What is the Dielectric Absorption Index and Polarization Index Test? Step by Step Procedure, Formulas & Examples

When working with electrical machines and equipment, testing the condition of insulation is one of the most important tasks an electrical engineer performs. Poor insulation can lead to equipment failure, costly downtime, and dangerous situations. Among the various insulation testing methods available today, the Dielectric Absorption Index (DAI) and Polarization Index (PI) tests stand out as reliable diagnostic tests.

These tests help engineers evaluate the health of insulation in transformers, motors, generators, cables, and other electrical equipment. Unlike simple spot reading tests, DAI and PI tests provide deeper insights into the actual condition of insulation materials.

In this technical guide, we will discuss everything you need to know about Dielectric Absorption Index (DAI) and Polarization Index (PI) tests along with step by step testing procedure, interpretation of test results, equipment required for testing, difference between DAI and PI test and their applications.

1. What is Insulation Resistance Testing?

Before we discuss DAI and PI tests, let us first understand the basic concept of insulation resistance testing. Insulation acts as a barrier that prevents current from flowing between conductors and the ground or between different conductors. Over time, insulation can degrade due to moisture, contamination, thermal stress, mechanical damage, and aging.

Insulation resistance testing involves applying a DC voltage across the insulation and measuring the resistance it offers. A megohmmeter (commonly called a Megger) is the standard instrument used for this purpose. The test voltage usually ranges from 500V to 10,000V depending on the equipment rating.

When DC voltage is applied to insulation, three types of currents flow through it:

• Capacitive Charging Current: This current flows immediately when voltage is applied. It charges the capacitance of the insulation and decreases rapidly to almost zero within seconds.
• Absorption Current (Polarization Current): This current flows as molecules within the insulation material align themselves with the electric field. It decreases gradually over time and may take several minutes to stabilize.
• Leakage Current (Conduction Current): This is the actual current that flows through the insulation due to impurities and moisture. In good insulation, this current is very small and remains constant.

2. What is Dielectric Absorption Index (DAI)?

The Dielectric Absorption Index is a ratio that compares insulation resistance values taken at two different time intervals during a single test. It measures how well the insulation absorbs and retains electrical charge.

DAI is specifically designed for short-duration tests and is useful when time is a constraint. This test is particularly helpful for evaluating smaller electrical equipment and components where the full polarization process happens relatively quickly.

2.1 How to Calculate Dielectric Absorption Index

The formula for calculating DAI is straightforward:

\(DAI = \frac{\text{Insulation Resistance at 60 seconds}}{\text{Insulation Resistance at 30 seconds}}\)

Or simply: \(DAI = \frac{IR_{60}}{IR_{30}}\)

For example, if you measure insulation resistance of a motor winding and get:

• Resistance at 30 seconds = 150 MΩ
• Resistance at 60 seconds = 200 MΩ

Then, \(DAI = \frac{200}{150} = 1.33\)

2.2 Interpretation of DAI Values

The DAI value tells you about the condition of insulation. Here is a general guideline for interpreting DAI results:

A DAI value below 1.0 indicates that insulation resistance is actually decreasing with time. This is a serious warning sign that the insulation has excessive moisture or contamination.

3. What is Polarization Index (PI)?

The Polarization Index is another ratio-based test that provides a more thorough assessment of insulation health. It uses a longer test duration compared to DAI, which allows for more complete polarization of insulation molecules.

PI testing is the preferred method for evaluating larger electrical machines such as motors, generators, and transformers. The extended test duration gives a clearer picture of the insulation behavior under sustained electrical stress.

3.1 How to Calculate Polarization Index

The formula for calculating PI is:

\(PI = \frac{\text{Insulation Resistance at 10 minute}}{\text{Insulation Resistance at 1 minute}}\)

Or simply: \(PI = \frac{IR_{10}}{IR_{1}}\)

For example, consider a generator winding test with these readings:

• Resistance at 1 minute = 500 MΩ
• Resistance at 10 minutes = 1500 MΩ

Then, \(PI = \frac{1500}{500} = 3.0\)

3.2 Interpretation of PI Values

The PI value provides valuable information about insulation quality. Here is the standard interpretation guide:

For rotating machines rated at Class F insulation (155°C) or higher, IEEE recommends a minimum PI value of 2.0 for safe operation.

