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Insulation Resistance (IR) Measurement Method

Test and Measurement Methods

Insulation Resistance (IR) Measurement

This page describes insulation resistance measurement details, including types of IR measurements, iTIG IR measurement techniques, test conditions, and causes of measurement results. For a general description of IR measurement using the iTIG, see the Insulation Resistance (IR) Measurement Summary.

Measuring Megohms

Insulation resistance (IR) is among the most common motor measurements. It is also has more types of currents than some users realize.  In its most basic form, an insulation resistance measurement is done with a hand-crank meter that measures megohms . An advanced tester plots megohms over a period of 10 minutes or more and displays voltage, leakage current, DAR, and PI ratios.  Learn more about DAR and PI ratios.

In an IR or megohm measurement, the voltage applied and the total leakage current are measured between the windings and the motor frame/ground. Ohms law:

{R}=\frac{V}{I}

is applied to calculate the resistance in megohms, where R is resistance in megohms, V is voltage applied in volts, and I is the total resulting current in microamperes (µA).

A temperature correction factor is applied to correct the megohm measurement at present temperature to what it would be at a standard temperature. According to IEEE 43 and ANSI/EASA standards, the standard temperature is 40°C.

The leakage current of a used motor is often surface current running in the dirt on the outside of the windings. The dirt contains particles of dust, oil, grease, moisture, etc. Conduction current that flows through weak ground insulation to ground is often dwarfed by the surface currents. Therefore, the insulation resistance measurement or megohm measurement is sometimes referred to as the dirt test. Megohms tend to drop with increasing amounts of dirt.

A megohm measurement on new motors is often not interesting other than to check that there are no direct shorts to ground. Users will often go directly to a hipot test.

Currents Involved in Megohm, DAR, and PI Measurements

Definitions

Current Types

IC—Capacitive

A capacitive inrush current brings the potential in the motor up to the test voltage by charging it. This current drops quickly and reaches zero within a few seconds after the test voltage is reached. For large motors with high capacitance, the inrush current is high. Total leakage current failure limits must be set high enough to avoid tripping the limit during this initial phase of the test. For more information on the subject of capacitive inrush current and how to avoid tripping a limit, see DC hipot test methods.

IA—Absorption

Absorption current polarizes the insulation.  This current also goes to zero or very near zero within 30 seconds to 1 min in random wound motors.  Form wound motors take much longer due to the layers of insulation used between turns.  The change in absorption current over time is what is used to calculate PI and DAR ratios in an insulation resistance measurement.

IG—Conductance

Conductance current flows between the copper conductors and ground through the bulk of the insulation. This current is usually zero if the motor is new or undamaged. As the motor insulation ages and cracks or is damaged conductance current may flow depending on the test voltage applied. Conductance current tends to accelerate with increasing voltage. This current is sometimes referred to as leakage current or as part of the leakage current.

IL—Surface Leakage

According to IEEE 43, surface leakage is the current flowing in the dirt on the surface of the windings to ground. It is called surface conduction current in other standards. A dirtier motor has higher leakage current and a lower megohm result. There may be an increase in the surface leakage current on motors with a stress-control coating on the the end-windings. After 1 minute with a random wound motor or 5-10 minutes with a form wound motor, the surface leakage current is typically the only current remaining unless the insulation is weak or damaged.

IT—Total

The total current is the sum of the 4 currents. An motor and insulation tester measures total current. The total current equals or is very close to the surface leakage current at the end of the insulation resistance measurement. This gives the operator a good measure of how dirty or contaminated the motor. It also alerts the operator to a possible catastrophic connection from the windings to ground.

IR Measurement Considerations

Leakage Current as a Function of Time

To determine if the leakage current is primarily a surface current or also contains conductance current, one must do a step voltage test or ramp test. See information below on minimum megohm levels. Note that these tests can be done at voltages lower than the normal DC hipot test voltage in order to find conduction current.

Insulation Resistance Relative Current

Tracking Megohm Measurements Over Time

Megohm measurements are tracked over time to help determine when a motor or generator should be reconditioned. This is done automatically with the iTIG motor analyzer. Especially for larger motors, other insulation resistance measurements such as DAR or PI measurements are used in reconditioning assessments. Additional tests are DC hipot, step voltage/ramp tests, surge tests and partial discharge measurement.

Motors with Low IR Measurement Readings

ANSI/AR100-2015 and IEEE 43-2013 both recommend that motors with low insulation resistance measurement readings should not be subjected to high-voltage testing.

