Polarization Index Calculator

A polarization index (PI) calculation compares insulation resistance measured after one minute with insulation resistance measured after ten minutes. The calculator produces the polarization index ratio, the difference from a selected reference value, and a clear reference comparison.

PI is used when evaluating the condition of electrical insulation under a timed insulation-resistance test. It is commonly associated with motors, generators, transformers, cables, and other equipment where insulation resistance is measured over time. A rising resistance reading during the test produces a PI greater than 1; the amount of rise is expressed by the ratio.

The calculation does not establish equipment acceptability by itself. The applicable maintenance procedure, equipment manufacturer guidance, test voltage, temperature, humidity, equipment condition, and site testing requirements determine how the result is used.

Insulation Resistance Over Time

An insulation-resistance test applies a controlled DC test voltage and records resistance in megohms. The 1-minute reading provides an early value, while the 10-minute reading shows how resistance changes as the test continues.

The calculator uses these interface fields:

Field	Electrical purpose
1-minute resistance	Insulation resistance recorded at one minute, in MOhm
10-minute resistance	Insulation resistance recorded at ten minutes, in MOhm
Reference PI	Comparison ratio specified by the testing procedure or equipment context
Polarization index	Calculated ratio of the 10-minute resistance to the 1-minute resistance
Reference margin	Difference between the calculated PI and the Reference PI
Reference comparison	Indicates whether the calculated PI is at or above the entered reference

Both resistance inputs must represent the same equipment, test setup, and unit basis. Entering one reading in MOhm and another in a different resistance unit would make the ratio invalid unless the values are converted first.

Polarization Index Formula

The calculator applies the following ratio:

\(\displaystyle \text{Polarization Index} = \frac{\text{10-minute resistance}}{\text{1-minute resistance}}\)

The reference comparison is:

\(\displaystyle \text{Reference margin} = \text{Polarization index} - \text{Reference PI}\)

A nonnegative margin produces the status:

The entered polarization index is at or above the comparison reference.

Because PI is a ratio, the resistance units cancel mathematically. The interface uses MOhm so the entered values remain consistent with typical insulation-resistance test records.

Calculation Example

Enter the following readings:

Input	Value
1-minute resistance	100 MOhm
10-minute resistance	250 MOhm
Reference PI	2

Calculate the polarization index:

\(\displaystyle \text{PI} = \frac{250\ \text{MOhm}}{100\ \text{MOhm}} = 2.5\)

Calculate the reference margin:

\(\displaystyle \text{Reference margin} = 2.5 - 2.0 = 0.5\)

The resulting worksheet values are:

Result	Value
Polarization index	2.5
Reference margin	0.5
Reference comparison	At or above reference

The 10-minute resistance in this example is 2.5 times the 1-minute resistance. With a Reference PI of 2, the result is 0.5 above that reference.

Field Test Boundaries

A PI result is valid only to the extent that the underlying readings are valid. Record the equipment identity, test voltage, test duration, ambient conditions, insulation temperature, equipment operating history, and test instrument used with the readings.

Do not use the ratio alone to diagnose a specific insulation defect or to authorize energization. A maintenance decision may require review of the absolute insulation-resistance values, prior test records, equipment condition, manufacturer instructions, and the governing maintenance or acceptance procedure. The Reference PI entered in the calculator is a comparison value; it does not replace those requirements.

Related calculations: Insulation Resistance Calculator, Continuity Test Calculator.