Insulation Resistance (IR) Values


The measurement of insulation resistance is a common routine test performed on all types of electrical wires and cables. As a production test, this test is often used as a customer acceptance test, with minimum insulation resistance per unit length often specified by the customer. The results obtained from IR Test are not intended to be useful in finding localized defects in the insulation as in a true HIPOT test, but rather give information on the quality of the bulk material used as the insulation.

Even when not required by the end customer, many wire and cable manufacturers use the insulation resistance test to track their insulation manufacturing processes, and spot developing problems before process variables drift outside of allowed limits.

 Selection of IR Testers (Megger):

  • Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available.
  • The recommended ratings of the insulation testers are given below:
Voltage Level IR Tester
650V 500V DC
1.1KV 1KV DC
3.3KV 2.5KV DC
66Kv and Above 5KV DC

 Test Voltage for Meggering:

  • When AC Voltage is used, The Rule of Thumb is Test Voltage (A.C) = (2X Name Plate Voltage) +1000.
  • When DC Voltage is used (Most used in All Megger), Test Voltage (D.C) = (2X Name Plate Voltage).
Equipment / Cable Rating DC Test Voltage
24V To 50V 50V To 100V
50V To 100V 100V To 250V
100V To 240V 250V To 500V
440V To 550V 500V To 1000V
2400V 1000V To 2500V
4100V 1000V To 5000V

 Measurement Range of Megger:

Test voltage Measurement Range
250V DC 0MΩ to 250GΩ
500V DC 0MΩ to 500GΩ
1KV DC 0MΩ to 1TΩ
2.5KV DC 0MΩ to 2.5TΩ
5KV DC 0MΩ to 5TΩ

 Precaution while Meggering:

Before Meggering:

  • Make sure that all connections in the test circuit are tight.
  • Test the megger before use, whether it gives INFINITY value when not connected, and ZERO when the two terminals are connected together and the handle is rotated.

During Meggering:

  • Make sure when testing for earth, that the far end of the conductor is not touching, otherwise the test will show faulty insulation when such is not actually the case.
  • Make sure that the earth used when testing for earth and open circuits is a good one otherwise the test will give wrong information
  • Spare conductors should not be meggered when other working conductors of the same cable are connected to the respective circuits.

After completion of cable Meggering:

  • Ensure that all conductors have been reconnected properly.
  • Test the functions of Points, Tracks & Signals connected through the cable for their correct response.
  • In case of signals, aspect should be verified personally.
  • In case of points, verify positions at site. Check whether any polarity of any feed taken through the cable has got earthed inadvertently.

Safety Requirements for Meggering:

  • All equipment under test MUST be disconnected and isolated.
  • Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.
  • Never use Megger in an explosive atmosphere.
  • Make sure all switches are blocked out and cable ends marked properly for safety.
  • Cable ends to be isolated shall be disconnected from the supply and protected from contact to supply, or ground, or accidental contact.
  • Erection of safety barriers with warning signs, and an open communication channel between testing personnel.
  • Do not megger when humidity is more than 70 %.
  • Good Insulation: Megger reading increases first then remain constant.
  • Bad Insulation: Megger reading increases first and then decreases.
  • Expected IR value gets on Temp. 20 to 30 decree centigrade.
  • If above temperature reduces by 10 degree centigrade, IR values will increased by two times.
  • If above temperature increased by 70 degree centigrade IR values decreases by 700 times.

How to use Megger:

  • Meggers is equipped with three connection Line Terminal (L), Earth Terminal (E) and Guard Terminal (G).

  • Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The “Guard” terminal is provided for special testing situations where one resistance must be isolated from another. Let’s us check one situation where the insulation resistance is to be tested in a two-wire cable.
  • To measure insulation resistance from a conductor to the outside of the cable, we need to connect the “Line” lead of the megger to one of the conductors and connect the “Earth” lead of the megger to a wire wrapped around the sheath of the cable.

  • In this configuration the Megger should read the resistance between one conductor and the outside sheath.
  • We want to measure Resistance between Conductor- 2To Sheaths but Actually Megger measure resistance in parallel with the series combination of conductor-to-conductor resistance (Rc1-c2) and the first conductor to the sheath (Rc1-s).
  • If we don’t care about this fact, we can proceed with the test as configured. If we desire to measure only the resistance between the second conductor and the sheath (Rc2-s), then we need to use the megger’s “Guard” terminal.

