Engineering guide

    Insulation Resistance PI/DAR

    Two ratios of timed insulation-resistance readings, and what they can and cannot tell you.

    KiloVolt Engineering Toolkit

    A megohmmeter reading changes with time

    An insulation-resistance tester applies a DC test voltage to an isolated item and measures the resistance over time. The reading usually rises as the insulation absorbs charge. Two simple ratios describe that change between fixed times: the polarization index (PI) and the dielectric absorption ratio (DAR).

    1. What is measured

    Insulation is not an ideal resistor. During a DC test the current includes charging, absorption and leakage components; as the first two fade, the indicated resistance rises. The result depends on the item, the test voltage, temperature, humidity, contamination and the instrument. The raw readings are valuable on their own, and a high ratio is never automatic proof of safety.

    2. Polarization index

    PI=R10 minR1 minPI=\frac{R_{10\,\mathrm{min}}}{R_{1\,\mathrm{min}}}

    Both readings come from the same test at the specified times. With 150 MΩ at one minute and 600 MΩ at ten minutes, PI = 4.00; the ratio has no unit as long as both readings use the same unit. IEEE 43 cautions that for rotating machines with a one-minute reading above about 5000 MΩ, PI may no longer be a useful indicator; the calculator flags that case rather than invent a PASS.

    3. Dielectric absorption ratio

    DAR30=R60 sR30 s,DAR15=R60 sR15 sDAR_{30}=\frac{R_{60\,s}}{R_{30\,s}},\qquad DAR_{15}=\frac{R_{60\,s}}{R_{15\,s}}

    The time basis is written next to the result: a DAR with a 15-second denominator is not interchangeable with one using 30 seconds. With 90 MΩ at 30 s and 150 MΩ at 60 s, DAR30 = 150/90 ≈ 1.667.

    4. What the test tells you

    A resistance that rises with time can be consistent with absorption in some insulation types, but the link between PI or DAR and condition depends on the technology, contamination, service history and environment. A single value is not a universal acceptance or energisation criterion. Keep consistent procedures and track the raw 1-minute and 10-minute readings, the test voltage and the temperature over time.

    5. Preparation and safety

    Before connecting a high-voltage insulation tester, qualified personnel isolate the item, prove the absence of voltage, check the equipment instructions, and protect sensitive electronics, surge protection and converters. After a DC test the item can hold a dangerous stored charge: discharge it with the approved method and confirm the discharge. KiloVolt does not replace a test plan, the manufacturer's guidance or the safety rules.

    6. Questions

    • Does PI = 2 mean a motor is good? No: criteria depend on the insulation construction and the test procedure, and a ratio without the raw readings and conditions can mislead.
    • Can I enter GΩ? Yes, as long as all readings use the same unit; the conversion is automatic.
    • Can a 10-minute value be inferred from a 1-minute reading? No: PI needs the actual timed reading.
    • Can I test an energised electronics cabinet? No, and a DC test can be inappropriate for connected electronics even when de-energised: follow the equipment requirements.

    Megger instrument guide · IEEE 43 (rotating machinery; access may require a licence)

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