4. Why Do DAI and PI Values Increase Over Time in Good Insulation?

This is an interesting phenomenon that every engineer should understand. When you apply DC voltage to good insulation, the resistance reading increases over time. This happens because:

1. The capacitive charging current dies out very quickly (within first few seconds)
2. The absorption current gradually decreases as polarization reaches saturation
3. Only the small leakage current remains after some time

In good insulation, the leakage current is minimal. As the absorption current decreases the overall current flow reduces. Since resistance is inversely proportional to current (at constant voltage), the measured resistance increases.

In poor or contaminated insulation, the leakage current is high and dominates the measurement. This masks the decreasing absorption current and results in flat or decreasing resistance readings. Hence, a low or declining PI/DAI value indicates insulation problems.

5. Difference Between DAI and PI Tests

While both tests follow the same principle, there are some key differences:

6. Equipment Required for DAI and PI Testing

The primary equipment needed for these tests includes:

1. Megohmmeter (Insulation Resistance Tester): Modern digital megohmmeters come with built-in timers and can automatically calculate DAI and PI values. Select a megger with appropriate voltage rating for your equipment.
2. Test Leads: Use high-quality insulated test leads with proper ratings. The leads should be free from cracks and damage.
3. Guard Lead: This helps eliminate surface leakage currents that can affect measurement accuracy.
4. Timer: If using an analog megger without built-in timer, keep a stopwatch handy.
5. Personal Protective Equipment: Safety gloves, safety glasses, and insulated footwear are necessary.

7. Step-by-Step Procedure for Conducting DAI and PI Tests

Here is a practical guide for performing these tests on a motor winding:

7.1 Preparation Phase

1. Disconnect the motor from the power supply and lock out/tag out the circuit
2. Disconnect all external connections including power cables and control wiring
3. Discharge any residual charge by grounding the windings for several minutes
4. Clean the terminal connections to remove dirt and moisture
5. Record the ambient temperature and humidity
6. Select appropriate test voltage based on motor rating (typically 500V for motors up to 1000V rating, 1000V for motors up to 2500V rating)

7.2 Testing Phase

1. Connect the megger positive lead to the winding terminal
2. Connect the megger negative/earth lead to the motor frame/ground
3. Connect the guard terminal if surface leakage is suspected
4. Start the test and begin timing
5. Record resistance readings at 30 seconds, 60 seconds, 1 minute (if not same as 60s), and 10 minutes
6. Keep the test voltage constant throughout the test duration
7. After completing the test, remove the voltage and ground the winding to discharge stored energy

7.3 Calculation Phase

Calculate DAI using 60-second and 30-second readings
Calculate PI using 10-minute and 1-minute readings
Compare results with standard values and previous test records

8. Practical Example: Testing a 415V Induction Motor

Let us work through a real example. Suppose you are testing a 50 HP, 415V induction motor that has been in service for 5 years. You use a 500V megger and record these readings:

• At 30 seconds: 120 MΩ
• At 60 seconds: 180 MΩ
• At 1 minute: 180 MΩ
• At 10 minutes: 420 MΩ

Calculations:

• DAI = 180 / 120 = 1.5
• PI = 420 / 180 = 2.33

Interpretation:

• The DAI value of 1.5 indicates questionable insulation (borderline between questionable and good)
• The PI value of 2.33 indicates good insulation

In this case, while the PI shows acceptable condition, the lower DAI suggests some initial absorption issues. The recommendation would be to continue monitoring and schedule the motor for detailed inspection during the next planned shutdown.

9. Factors Affecting DAI and PI Test Results

Several factors can influence the accuracy and reliability of these tests:

9.1 Temperature

Temperature has a major impact on insulation resistance measurements. As temperature increases, insulation resistance decreases. A general rule states that insulation resistance halves for every 10°C rise in temperature. Always record the temperature during testing and correct readings to a standard reference temperature (usually 40°C) when comparing historical data.

9.2 Humidity

Moisture is one of the biggest enemies of electrical insulation. High humidity can lower insulation resistance and affect DAI/PI values. If possible, conduct tests in controlled humidity conditions. If the equipment has been exposed to moisture, dry it out before testing.

