Insulation Resistance (IR) Test for Transformers

The Insulation Resistance (IR) Test is conducted to assess the condition of insulation in a transformer’s windings and ensure there are no breakdowns due to moisture, dirt, or aging.

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Test Equipment Required

Megger (Insulation Resistance Tester) – 500V, 1000V, 2500V, or 5000V depending on transformer rating.

HV Gloves & PPE – for safety.

Multimeter – to confirm no voltage before testing.

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Test Procedure

1. Pre-Test Preparations

Isolate the Transformer: Disconnect the transformer from the power supply and ensure it is properly de-energized.

Short-Circuit the Windings: If required, short-circuit the secondary winding to avoid floating voltages.

Clean Surface & Terminals: Remove any dust, moisture, or contaminants from bushings and terminals.

Measure Temperature & Humidity: IR readings are affected by temperature and humidity. If temperature correction is needed, use a standard correction chart.

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2. Insulation Resistance Measurement

Perform the test between the following points:

HV Winding to LV Winding

HV Winding to Earth (Ground)

LV Winding to Earth (Ground)

Step-by-Step Testing

1. Connect the Megger:

The Positive lead (+) to the winding under test.

The Negative lead (-) to the ground (earth) or another winding.

2. Set the Test Voltage:

Below 1 kV transformers → 500V

1 kV to 5 kV transformers → 1000V

5 kV to 15 kV transformers → 2500V

Above 15 kV transformers → 5000V

3. Press the Test Button and hold for at least 60 seconds.

4. Note the Insulation Resistance Value (MΩ) after 15s, 30s, and 60s.

5. Stop the Test and Discharge the Transformer:

After testing, safely discharge the winding by connecting it to the ground for a few seconds.

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3. Acceptance Criteria

New Transformers: IR should be above 1000 MΩ (depends on transformer rating).

In-Service Transformers: A reading above 1 MΩ per kV of operating voltage is acceptable.

Polarization Index (PI) Calculation: PI = IR(60s) / IR(10s)

PI > 2.0 → Good insulation

PI between 1.5 and 2.0 → Moderate condition

PI < 1.5 → Poor insulation (possible moisture or contamination)

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4. Precautions & Safety Tips

Always de-energize the transformer before testing.

Ensure there is no residual charge before and after the test.

Do not touch the terminals during or after the test until proper discharge.

Perform tests in dry conditions for accurate results.