Licchavi Lyceum


Licchavi Lyceum

Tan Delta Test

Tan Delta Test is also known as the Dissipation Factor (DF) test. It is a diagnostic test performed on insulation systems of electrical equipment to assess the quality and condition of the insulation. The test is primarily conducted on high-voltage equipment, such as transformers, cables, motors, and generators, to measure the dielectric loss and evaluate the insulation integrity. It helps detect any insulation defects, such as moisture ingress, aging, contamination, or degradation, which can lead to insulation breakdown or equipment failure.

Tan Delta Test
Tan Delta Test

What is measured in the Tan Delta Test?

The Tan Delta Test measures the tangent of the phase angle difference between the voltage and current waveforms in an insulation system when subjected to an alternating voltage. The test evaluates the power dissipation or energy losses occurring within the insulation.

Tan δ = IR/IC


Tan δ = P / Q


  • tan δ is the tangent of the phase angle difference between the voltage and current waveforms.
  • P represents the resistive or active power dissipated in the insulation.
  • Q represents the reactive power associated with the energy stored and released by the insulation.

During the test, a voltage is applied across the insulation system, and the current flowing through the insulation is measured. The phase angle difference between the voltage and current waveforms is analyzed. The tangent of this phase angle is called the tan delta (tan δ) or the dissipation factor (DF).

The tan delta value represents the ratio of the resistive losses (active power) to the reactive losses (reactive power) in the insulation. It indicates the effectiveness of the insulation in storing and releasing energy. A low tan delta value indicates a high-quality insulation system with low losses, while a high tan delta value suggests the presence of insulation issues, such as moisture, contamination, or degradation.

Utility of the Result

By comparing the tan delta values obtained with the industry standards, the technical experts can identify any changes or potential problems in the insulation system. Tan Delta Testing helps in predicting and preventing insulation failures, allowing for proactive maintenance and avoiding costly equipment downtime.

Acceptable Limits

Insulation Type Tan Delta (tan δ) Limit
Oil-filled Transformers ≤ 0.005 to 0.015
Dry-type Transformers ≤ 0.002 to 0.006
Power Cables ≤ 0.001 to 0.005
Motors and Generators ≤ 0.002 to 0.010
Switchgear and Circuit Breakers ≤ 0.001 to 0.010

The acceptable limits for Tan Delta values can vary depending on the specific application, industry standards, and the condition of the insulation. The values provided in the table are general guidelines and may differ in specific cases. It’s important to refer to relevant standards, equipment specifications, and manufacturer’s recommendations to determine the acceptable limits for Tan Delta values specific to your application.

Enter the values of Active Power and Reactive Power in the below program and you will get the Tan Delta Value along with the sample graph representing the curve of Tan Delta of a healthy insulation system.

Tan Delta Calculator and Graph

Tan Delta Calculator and Graph

What is the procedure to do the Tan Delta Test?

  1. Safety Precautions:
    • Ensure that all appropriate safety measures are in place, including wearing personal protective equipment (PPE) and following electrical safety guidelines.
    • Disconnect the equipment from the power source and make sure it is properly de-energized before proceeding.
  2. Test Setup:
    • Set up the Tan Delta Test equipment, which typically includes a Tan Delta Analyzer or a Power Factor Test Set.
    • Ensure that the test equipment is calibrated and functioning correctly.
  3. Connect the Test Equipment:
    • Connect the test equipment to the equipment under test (EUT) following the manufacturer’s instructions.
    • Ensure proper grounding and isolation of the EUT.
  4. Test Configuration:
    • Configure the test equipment to the appropriate settings, such as the test voltage level and frequency, based on the specifications of the EUT.
  5. Apply the Test Voltage:
    • Gradually apply the test voltage to the EUT using the test equipment.
    • Monitor the voltage level to ensure it remains within the specified range.
  6. Measure the Current:
    • Measure the current flowing through the insulation of the EUT using the test equipment.
    • Ensure accurate current measurement by following the equipment’s instructions and using appropriate probes.
  7. Analyze the Waveforms:
    • Observe and analyze the voltage and current waveforms displayed on the test equipment.
    • Measure the phase angle difference (δ) between the voltage and current waveforms.
  8. Calculate the Tan Delta:
    • Calculate the Tan Delta (tan δ) value using the formula tan δ = P / Q, where P represents the resistive power and Q represents the reactive power.
    • Some advanced test equipment may provide automatic calculation of the Tan Delta value.
  9. Record the Results:
    • Record the measured Tan Delta value, along with any other relevant parameters or observations.
    • Document the test setup details, including voltage level, frequency, and test conditions.
  10. Compare with Accepted Values:
    • Compare the measured Tan Delta value with the accepted values or industry standards specific to the type and condition of the insulation.
    • Evaluate the results and identify any deviations or potential issues.
  11. Interpretation and Recommendations:
    • Interpret the test results based on the comparison with accepted values, previous measurements, or equipment specifications.
    • Determine if further action is required, such as maintenance, repairs, or additional diagnostic tests.
  12. Safety Measures and Equipment Disconnection:
    • After completing the test, ensure proper safety measures are followed, including equipment disconnection and securing the EUT.

It’s important to note that the above procedure is a general guideline. The specific steps and requirements may vary depending on the type of equipment, the test equipment used, and any applicable industry standards or guidelines. Always refer to the manufacturer’s instructions and follow established safety protocols while conducting the Tan Delta Test.

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