# How to measure insulation resistance of a motor

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## Winding insulation resistance

If the motor is not put into operation immediately upon arrival, it is important to protect it against external factors like moisture, high temperature and impurities in order to avoid damage to the insulation. Before the motor is put into operation after a long period of storage, you have to measure the winding insulation resistance.

If the motor is kept in a place with high humidity, a periodical inspection is necessary.

It is practically impossible to determine rules for the actual minimum insulation resistance value of a motor because resistance varies according to method of construction, condition of insulation material used, rated voltage, size and type. In fact, it takes many years of experience to determine whether a motor is ready for operation or not.

A general rule-of-thumb is 10 Megohm or more.

 Insulation resistance value Insulation level 2 Megohm or less Bad 2-5 Megohm Critical 5-10 Megohm Abnormal 10-50 Megohm Good 50-100 Megohm Very good 100 Megohm or more Excellent

The measurement of insulation resistance is carried out by means of a megohmmeter – high resistance range ohmmeter. This is how the test works: DC voltage of 500 or 1000 V is applied between the windings and the ground of the motor.

During the measurement and immediately afterwards, some of the terminals carry dangerous voltages and MUST NOT BE TOUCHED.

Now, three points are worth mentioning in this connection: Insulation resistance, Measurement and Checking.

### 1. Insulation resistance

• The minimum insulation resistance of new, cleaned or repaired windings with respect to ground is 10 Megohm or more.
• The minimum insulation resistance, R, is calculated by multiplying the rated voltage Un, with the constant factor 0.5 Megohm/kV.

For example: If the rated voltage is 690 V = 0.69 kV, the minimum insulation resistance is: 0.69 kV x 0.5 Megohm/kV = 0.35 Megohm

### 2. Measurement

• Minimum insulation resistance of the winding to ground is measured with 500 V DC. The winding temperature should be 25°C ± 15°C.
• Maximum insulation resistance should be measured with 500 V DC with the windings at a operating temperature of 80 – 120°C depending on the motor type and efficiency.

### 3. Checking

• If the insulation resistance of a new, cleaned or repaired motor that has been stored for some time is less then 10 Mohm, the reason might be that the windings are humid and need to be dried.
• If the motor has been operating for a long period of time, the minimum insulation resistance may drop to a critical level. As long as the measured value does not fall below the calculated value of minimum insulation resistance, the motor can continue to run.

However, if it drops below this limit, the motor has to be stopped immediately, in order to avoid that people get hurt due to the high leakage voltage.

Reference: Grudfos – Motor Book

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#### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

1. mikegen
Feb 14, 2017

This article is really helpful to me and solved half of my worries concerning resistance insulation. But there is something that is not clear to me or maybe I don’t understand it. Well, its base on resistance in ohms. please, tell me the general rules guiding winding resistance in ohms.

I will be very glad if you can solve this worries of mine.

Thanks.

2. Jerry
Oct 20, 2016

My candit option is the article have been so explicit, however, you have said the IR, varies, depend on the type of material used, the size… Is it in kW? Pl use another example, because, .35mohms/kV, is not clear to me. I want to know how to calculate the minimum IR, of an electric motor.

3. Tsvetan
Mar 08, 2016

Is it common to test the resistance between the windings as wells as the one between winding and ground?

4. ketan
Oct 12, 2015

Nice ,articles provides good practical knowledge

5. Emeka
Sep 21, 2015

75HP (55KW) motor has 0.1M ohms L1 – L2 – L3 conductors and 0.0M ohm conductors to GND after three idle years. It was cleaned, dried by 1000W lamp heat and re-vanished to get 2.2G ohms conductors to GND. Thereafter, motor draws currents 22A on NO-Load and 149A when coupled to 75HP Fire water pump which caused over heat to 250Kw and 350Kw power generator. What is the fault of the above procedure and possible solution?

6. Ernest Anderson
Jul 11, 2015

7. murat baykal
Mar 10, 2015

Very useful artciles are take place here. As an unexperienced engineer those articles are very educating and informative.