  • Connecting the “Guard” terminal to the first conductor places the two conductors at almost equal potential. With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the Megger’s resistance indication will be based exclusively on the current through the second conductor’s insulation, through the cable sheath, and to the wire wrapped around, not the current leaking through the first conductor’s insulation.
  • The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment. For example consider a two core cable with a sheath. As the diagram below shows there are three resistances to be considered.

  • If we measure between core B and sheath without a connection to the guard terminal some current will pass from B to A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cable cores will be at very nearly the same potential and thus the shunting effect is eliminated.

(1) IR Values For Electrical Apparatus & Systems:

(PEARL Standard / NETA MTS-1997 Table 10.1)

Max.Voltage Rating Of Equipment Megger Size

Min.IR Value


250 Volts

500 Volts

25 MΩ

600 Volts

1,000 Volts

100 MΩ

5 KV

2,500 Volts

1,000 MΩ

8 KV

2,500 Volts

2,000 MΩ

15 KV

2,500 Volts

5,000 MΩ

25 KV

5,000 Volts

20,000 MΩ

35 KV

15,000 Volts

100,000 MΩ

46 KV

15,000 Volts

100,000 MΩ

69 KV

15,000 Volts

100,000 MΩ

One Meg ohm Rule for IR Value for Equipment:

  • Based upon equipment rating:
  • < 1K V = 1 MΩ minimum
  • >1KV = 1 MΩ /1KV

As per IE Rules-1956:

  • At a pressure of 1000 V applied between each live conductor and earth for a period of one minute the insulation resistance of HV installations shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards.
  • Medium and Low Voltage Installations- At a pressure of 500 V applied between each live conductor and earth for a period of one minute, the insulation resistance of medium and low voltage installations shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards] from time to time.

As per CBIP specifications the acceptable values are 2 Mega ohms per KV

(2) IR Value for Transformer:

  • Insulation resistance tests are made to determine insulation resistance from individual windings to ground or between individual windings. Insulation resistance tests are commonly measured directly in megohms or may be calculated from measurements of applied voltage and leakage current.
  • The recommended practice in measuring insulation resistance is to always ground the tank (and the core). Short circuit each winding of the transformer at the bushing terminals. Resistance measurements are then made between each winding and all other windings grounded.

  • Windings are never left floating for insulation resistance measurements. Solidly grounded winding must have the ground removed in order to measure the insulation resistance of the winding grounded. If the ground cannot be removed, as in the case of some windings with solidly grounded neutrals, the insulation resistance of the winding cannot be measured. Treat it as part of the grounded section of the circuit.
  • We need to test winding to winding and winding to ground ( E ).For three phase transformers, We need to test winding ( L1,L2,L3 ) with substitute Earthing for Delta transformer or winding ( L1,L2,L3 ) with earthing ( E ) and neutral ( N ) for wye transformers.

IR Value for Transformer

(Ref: A Guide to Transformer Maintenance by. JJ. Kelly. S.D Myer)

Transformer Formula
1 Phase Transformer IR Value (MΩ) = C X E / (√KVA)
3 Phase Transformer (Star) IR Value (MΩ) = C X E (P-n) / (√KVA)
3 Phase Transformer (Delta) IR Value (MΩ) = C X E (P-P) / (√KVA)
Where C= 1.5 for Oil filled T/C with Oil Tank, 30 for Oil filled T/C without Oil Tank or Dry Type T/C.
  •  Temperature correction Factor (Base 20°C):
Temperature correction Factor



Correction Factor

























  • Example: For 1600KVA, 20KV/400V,Three Phase Transformer
  • IR Value at HV Side= (1.5 x 20000) / √ 1600 =16000 / 40 = 750 MΩ at 200C
  • IR Value at LV Side = (1.5 x 400 ) / √ 1600= 320 / 40 = 15 MΩ at 200C
  • IR Value at 300C =15X1.98= 29.7 MΩ

