1 GΩ – 5 GΩ
Very Large 5 GΩ+

If IR is low, reasons may be:

Moisture

Oil contamination

Carbon tracking

Internal fault

---

📌 6. Polarization Index (PI)

PI = IR at 10 min ➗ IR at 1 min

PI Value Condition

> 2 Excellent insulation
1.5 – 2 Good
< 1 Bad → Winding needs drying

---

📌 7. After the Test

Earth all terminals again

Restore all connections

Transformer safe for further tests/service

15/11/2025

Classification of transformer

15/11/2025

ELECTRICAL FORMULAS

11/11/2025

Busbar maintenance

03/11/2025

Who to my work. Install 850

03/11/2025

Who to my work

11/10/2025

Links open for H. V side transformer