Insulation Resistance Test of Transformers: Procedure and Importance Explained

Transformers play a crucial role in electrical power systems, providing the necessary voltage transformations for efficient transmission and distribution of electricity. To ensure their reliable and safe operation, regular maintenance and testing are essential. One critical test performed on transformers is the insulation resistance test, which assesses the condition of the transformer's insulation system. In this comprehensive article, we will delve into the occasions when the insulation resistance test is conducted and outline the detailed procedure involved in conducting the test. The importance of this test in ensuring the optimal performance and longevity of transformers will also be highlighted.

Insulation Resistance Test of Transformers

Occasions for Conducting the Insulation Resistance Test

The insulation resistance test is performed on transformers in various scenarios to evaluate the integrity of the insulation system. Some of the common occasions when this test is conducted include:

1.1. Routine Maintenance:

As part of regular maintenance practices, transformers undergo periodic insulation resistance testing. This helps in identifying any potential degradation or damage to the insulation system over time, allowing for timely repairs or replacement of faulty components. Routine tests are typically conducted annually or as recommended by the manufacturer or relevant standards.

1.2. Commissioning:

When a new transformer is installed, it is crucial to ensure that its insulation system is sound and capable of withstanding the operational stresses. Insulation resistance testing is performed during the commissioning process to validate the quality and reliability of the transformer's insulation before it is put into service.

1.3. Post Repair/Overhaul:

After a transformer undergoes significant repair or overhaul, it is vital to verify the integrity of the insulation system before returning it to operation. The insulation resistance test helps in confirming that the repaired or replaced insulation components meet the required standards and can withstand the electrical stresses encountered during operation.

1.4. Suspected Insulation Degradation:

If there are indications or suspicions of insulation degradation, such as abnormal operating conditions, overheating, or moisture ingress, an insulation resistance test can be performed to assess the condition of the insulation system. This helps in identifying potential insulation weaknesses or faults that may impact the transformer's performance or pose safety risks.

1.5. Regulatory Compliance:

Certain industries and regulatory bodies mandate insulation resistance testing as part of their compliance requirements. Utilities, industrial facilities, and other organizations must adhere to these regulations to ensure the safe and reliable operation of transformers and electrical systems.

Procedure for Conducting the Insulation Resistance Test

The insulation resistance test involves applying a high DC voltage across the transformer's insulation system and measuring the resulting leakage current. This test helps in evaluating the insulation's resistance to current flow and identifying any potential weaknesses or faults. The following steps outline the procedure for conducting the insulation resistance test:

2.1. Preparation:

Before initiating the test, ensure that all safety precautions are in place, and the transformer is properly isolated from the electrical supply. Follow the specific safety guidelines provided by the manufacturer or relevant standards. Gather the necessary test equipment, including a high-voltage DC source, a megohmmeter or insulation resistance tester, cables, and suitable personal protective equipment (PPE).

2.2. De-energization and Grounding:

Disconnect the transformer from the power supply and ensure it is de-energized. Ground all the transformer windings and take suitable precautions to discharge any residual electrical energy present in the system.

2.3. Test Connections:

Establish proper test connections between the high-voltage DC source, the insulation resistance tester, and the transformer windings. Follow the manufacturer's guidelines and ensure correct polarity connections.

1- OPen the HV and LV terminal including neutral.

2- Open the neutral ground first.

3- Note the oil Temperature of Power transformer.

4- Connect the black and red lead of set to power transformer winding as in figure below;-

Insulation Resistance Test of Transformers

Insulation Resistance Test of Transformers


Insulation Resistance Test of Transformers


2.4. Procedure for test:

  •  Press ON / TEST button of set the screen will display.
  •  Select the desired voltage by pressing 500v, 1kv,2.5 KV or 5 KV button
  •  Press the ON / TEST button again the test will start.
  •  To stop the timer press (after 15sec,60sec etc) STOP button the test will Stop.
  •  Note:- If beep or low MΩ display on screen during test press the STOP button this shows       winding is short.
  •  The reading is taken in MΩ.
  •  The insulation reading is corrected at 20 °C temperature
  •  When the temperature increases 10°C than 20 °C IR value reading is half and decrease in 10°C     IR is double.

2.5. Pre-Test Checks:

Perform pre-test checks on the test equipment to verify its functionality and calibration. Inspect the test leads, cables, and clamps for any damages or defects that could affect the accuracy of the test results.

2.6. Test Voltage Application:

Gradually apply the high-voltage DC test voltage to the transformer windings using the high-voltage DC source. The applied voltage should be within the specified range recommended by the manufacturer or relevant standards.

2.7. Measurement:

Using the insulation resistance tester or megohmmeter, measure the leakage current flowing through the insulation system. Record the readings for each winding and compare them against the acceptable limits specified by the manufacturer or industry standards.

2.8. Polarization Index (PI) and Dielectric Absorption Ratio (DAR):

Optionally, additional tests such as the Polarization Index (PI) and Dielectric Absorption Ratio (DAR) can be performed using the same test setup. These tests provide further insights into the condition of the insulation system and help in detecting moisture or contamination-related issues.

2.9. Post-Test Procedures:

After completing the measurements, gradually reduce and remove the test voltage from the transformer windings. Disconnect the test equipment, ensuring safety measures are followed. Analyze the test results and compare them with the acceptable values. If any anomalies or deviations are observed, further investigation or necessary corrective actions should be taken.

Importance of the Insulation Resistance Test

The insulation resistance test is a critical diagnostic tool for transformers, offering several benefits and ensuring their optimal performance and longevity. The importance of this test can be summarized as follows:

3.1. Early Detection of Insulation Deterioration:

By regularly conducting insulation resistance tests, potential insulation deterioration can be detected at an early stage. This allows for timely repairs or replacement of faulty insulation components, preventing further damage to the transformer and minimizing the risk of unexpected failures.

3.2. Prevention of Catastrophic Failures:

Insulation failures in transformers can lead to catastrophic consequences, including equipment damage, power outages, and safety hazards. By assessing the condition of the insulation system through insulation resistance testing, the risk of such failures can be mitigated, ensuring the reliability and continuity of power supply.

3.3. Validation of Insulation Integrity:

During the commissioning of new transformers or after major repairs, insulation resistance testing provides validation of the insulation system's integrity. This ensures that the transformer is fit for service and can withstand the electrical stresses encountered during operation.

3.4. Compliance with Standards and Regulations:

Insulation resistance testing is often required by industry standards, regulatory bodies, and insurance companies. By conducting this test, organizations can demonstrate compliance and fulfill their legal and safety obligations.

3.5. Diagnostic Insights into Transformer Health:

Insulation resistance testing, along with additional tests such as PI and DAR, provides valuable diagnostic insights into the overall health of transformers. By monitoring insulation resistance trends over time, potential issues such as moisture ingress, contamination, or aging can be identified, enabling proactive maintenance strategies.


The insulation resistance test plays a pivotal role in assessing the condition of the insulation system in transformers. By conducting this test on various occasions, including routine maintenance, commissioning, post-repair, suspected insulation degradation, and regulatory compliance, transformer operators can ensure the optimal performance and reliability of their assets. Following a comprehensive procedure that involves proper preparation, test connections, voltage application, measurement, and post-test analysis, the insulation resistance test provides valuable insights into the insulation integrity, allowing for early detection of insulation deterioration and prevention of catastrophic failures. Organizations that prioritize insulation resistance testing not only comply with industry standards and regulations but also proactively manage their transformer assets, ensuring a safe and uninterrupted power supply.

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