Option C: Polarization index Explanation & Concepts: The Polarization Index (PI) is a diagnostic test used to evaluate the condition of electrical insulation in transformers, motors, and generators. It is defined as the ratio of the insulation resistance measured after 10 minutes ($R_{600}$ or commonly $R_{60}$ in older industrial notation for total time) to the resistance measured after 1 minute ($R_{15}$ or $R_1$). Working Principle: When a constant DC voltage is applied to insulation, the measured current consists of capacitive, absorption, and conduction components. Initially, the charging current is high, but it decays over time as the polarization (absorption) current decreases. A good insulation system shows a continuous increase in resistance over time because the polarization process stabilizes, resulting in a PI ratio greater than 1.0, typically indicating a healthy, dry, and clean dielectric. Key Formulas: $PI = \frac{R_{10min}}{R_{1min}}$ — Standard definition of Polarization Index; $DAR = \frac{R_{1min}}{R_{30sec}}$ — Dielectric Absorption Ratio Key Points: PI value > 2.0 is generally considered good for Class A insulation.; PI value between 1.0 and 2.0 indicates questionable insulation.; PI value; The test is performed using a high-voltage Megohmmeter (Megger). Advantages: Non-destructive testing method; Effective at detecting moisture or aging in insulation Disadvantages: Highly temperature sensitive; Cannot detect localized spot defects (pinholes) Applications: Preventive maintenance of HV rotating machinery; Condition monitoring of power transformers Additional Info: IEEE 43 standard provides detailed guidelines for testing insulation resistance of rotating machinery.. Option A is a distractor, resistance ratio is a broad term, not a specific diagnostic metric.. Option B (Turns ratio) relates to transformer winding turn counts, not insulation condition.. Option D (Megger resistance ratio) is technically redundant as Megger is the instrument used to calculate PI. Answer: C is correct — The Polarization Index is the ratio of insulation resistance measured at 10 minutes to the resistance measured at 1 minute, serving as an indicator of insulation health. Exam Tip: Always remember that while PI is an excellent indicator of insulation health, it is highly dependent on temperature; ensure insulation temperature is corrected to a reference base (usually 40°C) before comparison.

Option D: Megger resistance ration

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R60/R15 is known as

Resistance ratio

Turns ratio

Polarization index

Megger resistance ration

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Option C

Polarization index

Quick Summary: The Polarization Index (PI) is a diagnostic test used to evaluate the condition of electrical insulation in transformers, motors, and generators. It is defined as the ratio of the insulation resistance measured after 10 minutes ($R_{600}$ or commonly $R_{60}$ in older industrial notation for total time) to the resistance measured after 1 minute ($R_{15}$ or $R_1$).

💡 Explanation

The Polarization Index (PI) is a diagnostic test used to evaluate the condition of electrical insulation in transformers, motors, and generators. It is defined as the ratio of the insulation resistance measured after 10 minutes ( $R_{600}$ or commonly $R_{60}$ in older industrial notation for total time) to the resistance measured after 1 minute ( $R_{15}$ or $R_1$ ).

🔢 Key Formulas

$PI = \frac{R_{10min}}{R_{1min}}$ — Standard definition of Polarization Index

$DAR = \frac{R_{1min}}{R_{30sec}}$ — Dielectric Absorption Ratio

⚙️ Working Principle

When a constant DC voltage is applied to insulation, the measured current consists of capacitive, absorption, and conduction components. Initially, the charging current is high, but it decays over time as the polarization (absorption) current decreases. A good insulation system shows a continuous increase in resistance over time because the polarization process stabilizes, resulting in a PI ratio greater than 1.0, typically indicating a healthy, dry, and clean dielectric.

📌 Key Points

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PI value > 2.0 is generally considered good for Class A insulation.
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PI value between 1.0 and 2.0 indicates questionable insulation.
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PI value < 1.0 often indicates moisture, contamination, or thermal degradation.
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The test is performed using a high-voltage Megohmmeter (Megger).

✅ Advantages

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Non-destructive testing method
▸
Effective at detecting moisture or aging in insulation

❌ Disadvantages / Limitations

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Highly temperature sensitive
▸
Cannot detect localized spot defects (pinholes)

🛠️ Applications / Uses

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Preventive maintenance of HV rotating machinery
▸
Condition monitoring of power transformers

📄 Additional Information

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IEEE 43 standard provides detailed guidelines for testing insulation resistance of rotating machinery.
▸
Option A is a distractor, resistance ratio is a broad term, not a specific diagnostic metric.
▸
Option B (Turns ratio) relates to transformer winding turn counts, not insulation condition.
▸
Option D (Megger resistance ratio) is technically redundant as Megger is the instrument used to calculate PI.

📊 Diagram / Illustration

✅

C is correct — The Polarization Index is the ratio of insulation resistance measured at 10 minutes to the resistance measured at 1 minute, serving as an indicator of insulation health.

Core Concepts Used

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Dielectric Absorption Insulation Resistance Preventive Maintenance

💡 EXAM TIP

Always remember that while PI is an excellent indicator of insulation health, it is highly dependent on temperature; ensure insulation temperature is corrected to a reference base (usually 40°C) before comparison.

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