Insulation Resistance of Transformer Coil


Coil  Voltage

Megger Size


Min.IR Value Liquid Filled T/C


Min.IR Value Dry Type T/C

0 – 600 V


100 MΩ

500 MΩ

600 V To 5KV


1,000 MΩ

5,000 MΩ

5KV To 15KV


5,000 MΩ

25,000 MΩ

15KV To 69KV


10,000 MΩ

50,000 MΩ

 IR Value of Transformers:

Voltage Test Voltage (DC)  LV side Test  Voltage (DC) HV side Min IR Value
415V 500V 2.5KV 100MΩ
Up to 6.6KV 500V 2.5KV 200MΩ
6.6KV to 11KV 500V 2.5KV 400MΩ
11KV to 33KV 1000V 5KV 500MΩ
33KV to 66KV 1000V 5KV 600MΩ
66KV to 132KV 1000V 5KV 600MΩ
132KV to 220KV 1000V 5KV 650MΩ

 Steps for measuring the IR of Transformer:

  • Shut down the transformer and disconnect the jumpers and lightning arrestors.
  • Discharge the winding capacitance.
  • Thoroughly clean all bushings
  • Short circuit the windings.
  • Guard the terminals to eliminate surface leakage over terminal bushings.
  • Record the temperature.
  • Connect the test leads (avoid joints).
  • Apply the test voltage and note the reading. The IR. Value at 60 seconds after application of the test voltage is referred to as the Insulation Resistance of the transformer at the test temperature.
  • The transformer Neutral bushing is to be disconnected from earth during the test.
  • All LV surge diverter earth connections are to be disconnected during the test.
  • Due to the inductive characteristics of transformers, the insulation resistance reading shall not be taken until the test current stabilizes.
  • Avoid meggering when the transformer is under vacuum.

Test Connections of Transformer for IR Test (Not Less than 200 MΩ):

  • Two winding transformer:
  1. (HV + LV) – GND
  2. HV – (LV + GND)
  3. LV – (HV + GND)
  • Three winding transformer:
  1. HV – (LV + TV + GND)
  2. LV – (HV + TV + GND)
  3. (HV + LV + TV) – GND
  4. TV – (HV + LV + GND)
  • Auto transformer (two winding):
  1. (HV + LV) – GND
  • Auto Transformer (three winding):
  1. (HV + LV) – (TV + GND)
  2. (HV + LV + TV) – GND
  3. TV – (HV + LV + GND)

For any installation, the insulation resistance measured shall not be less than:

  • HV – Earth 200 M Ω
  • LV – Earth 100 M Ω
  • HV – LV 200 M Ω

 Factors affecting on IR value of Transformer

The IR value of transformers are influenced by

  • surface condition of the terminal bushing
  • quality of oil
  • quality of winding insulation
  • temperature of oil
  • duration of application and value of test voltage

(3) IR Value for Tap Changer:

  • IR between HV and LV as well as windings to earth.
  •  Minimum IR value for Tap changer is 1000 ohm per volt service voltage

 (4) IR Value for Electric motor:

For electric motor, we used a insulation tester to measure the resistance of motor winding with earthing ( E ).

  • For rated voltage below 1KV, measured with a 500VDC Megger.
  • For rated voltage above 1KV, measured with a 1000VDC Megger.
  • In accordance with IEEE 43, clause 9.3, the following formula should be applied.
  • Min IR Value (For Rotating Machine) =(Rated voltage (v) /1000) + 1

As per IEEE 43 Standard 1974,2000

IR Value in MΩ
IR (Min) = kV+1 For most windings made before about 1970, all field windings, and others not described below
IR (Min) = 100 MΩ For most dc armature and ac windings built after about 1970 (form wound coils)
IR (Min) = 5 MΩ For most machines with random -wound stator coils and form-wound coils rated below 1kV
  • Example-1: For 11KV, Three Phase Motor.
  • IR Value =11+1=12 MΩ but as per IEEE43 It should be 100 MΩ
  • Example-2: For 415V,Three Phase Motor
  • IR Value =0.415+1=1.41 MΩ but as per IEEE43 It should be 5 MΩ.
  • As per IS 732 Min IR Value of Motor=(20XVoltage(p-p/(1000+2XKW))

