High Accuracy Insulation Resistance (IR) Testing

The iTIG III features highly accurate Insulation Resistance (IR) testing.  This is made possible by the highly accurate leakage current measurement.  Leakage current accuracy is 2% with a 10pA resolution. This means you see 5 digits after the decimal and the tester will calculate MegOhms with currents down to 0.0005 µA.

The iTIG III has a minimum current limit of 0.0005 µA.

When the current is low, small changes in leakage currents mean big differences in IR values such as MegOhms, Polarization Index (PI), and Dielectric Absorption Ratio (DAR).   For example, if the test voltage is 1000 V,  IR can be calculated up to 2000 GΩ.

 

IR = V/I = 1000V / 0.0005µA = 2000GΩ

 

Contrast that with testers only accurate to 0.1 µA (100nA). In this case, IR can only be calculated up to 10 GΩ (a difference by a factor of 200 ! ).

 

IR = V/I = 1000V / 0.1µA = 10GΩ

 

Let’s take a look at an example using PI (Polarization Index).  The iTIG III has a minimum leakage current limit of 0.0005µA (0.5 nA).  For motors with low leakage currents, if the current falls below the minimum current limit, the PI may be inaccurate or not calculated (NC).  Let’s compare two testers to see how this limit affects the PI value.

TesterX has a minimum leakage current limit of 0.1µA (100nA).  Let’s say both testers in this example measure current at 1 minute as 0.1µA.  At 10 minutes, TesterX measures 0.05µA which is under its current limit and that means the measurement is not accurate.

The iTIG III measures a current of 0.02µA, well above its current limit where measurements are accurate.  With a 1000V test voltage, PI is calculated as follows.

TesterX
iTIG III
TesterX at 1 min.
iTIG III at 1 min.
TesterX at 10 min. PI = 2
iTIG III at 10 min. PI = 5
TesterX iTIG III
1 min.:  1000V/0.1µA = 10 GΩ

10 min.:  1000V/0.05µA = 20 GΩ

PI = 20 GΩ/10 GΩ = 2

1 min.:  1000V/0.1µA = 10 GΩ

10 min.:  1000V/0.02µA = 50 GΩ

PI = 50 GΩ/10 GΩ = 5

 

The effect on PI is dramatic.  In physical terms, 0.05µA vs 0.02µA is a small difference in current but results in a big difference in the PI value.  Electrom’s iTIG III rivals dedicated megohm testers when it comes to accurate measurements and PI calculations at very low current levels.  If you had to report the above PI values to your customer, which instrument would you trust?