Polarization index (PI) test
Polarization index (PI) test
The Polarization Index (PI) Test is the most well-known application of the time-resistance method. Readings are taken at one minute and at ten minutes. The one-minute reading is divided by the ten-minute reading to provide a ratio. The result is a single number and can be considered independent of temperature since the thermal mass of the equipment being tested tends to be so great that the overall cooling which takes place during the 10 minutes of the test is negligible.
In general, a low PI ratio indicates little change, hence poor insulation, while a high PI ratio indicates the opposite. References to typical PI values are common in the literature, which makes this test very easy and readily employed. However, we say “in general” because there are materials that exhibit very little or no dielectric absorption. Carrying out a test on these materials would then produce a result very close to 1. An example is oil filled transformers, because behaviour of liquid insulation is physically different from solid insulation.
The PI is particularly useful because it can be performed on even the largest equipment, and yields a self-contained evaluation based on relative readings rather than absolute values. When testing large equipment, be aware that no PI can be calculated with a tester of limited range, because “infinity” is not a number! For this type of testing, use an instrument that measures to the tera-ohm range. The largest and newest capital equipment can be readily tested to yield repeatable data for recording and subsequent trend evaluation.
Another benefit of the PI test is that it can provide an indication of insulation quality in ten minutes on very large pieces of equipment that might take an hour or more to fully charge and stabilize if doing a spot reading test.
The following chart highlights selected PI values and what they mean to the operator.
|
Insulation Condition |
10/1 Minute Ratio (PI) |
|
Dangerous |
Less than 1.00 |
|
Questionable |
1.00-2.00 |
|
Good |
2.00-4.00 |
|
Excellent |
Above 4.00 |
There are several caveats to consider about PI values:
- PI results of 1.00-2.00 would be considered acceptable for equipment with very low capacitance, such as short runs of house wiring. Also, oil-filled equipment might exhibit PI values in this range.
- Some very high values of PI (above 5) could indicate brittle or cracked insulation that could fail under shock or during starts.
- A sudden increase in PI greater than 20%, without any maintenance having been performed, should serve as a warning. Insulation may hold its value for long periods but is not likely to dramatically improve all by itself.
The PI Test is critical to maintaining large electrical equipment. It provides proven results that are independent of temperature and help dictate further maintenance activities. This test does have one drawback. Each test takes 10 minutes, which means 30 minutes on 3-phase applications. A cable with 5 cores would take 50 minutes.
Megger has developed the PI Predictor to shorten the PI test time. The PI Predictor is an algorithm that uses the first part of the insulation resistance curve to predict where the curve would be at the end of the full 10-minute test. The algorithm can begin predicting after approximately 3 minutes. It continues to predict the final curve until it is 100% confident in the result. It then shuts down the test and displays the result. The prediction takes between 3 and 7 minutes depending on conditions with an average of 5 minutes (compared to 10 for the full test).
The PI standards allow the user to run the PI test for a shorter time, meaning the PI Predictor does not contradict the existing standards. What it does achieve is that the test can be run as short as possible but with the results expressed as a 10-minute PI test, making historical comparisons possible.