IR measurement test voltage
Rated voltage of motor (V)*DC voltage to be applied
<1000500
1000–2500500–1000
2501–50001000–2500
5001–12,0002500–5000
>12,0005000–10,000
Rated line-to-line voltage for 3-phase AC machines, line-to-ground voltage for single-phase machines, rated direct voltage for DC machines or field windings
Recommended minimum insulation resistance values at 40°C
Minimum insulation resistance after 1 minuteTest specimen
IR = kV + 1 MΩMost windings made before about 1970, all field windings and others not described below
IR = 100 MΩMost AC windings built after about 1970 (form wound coils)
IR = 5 MΩMost machines with random-wound stator coils, form wound coils rated below 1 kV, and DC armatures

Temperature Compensation Notes

The limits above are for windings at a temperature of 40°C. The megohm measurement results are temperature compensated because the windings usually are not at this temperature when tested. Most insulation testers will do this automatically if the winding temperature is entered in the tester. Resistance values must be temperature compensated when IR is tracked over time. The temperature must also be above the dew point for accurate comparisons of results.

The most common temperature compensating formula states that the insulation resistance drops by a factor of 50% for every 10°C increase in temperature. Therefore, it is clear that the insulation properties drop precipitously as the temperature rises.  IR of 10,000 megohms (10 Giga Ohms) at 20°C (~68°F) drops to 2,500 megohms at 40°C, and to 39 megohms at 100°C.

There are several other temperature compensation formulas.  The above formula is likely the most conservative. Different types of insulation systems in form wound motors have unique temperature characteristics. These are only obtained from the manufacturer of the motor.

The bottom line is temperature has a significant effect on insulation resistance and must be compensated for best results.

IR Measurement Considerations

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Question:  How much better is the result of Test #1 than Test #2 in the example IR measurement results shown below?

Example IR measurement results
Test #Test voltageLeakageIR
11000V0.01 μA10,000 MΩ (10 GΩ)
21000V0.02 μA5000 MΩ (5 GΩ)

Answer: It is difficult to know for certain. A difference of 0.01µA could be the result of a number of variables.  These variables might include temperature, changes in environmental conditions, electrical noise, or instability in the voltage or current.

Limitations of Measurement Result Interpretation

The difference in insulation resistance is high due to how resistance is calculated. The only physical change is the current and this change is  extremely small. Some insulation testers display leakage current to the third (thousandths), or even fourth decimal places with a resolution as low as 1nA or 1pA.  Electrom’s instruments calculate and display IR in teraohms (TΩ). Because their accuracy is too dependent on variables other than the leakage current being measured, decimal place digits are not specified for IR.

Other Advice and Tips from IEEE 43-2013

  • Before starting a measurement, the winding insulation should be discharged to avoid measurement errors.
  • For motors with a stress-control coating applied to the end-windings, there may be an increase in the surface leakage current and thereby lower megaohms (megohms or MΩ) than expected.
  • For winding temperatures below the dew point, it is impossible to predict the effect of condensation on the surface. Therefore, a correction to 40 °C for trend analysis introduces significant errors.
  • For directly water-cooled windings, the water should be removed and the internal circuit thoroughly dried. The winding manufacturer may have provided a means of measuring results of the insulation resistance measurement without need for the coolant water to be drained.
  • A minimum discharge time of four times the voltage application duration is recommended. All Electrom instruments discharge the motor through a resistor.  For motors with voltage less than 100V, connecting the winding directly to ground with the instrument ground lead or a shorting stick or jumper will complete the discharge immediately.  Any residual absorption charge takes longer to discharge.  Keep motors with absorption charges connected directly to ground if they are to be handled soon after a test.
  • Absorption discharge takes more than 30 minutes depending on the insulation type and physical size of the motor.
  • A significant decrease in insulation resistance (increase in measured current) with an increase in applied voltage is an indication of insulation problems in an insulation resistance measurement.
  • A steady increase in the IR with age indicates decomposition of the bonding of insulation materials, especially when they are  thermoplastic.
  • When a low PI occurs at temperatures above 60°C, taking a second measurement below 40°C and above the dew point is recommended as a check.
  • PI can be used to indicate when the drying process of insulation is complete.  This occurs when the PI exceeds the recommended minimum.
  • If the IR value at 40 °C is greater than 5000 MΩ, the PI is ambiguous and disregarded.

Polarization Index (PI) and Dielectric Absorption Ratio (DAR or DA)

PI = \frac{R_{10}}{R_{1}}

where

R10 is resistance in megohms at 10 minutes

R1 is resistance in megohms at 1 minute

and

R = \frac{V}{I}

so the PI formula reduces to:

PI = \frac{I_{1}}{I_{10}}

that is the current at 1 minute divided by the current at 10 minutes.

PI Measurement

A polarization index (PI) is the ratio of the megaohms (megohms) measured after 10 minutes divided by the megohms measured after 1 minute. This measurement is conducted primarily on form wound motors and generators. An instrument continuously records megohm data over 10 minutes. The resulting graph and PI ratio can then provide additional information about the winding insulation beyond the megohm number itself.