8. yphamngoc
Nov 16, 2014

How about the ratio one min per 15s and 10mins per 1 min?
Thanks

9. Michael okikiri
Nov 06, 2014

Am a bit confused on the illustrations on insulation resistance test. If 2 megaohm is considered bad base on the thumb rule and you calculated the minimum insulation resistance in the example to be 0.35megaohm is this not below the 2megaohms considered bad? Pls clarify

Michael okikiri

• tv
Nov 16, 2014

In general, it is considered as 2 M ohms. But for critical operation this can be lowered to 0.35 M Ohms as stated. Once the emergency situation is out, the cause of low insulation should be attended.

10. farai benie
Oct 30, 2014

11. awny
Oct 01, 2014

12. Mustafa Rabiul Hassan
Sep 18, 2014

13. nombuso
Sep 09, 2014

As a lecturer in electrical department I find this page very useful.

14. arif wahyudi
Sep 03, 2014

I think we can use EASA or IEEE std. 43, table 3

15. Todd Thomassen
Aug 02, 2014

Nice presentation!

16. Malesela Godfrey
Jul 06, 2014

What may be the cause when you meggar between L1 and L3 and get results of less than 0,5 Megohm? and when you meggar between other phases you get more than 0,5 Megohm even between phases and earth is 0,5 Megohm.

• vijay
Oct 21, 2014

L1 & L3 always should show zero resistance with megger. So, also between L1 & L2, L2 & L3. But, with respect to Earth the value should be higher the resistance higher the benefits. The symptoms that you wrote clearly shows that the windings are pretty bad. Requires overhauling.
Vijay

17. Nigel Chikanv
Jul 03, 2014

Thanks

18. VIKAS PRAKASH SINGH
Jun 28, 2014

very useful ………

19. Matias Azambuja
Jun 26, 2014

I agree with John that there is an contradiction about the minimum insulation resistance.

The example above is saying that 0,35Mohm is the minimum IR for a 690V motor. I don´t agree.

There is another point that I don´t know if I didn´t understand or if there is an error:

2 Measurement

Minimum insulation resistance of the winding to ground is measured with 500 V DC. The winding temperature should be 25°C ± 15°C.
Maximum insulation resistance should be measured with 500 V DC with the windings at a operating temperature of 80 – 120°C depending on the motor type and efficiency.

In my opinion there is no reason to measure de IR in 2 different temperatures. Also, when we increase the temperature the IR tend to reduce not increase because isolating material have a behavior different from conductive materials.

Jun 22, 2014

Muy buena nota! !!

21. ANIL KUMAR.M
Jun 19, 2014

It is good article sir,
Thank u

22. meenakshi sundaram balu
Jun 18, 2014

Very very useful for me

23. H Santa Rao
Jun 15, 2014

Hi John !

Good Morning.

It is because of An Electrical Equipment for that matter should run at High Value of IR.based on Operating Voltage and current as well.

i.e We can Charge an apparatus but we cannot take risk of continued charging an apparatus with minimal Safe Value of IR which may vary depends on Site Conditions.

for further details refer the standard Books on Electricity Utility.

Jun 13, 2014

Hi
I need some info about electromotor rtd’s. such as:wiring;how to work & position in electromors
Thanks

25. John McGraw
Jun 12, 2014

The above article reads partially as follows:

” The minimum insulation resistance of new, cleaned or repaired windings with respect to ground is 10 Megohm or more.
The minimum insulation resistance, R, is calculated by multiplying the rated voltage Un, with the constant factor 0.5 Megohm/kV.

For example: If the rated voltage is 690 V = 0.69 kV, the minimum insulation resistance is: 0.69 kV x 0.5 Megohm/kV = 0.35 Megohm”

There appears to be a contradiction here unless I am missing something. How can .35mOhm be ok if anything under 2 is Bad and the recommended is 10?

• haytham
Jun 21, 2014

Hi eng john , i think this calculated values it true when the applyed voltage 1000 V, otherwise your point is 5stars

26. sha baba
Jun 11, 2014

ya its fine

27. sajna soman
Jun 10, 2014

Very useful article sir. Thanks….

• sha baba
Jun 11, 2014

hi sanjana how do u do nice I also appreciate