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Winter 2002-2003 1

Megohm 101  or, The Fine Art

of Resistance Evaluation

by David C. LeClair 

Shermco Industries, Inc.

• Apply the test voltage andrecord insulation resistance atthe predetermined intervals.

• Remove test voltage.• Apply grounds to the motor for

a time equal to four times theapplied voltage duration.

The guidelines for testing mo-tors are generally consistent be-tween the different sources. Theydo not, however, agree on whatvalues make motor-insulation re-sistance good or bad.

Same Test, DifferentEvaluations

The one conclusion all of ourexperts come to depends on themagnitude of insulation resistancereadings: high insulation resis-tance is good, low insulation resis-tance is bad. But after this point ourreference materials start to diverge.The method in IEEE Std. 43-2000gives minimum insulation resis-

The insulation-resistance testing of motors is one of the first types of 

testing a NETA Level II technician masters. However, analyzing

the results can stress the minds of the most skilled technicians and

engineers. It requires a four-function calculator, understanding of Ohm’s

Law, knowledge of insulation materials, adherence to electrical safety

procedures, and checking the right reference manuals. All of this is im-

portant because your customer only wants to know, with a maximum

amount of confidence, “Is my motor good or bad?” The answer should

not be “Well, kind of,” or “It depends.”

The Insulation-Resistance Test ProcedureThe first step to evaluating motors is to check the reference materials

published by the experts. There are many good publications that dis-cuss insulation-resistance testing procedures, but I would suggest:

•  A Stitch in Time, #AVOB001 available from Megger. This is one of theoldest, best, and least expensive references on insulation-resistancetesting.

• IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery, IEEE Std 43-2000. The manufacturer of the motor you aretesting will most likely reference IEEE Std 43-2000.

• NETA’s Maintenance Testing Specifications and Acceptance Testing Speci-

 fications.

So what do you do? The test procedure prescribed by these sources is:

• Electrically isolate the motor to be tested.

• Record the nameplate information.

• Select the test voltage based on the nameplate voltage.

• Guard the test area with barricades, signs, and/or tape.

• Verify test set operation by performing open circuit and short circuittests on the output leads.

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2 NETA WORLD

tance in Table 3 based on age or type and nameplatevoltage. For windings made prior to about 1970 and afew other types, minimum insulation resistanceshould be at least one megohm for every thousandvolts (rated nameplate voltage) plus one megohm. Forlow-voltage motors the minimum insulation resis-tance is five megohms. Note that these values are formotors with their insulation system at a temperatureof 40 degrees Celsius (104 degrees Fahrenheit). The

Comparison Table, below, shows 40 degrees Celsiusand corrected 20 degrees Celsius values. The Electri-cal Apparatus Service Association (EASA) and mostmotor manufacturers adhere to this recommendationas well. The NETA recommended values in Table 10.1(the same for either the MTS-2001 or the ATS-1999)also differ from our other sources and seem somewhatconservative. Additionally, Table 10.1 is not specificto a motor temperature.

So which evaluation is correct?

Comparison Table:

Minimum RecommendedInsulation Resistance (megohms)

NETA IEEE Std 43-2000 IEEE Std 43-2000

Motor Voltage Table 10.1 Table 3 (at 40°C) Table 3 (at 20°C)

480 V, 3 Phase 100 5 12.5

4.2k V, 3 Phase 1000 5.2 13

The polarization index (PI) is the ten-minute read-ing divided by the one-minute reading. The dielectricabsorption ratio (DAR) is the one-minute reading di-vided by the half-minute reading. Both of these ratios

should be greater the 1.5 to get a satisfactory rating.However, the advancement of insulating materialsand methods limits the usefulness of the ratios. TheIEEE Std. 43-2000 cautions the use of values larger than5000 megohms for calculating the ratios. At these smallcurrents of less than a microampere, there are manyinfluences that lead to a poor PI ratio. It is not un-usual to achieve 5,000 megohms within the firstminute of testing.

Conclusion

I use the IEEE Std 43-2000 table only for emergen-cies. Any low insulation resistance results should bediscussed with your customer/owner and all otheroptions reviewed. When it comes to life safety sys-tems or critical motors I use NETA’s Table 10.1. Table10.1 was created by successful technicians with manyyears of experience. Motors having insulation resis-

tance values greater than the minimums specified inTable 10.1 generally are in good condition (clean anddry). The polarization index (PI) is a twenty to fiftyminute test — the ten minute test time when voltageis applied, then the motor must be grounded for atleast another forty minutes for it to reduce the residualvoltage on the motor to a safe value. This test is typi-cally not used for emergencies and trouble shootingchecks. The one-minute test (DAR) is sufficient to en-sure you are not energizing a faulted motor

In conclusion, remember the following tips whentesting motors:

• If you are thinking of keeping the test voltage on a

motor low to improve the resistance until a satis-factory level is achieved and that this will only makeyour readings better, remember that the motor con-dition has not changed. You have not dumpedenough energy in the motor to dry it out.

• Never perform an insulation resistance testing onhermetic chiller motors while they are undervacuum. Consult the manufacturer first. The testvoltage can cause damage to windings.

References:

IEEE Std 43-2000, IEEE Recommended Practice for Test-ing Insulation Resistance of Rotating Machinery, NewYork, NY: IEEE

IEEE Std 902-1998, IEEE Guide for Maintenance, Opera-tion, and Safety of Industrial and Commercial PowerSystems, New York, NY: IEEE

 A Stitch in Time, Biddle Instruments/Megger, Blue Bell,PA

 Acceptance Testing Specifications, NETA, 1999.

 Maintenance Testing Specifications, NETA, 2002

David C. LeClair received his Bachelor of Science Degree inElectrical Engineering from the University of Texas in 1981. Heworks in the field as an electrical engineer and has been employedat Shermco Industries, Inc. for the past 16 years. Prior to his presentposition Mr. LeClair worked for DOW Chemical USA in Freeport,Texas.