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What does тАШincreasing InR with time indicateтАЩ?
Healthy insulation
Poor insulation
Moderate insulation
None of these
Healthy insulation
Increasing Insulation Resistance (InR) with time during a DC high-voltage test (like a Megger test) indicates healthy insulation. This phenomenon occurs due to the dielectric absorption current, where the polarization of dipoles within the insulating material decays over time, causing the total current to decrease and the measured resistance to rise.
Increasing Insulation Resistance (InR) with time during a DC high-voltage test (like a Megger test) indicates healthy insulation. This phenomenon occurs due to the dielectric absorption current, where the polarization of dipoles within the insulating material decays over time, causing the total current to decrease and the measured resistance to rise.
ItтАЛ(t)=IcтАЛ+IaтАЛ(t)+ILтАЛ тАФ Total charging current components
R(t)=ItтАЛ(t)VтАЛ тАФ Time-dependent resistance calculation
When a constant DC voltage is applied across insulation, the total charging current ItтАЛ is the sum of capacitive current IcтАЛ, absorption current IaтАЛ, and leakage current ILтАЛ. IcтАЛ drops rapidly, IaтАЛ decays exponentially as molecules align with the electric field, and ILтАЛ remains constant for stable insulation. Since R=V/ItтАЛ, as IaтАЛ decays, the total current ItтАЛ decreases, leading to a steady increase in the calculated resistance value over time.
The Polarization Index (PI) is the ratio of insulation resistance at 10 minutes to that at 1 minute.
A PI value greater than 2 is generally considered indicative of good, dry insulation.
Constant InR readings suggest a potential issue with insulation moisture or contamination.
Moisture in insulation provides a conductive path that prevents the resistance from increasing effectively over time.
Early detection of moisture and contamination.
Non-destructive testing methodology.
Requires significant time for accurate measurement.
Environmental conditions (humidity/temperature) strongly influence readings.
Transformer winding insulation testing.
Rotating machine (motors/generators) stator health monitoring.
If InR remains constant or decreases over time, it often indicates surface leakage or internal moisture, categorizing the insulation as faulty or poor.
Option B is incorrect because poor or contaminated insulation typically results in a flat or decreasing resistance profile due to dominant leakage current.
A is correct тАФ An increasing insulation resistance trend over time signifies that the insulation is effectively polarizing and free from significant moisture or conduction paths.
Always remember that moisture is the primary enemy of insulation; a high PI (Polarization Index) is the standard test for verifying the absence of moisture in high-voltage equipment.