5.2 Polarization Index (PI) & Dielectric Absorption Ratio (DAR)

Polarization Index (PI)

The Polarization Index extends a single IR reading into a time-ratio diagnostic. Apply the test voltage continuously for 10 minutes, record the 1-minute and 10-minute resistance values, and divide:

PI = IR(10 min) / IR(1 min)

Good insulation keeps absorbing charge over time, so resistance rises steadily between minute 1 and minute 10; wet or contaminated insulation is dominated by conduction (leakage) current that is constant from the start, so resistance stays flat and the ratio approaches 1.0. IEEE Std 43-2013 (Clause 12.2, Table 3) sets the minimum PI by thermal class:

Worked Example

A 480 V motor is tested with a 1,000 V DC megger. The 1-minute reading is 150 MΩ; the 10-minute reading is 450 MΩ.

PI = 450 / 150 = 3.0

That is well above the 2.0 minimum for Class B and higher — good insulation. If the same motor read 180 MΩ at 1 minute and 200 MΩ at 10 minutes, PI = 1.11, which lands in the questionable-to-dangerous band and warrants investigation before service.

The 5,000 MΩ Caveat

IEEE 43 says PI is not applicable when the 1-minute IR exceeds 5,000 MΩ. At that level the absorption current is masked by the instrument's measurement floor and the denominator is dominated by geometry; the ratio becomes noise. In that case, treat the high absolute IR as the pass criterion and do not use PI. PI is also not applicable to non-insulated field windings or DC armatures with exposed commutators.

PI Interpretation Table

Dielectric Absorption Ratio (DAR)

The Dielectric Absorption Ratio is the short-time cousin of PI. Instead of 10 minutes, DAR uses the first minute:

DAR = IR(60 s) / IR(30 s)

Because the absorption current decays fastest in the first seconds, a 60/30 ratio still captures the rise for dry insulation but finishes in one minute instead of ten. DAR is the right tool when a 10-minute test is impractical — small motors, field work under time pressure, or equipment where holding voltage for 10 minutes is logistically difficult.

DAR Interpretation

A DAR below 1.25 is cause for investigation; a minimum DAR of 1.25 is sometimes required for new equipment when PI cannot be run. DAR is typically lower than PI because the measurement window is shorter, so a DAR of 1.6 is a stronger result than a PI of 1.6.

When DAR Is Used vs PI

• Use PI for in-plant acceptance and maintenance of medium-voltage rotating machinery where 10 minutes is feasible and IEEE 43 compliance is required.
• Use DAR for quick field checks, small motors, trend comparisons on low-voltage equipment, or any case where holding the test for 10 minutes is not practical.
• DAR is temperature- and size-independent, which makes it useful for comparative diagnostics across different machines where absolute IR values are not directly comparable.

Both ratios are independent of temperature in a way the absolute IR is not — that is their diagnostic power. A 600 MΩ reading at 40°C and a 300 MΩ reading at 50°C may represent the same insulation condition, but a PI of 3.0 means the same thing on both machines.

What PI and DAR Reveal: Absorption vs Conduction

When you apply DC voltage to insulation, three currents flow: geometric capacitance current (decays in milliseconds), absorption current (decays over minutes, as dipoles in the dielectric align), and conduction (leakage) current (constant, driven by moisture, contamination, or carbonization). The first two are healthy and reversible; the third is the enemy.

A rising IR-vs-time curve — high PI or high DAR — means absorption current dominates and conduction is small: the insulation is dry and sound. A flat curve — PI near 1.0, DAR near 1.0 — means conduction dominates: the dielectric is wet, dirty, or degraded. That is the entire diagnostic content of PI and DAR in one sentence.

Plateau Interpretation

The 10-minute IR trace of healthy insulation rises and then plateaus as absorption current decays to near zero; the steady-state reading is essentially pure leakage. A trace that plateaus early (by 2 to 3 minutes) at a low value indicates high leakage; a trace still rising at 10 minutes indicates strong absorption and good insulation. The shape of the curve, not just the two endpoints, is informative — which is why modern meggers plot and store the full 10-minute trace rather than just capturing two numbers.

Step-Voltage IR Trending

A step-voltage ("megger ramp") test applies progressively higher DC voltage in equal steps — for example 500 V, 1,000 V, 1,500 V, 2,000 V — holding each step long enough to record leakage current. Healthy insulation shows linear, stable leakage at each step: doubling voltage roughly doubles leakage current, so resistance stays constant. A non-linear knee-point where leakage current rises disproportionately at a higher step signals the onset of insulation breakdown — contamination, a void, or carbonized tracking. A declining IR trend (resistance dropping as voltage rises) is the classic signature of degradation and can identify failing insulation earlier than a single-voltage PI test. Compare the knee voltage to prior tests; a knee that walks backward over years is an early warning that the dielectric is approaching failure.

When DAR Is Used vs PI — Quick Reference

Do not run a 10-minute PI on a winding with IR below the IEEE 43 minimum — investigate and dry first. And do not invert the ratio: PI is always 10-minute over 1-minute, never the reverse. Inverting the ratio is the most common PI calculation error on the Level II exam.