DA Measurement

The dielectric absorption ratio (DAR) or DA, is the ratio of the megohms at 1 minute divided by the megohms at 30 seconds. When the measured leakage current stabilizes within 1 minute, operators typically use the DA measurement. If this happens, the 10 minute PI measurement is useless because the ratio is 1.

Common DAR and PI values used in the literature and by manufacturers of test equipment for assessing the insulation conditions are:

Common DAR and PI values
DARPIInsulation condition
<1.251.0–2.0Questionable
1.25–1.62.0–4.0Good
>1.6>4.0Excellent

It is not necessary to make a temperature correction since both DAR and PI are ratios. It is recommended that motors with low insulation resistance readings not be subjected to high voltage testing.

IEEE 43-2013

Because the changes in currents may be very small, the IEEE 43-2013 states:

“When the insulation resistance reading obtained after the voltage has been applied for 1 minute (IR1) is higher than 5000 MΩ, based on the magnitude of applied direct voltage, the total measured current (IT) can be in the sub micro-ampere range. At this level of required test instrument sensitivity, small changes in the supply voltage, ambient humidity, test connections, and other non-related components can greatly affect the total current measured during the 1–10 minute interval required for a PI measurement. Because of these phenomena, when the IR is higher than 5000 MΩ, the PI may or may not be an indication of the insulation condition and is therefore not recommended as an assessment tool.”

In summary, for motors with no or little absorption current where the total leakage current stabilizes within 1 minute, PI values are close to or equal to 1.  In this case, PI is not a proper evaluation tool. This is often the case for random wound rotating equipment.

Minimum IEEE 43-2013 PI ratings are shown below.

Minimum IEEE 43-2013 PI ratings
Thermal class ratingMinimum PI
Class 105 (A)1.5
Class 130 (B) and above2

High Accuracy Insulation Resistance (IR) Measurement

The iTIG features highly accurate insulation resistance (IR) measurement. 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 calculate megohms with currents down to 0.0005 µA.

When the current is low, small changes in leakage currents mean big differences in IR values such as megohmspolarization index (PI), and dielectric absorption ratio (DAR). For example, if the test voltage is 1000 V,  IR can be calculated up to 2000 GΩ.

iTIG megohm test screen at 1000V, 0.00050 µA; 2000 GΩ, 500,000 MΩ
The iTIG has a minimum current limit of 0.0005 µA.
IR = \frac{V}{I} = \frac{1000V}{0.0005µA} = 2000GΩ

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

IR = \frac{V}{I} = \frac{1000V}{0.1µA} = 10GΩ

High Accuracy Insulation Resistance (IR) Measurement

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Measuring the PI (Polarization Index)

Here is an example using the polarization index (PI).  The iTIG 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).  Two testers are compared in this example to demonstrate how this limit affects the accuracy of the PI value.

Assume both testers in this example measure current at 1 minute as 0.1µA. 

  • The Comparison Tester has a minimum leakage current limit of 0.1µA (100nA).
    • At 10 minutes, the Comparison Tester measures 0.05µA, which is under its current limit.
    • Therefore the Comparison Tester measurement is not accurate.
  • The iTIG has a minimum leakage current of 0.0005µA (0.5 nA).
    • At 10 minutes, the iTIG measures 0.02µA, which is above the iTIG’s current limit.
    • Therefore the iTIG measurement is in the accurate range.

With a 1000V test voltage, the PI is calculated as shown in the chart at right.

The effect of the difference between 0.1 µA (100nA) and 0.0005µA (0.5 nA) accuracy on PI calculation is dramatic.  In physical terms, 0.05µA vs 0.02µA is a small difference in current. However, the result is a big difference in the PI value.  The Electrom iTIG rivals dedicated megohm testers when it comes to accurate measurements and PI calculations at very low current levels. 

If you had to report these PI values to your customer, which instrument would you trust?

Instrument Comparison Result

Screen Display for 1 Minute Measurement

Screen Display for 10 Minute Measurement

1 Minute Value

10 Minute Value

PI Value

Generic tester profile image
Comparison Tester.
Tester X at 1 Minute
Comparison Tester at 1 minute.
Test X at 10 minutes, PI=2
Comparison Tester at 10 minutes, PI=2.

Comparison Tester 1 minute value: 1000V/0.1µA = 10 GΩ

Comparison Tester 10 minute value: 1000V/0.05µA = 20 GΩ

Comparison Tester PI value: 20 GΩ/10 GΩ = 2

iTIG tester for IR measurement.
Electrom iTIG.
iTIG at 1 minute
iTIG at 1 minute.
iTIG at 10 minutes, PI=5
Electrom iTIG at 10 minutes, PI=5.

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