Wednesday, July 30, 2014

Preventive Maintenance test with Insulation Resistance Test, Part 2

Part 1 covers the introduction of the insulation resistance test in preventive maintenance can be found here.
What are the test methods for insulation resistance test?
There are three types of tests for measuring insulation resistance.
  • Spot reading
  • Time-resistance
  • Step voltage
Each test applies its own methodology that focuses on a specific insulating property of the devices being tested. Users need to choose the one that best fits the test requirements.

Spot test
A test voltage is applied for a short interval until a stable reading is achieved, or for a fixed period of time, normally 60 seconds or less. The reading is collected at the end of the test. This test is normally performed for Go/NoGo testing or historical records. Temperature and humidity variations may affect the readings and have to be compensated for if necessary.

For the historical record, a curve is plotted based on the history of the readings. Observation of the trend is taken over a period of time, normally over years or months. 
Figure 3 For an effective monitoring the equipment insulation resistance, the insulation resistance values collected at each test interval should be plotted at the graph to track it’s trend

This test is suitable for a device with a small or negligible capacitance effect, e.g. short wiring run.

Time-resistance test
Successive readings are taken at a specific time, typically every few minutes, and difference in readings compared. Good insulation will show a continual increase in the resistance value. If the reading is stagnant and it does not increase as expected, the insulation may be weak and attention may be needed. Moist and contaminated insulation may lower resistance readings since they will increase the leakage current during testing. The temperature influence on this test is negligible as long as there is no significant temperature change in the device under test.
This test is suitable for the predictive and preventive maintenance of rotating machines.
The polarization index (PI) and dielectric absorption ratio (DAR) are commonly used to quantify the time-resistance test result.
Figure 2 Curve plot of a time-resistance test made on a motor winding with Keysight Handheld Meter Logger software. Good insulation shows a continual increase in resistance, the trend line should be in inverse exponential

The polarization index is defined as the ratio of the 10 minute resistance value to the 1 minute resistance value. The interpretation of the value is shown in Table 1. The IEEE Std 43-2000 recommends the minimum value of PI for AC and DC rotating machinery in thermal class B, F and H as 2.0, and the minimum PI value for class A equipment is 1.5.
NOTE: Some new insulation systems have a faster response to the insulation test. They usually start with test result at GΩ range yielding a PI between 1 and 2. In these cases, the PI calculation may be disregard. According to the IEEE Std 43-2000, if the 1 minute insulation resistance is above 5 GΩ, the calculated PI may not be meaningful.
Dielectric absorption ratio is referred to the ratio of the 60 second resistance value to the 30 second resistance value. The interpretation of the value is shown in Table 1.
DAR is suitable for devices with insulation materials in which the absorption current decreases quickly.

Table 1. PI and DAR test result interpretation

Insulation condition
PI value
DAR value
< 2
< 1.25
2 to 4
< 1.6
> 4
> 1.6

Different voltage levels are applied in steps to the device under test. The recommended ratio of the test voltage is 1:5. The test at each step is same length, usually 60 seconds, and goes from low to high. This test is normally used at test voltages lower than the rated voltage of the equipment. The rapid increase of the test voltage level creates additional stress on the insulation and causes the weak point to fail, subsequently leading to a lower resistance value.
This test is particularly useful when the rated voltage of the equipment is higher than the available test voltage generated by the insulation resistance tester.

In the last article (Part 3), I will cover the test voltage selection, minimum insulation resistance, as well as the safety consideration for the insulation resistance test.