9.3 Contamination

Dust, oil, carbon deposits, and chemical contamination on insulation surfaces can create leakage paths. Clean the equipment before testing to get accurate results.

9.4 Previous Energization

If the equipment was recently energized, residual charge can affect initial readings. Ground the equipment properly before testing.

9.5 Test Voltage

Using incorrect test voltage can give misleading results. Follow manufacturer recommendations or industry standards for selecting test voltage.

10. Applications of DAI and PI Testing

These tests find wide application across various electrical equipment:

• Electric Motors: Both AC and DC motors are regularly tested using PI method. This helps identify winding insulation degradation before failure occurs.
• Generators: Large generators in power plants undergo regular PI testing as part of predictive maintenance programs.
• Power Transformers: Transformer windings are tested to check insulation between windings and between windings and core/tank.
• High Voltage Cables: Cable insulation is tested using these methods, especially before commissioning and after repairs.
• Switchgear: Bus bar insulation and circuit breaker components are evaluated using these tests.
• Rotating Exciters: Exciter windings require regular insulation monitoring using PI tests.

11. Advantages of DAI and PI Tests

These tests offer several benefits over simple spot resistance tests:

1. Temperature Independence: The ratio method minimizes temperature effects since both readings are taken under similar conditions
2. Trend Analysis: By recording values over time, you can track insulation degradation and plan maintenance
3. Moisture Detection: Low values quickly reveal moisture problems in insulation
4. Non-Destructive: These tests do not damage the insulation being tested
5. Simple to Perform: With modern digital meggers, conducting these tests is straightforward
6. Cost-Effective: The equipment is relatively inexpensive compared to other diagnostic methods

12. Limitations and Precautions

Despite their usefulness, these tests have some limitations:

1. Not for Wet Insulation: If insulation is known to be wet, dry it first. Testing wet insulation gives meaningless low readings.
2. Safety Hazards: The test voltages can be dangerous. Always follow safety procedures and never touch equipment during testing.
3. Capacitance Discharge: Large equipment stores significant charge after testing. Always ground the equipment before handling.
4. Surface Leakage: Dirty surfaces can give false low readings. Use guard terminals and clean equipment before testing.
5. Not Definitive: While these tests indicate insulation condition, they should be used alongside other diagnostic methods for a complete assessment.

13. Industry Standards and Guidelines

Several standards govern the practice of insulation resistance testing:

• IEEE 43: Recommended Practice for Testing Insulation Resistance of Electric Machinery
• IEEE 62: Guide for Diagnostic Field Testing of Electric Power Apparatus
• IEC 60085: Electrical Insulation – Thermal Evaluation and Designation
• NEMA MG1: Motors and Generators

14. Record Keeping and Trending

One of the most valuable aspects of DAI and PI testing is the ability to track insulation condition over time. A single test gives you a snapshot of current condition. But when you compare results from multiple tests conducted over months or years, you can identify gradual deterioration.

Best practices for record keeping include:

• Record test date and time
• Note ambient temperature and humidity
• Document test voltage used
• Record all time-stamped resistance readings
• Calculate and record DAI and PI values
• Note any observations about equipment condition
• Compare with previous test results
• Flag any values below acceptable thresholds

15. When Should You Perform These Tests?

The frequency of testing depends on several factors:

Commissioning: All new equipment should be tested before energization
After Installation: Test cables and machines after installation work
After Repairs: Always test after any repair work on insulation
Periodic Maintenance: Annual testing is common for large motors and generators
After Exposure to Moisture: Test if equipment has been exposed to flooding or high humidity
Before Re-Energization: Test equipment that has been idle for extended periods

16. Conclusion

The Dielectric Absorption Index and Polarization Index tests are powerful tools in the electrical engineer’s toolkit. They provide valuable insights into insulation health that simple spot readings cannot offer.

Regular testing and proper record keeping enable predictive maintenance strategies that reduce unexpected failures and extend equipment life. While these tests should not be used in isolation, they form an important part of a complete insulation diagnostic program.

17. Frequently Asked Questions (FAQs)