Question 67

The Correct Answer is D **Explanation for Option D (Correct Answer):** The Polarization Index (PI) test is a standard, non-destructive diagnostic test used to assess the condition of electrical insulation in high-voltage equipment such as motors, generators, cables, and transformers. The PI value is calculated by observing how the insulation resistance (IR) changes over a fixed period of time (typically 10 minutes). The polarization index is formally defined as the ratio of the insulation resistance measured after ten minutes ($IR_{10\text{ min}}$) to the insulation resistance measured after one minute ($IR_{1\text{ min}}$). $$PI = \frac{IR_{10\text{ min}}}{IR_{1\text{ min}}}$$ A healthy insulation system will continue to absorb charge slowly over time (polarization absorption current dominates the leakage current), causing the measured resistance to rise significantly between the one-minute and ten-minute marks, resulting in a PI value typically greater than 2.0. A low PI value suggests that the insulation is severely contaminated or degraded. **Explanation for Incorrect Options:** * **A) The polarization index is the ratio of the insulation resistance taken at one minute to the insulation resistance taken at ten minutes.** This definition is the inverse of the standard PI formula. Since resistance should increase over time in healthy insulation, this ratio would result in a value less than 1 for good insulation, which is not how the PI is defined or interpreted. * **B) The polarization index is the insulation resistance taken at ten minutes.** The PI is a dimensionless ratio, not a resistance value itself. While the 10-minute resistance value is crucial, it is only one component of the PI calculation. * **C) The polarization index is the ratio of the insulation resistance taken at thirty minutes to the insulation resistance taken at one minute.** Although some specialized or older standards might use different time intervals for specific diagnostic tests (like the 30-minute reading for the Dielectric Absorption Ratio in some contexts), the universally accepted standard definition for the Polarization Index (PI) is the ratio of the 10-minute reading to the 1-minute reading.