IR Value of Motor as per NETA ATS 2007. Section 7.15.1

Motor Name Plate (V) Test Voltage Min IR Value
250V 500V DC 25 MΩ
600V 1000V DC 100MΩ
1000V 1000V DC 100MΩ
2500V 1000V DC 500MΩ
5000V 2500V DC 1000MΩ
8000V 2500V DC 2000MΩ
15000V 2500V DC 5000MΩ
25000V 5000V DC 20000MΩ
34500V 15000V DC 100000MΩ

IR Value of Submersible Motor:

IR Value of Submersible Motor

Motor Out off Well (Without Cable) IR Value
New Motor 20 MΩ
A used motor which can be reinstalled 10 MΩ
Motor  Installed in Well (With Cable)  
New Motor 2 MΩ
A used motor which can be reinstalled 0.5 MΩ

 (5) IR Value for Electrical cable and wiring:

  • For insulation testing, we need to disconnect from panel or equipment and keep them isolated from power supply. The wiring and cables need to test for each other ( phase to phase ) with a ground ( E ) cable. The Insulated Power Cable Engineers Association (IPCEA) provides the formula to determine minimum insulation resistance values.
  • R = K x Log 10 (D/d)

  • R =IR Value in MΩs per 1000 feet (305 meters) of cable.
  • K =Insulation material constant.( Varnished Cambric=2460, Thermoplastic Polyethlene=50000,Composite Polyethylene=30000)
    D =Outside diameter of conductor insulation for single conductor wire and cable
  • ( D = d + 2c + 2b diameter of single conductor cable )
    d – Diameter of conductor
    c – Thickness of conductor insulation
    b – Thickness of jacket insulation

HV test on new XLPE cable (As per ETSA Standard)

Application Test Voltage Min IR Value
New cables – Sheath 1KV DC 100 MΩ
New cables – Insulation 10KV DC 1000 MΩ
After repairs – Sheath 1KV DC 10 MΩ
After repairs – Insulation 5KV DC 1000MΩ

11kV and 33kV Cables between Cores and Earth (As per ETSA Standard)

Application Test Voltage Min IR Value
11KV New cables – Sheath 5KV DC 1000 MΩ
11KV After repairs – Sheath 5KV DC 100 MΩ
33KV no TF’s connected 5KV DC 1000 MΩ
33KV with TF’s connected. 5KV DC 15MΩ

IR Value Measurement (Conductors to conductor (Cross Insulation))

  • The first conductor for which cross insulation is being measured shall be connected to Line terminal of the megger. The remaining conductors looped together (with the help of crocodile clips) i. e. Conductor 2 and onwards, are connected to Earth terminal of megger. Conductors at the other end are left free.
  • Now rotate the handle of megger or press push button of megger. The reading of meter will show the cross Insulation between conductor 1 and rest of the conductors. Insulation reading shall be recorded.
  • Now connect next conductor to Line terminal of the megger & connect the remaining conductors to earth terminal of the megger and take measurements.

IR Value Measurement (Conductor to Earth Insulation)

  • Connect conductor under test to the Line terminal of the megger.
  • Connect earth terminal of the megger to the earth.
  • Rotate the handle of megger or press push button of megger. The reading of meter will show the insulation resistance of the conductors. Insulation reading shall be recorded after applying the test voltage for about a minute till a steady reading is obtained.

 IR Value Measurements:

  • If during periodical testing, insulation resistance of cable is found between 5 and 1 /km at buried temperature, the subject cable should be programmed for replacement.
  • If insulation resistance of the cable is found between 1000 and 100 /km, at buried temperature, the subject cable should be replaced urgently within a year.
  • If the insulation resistance of the cable is found less than 100 kilo ohm/km., the subject cable must be replaced immediately on emergency basis.

 (6) IR Value for Transmission / Distribution Line:

Equipment.                Megger Size Min IR Value
S/S .Equipments 5 KV 5000MΩ
EHVLines. 5 KV 10MΩ
H.T. Lines. 1 KV 5MΩ
LT / Service Lines. 0.5 KV 5MΩ

(7) IR Value for Panel Bus:

  • IR Value for Panel = 2 x KV rating of the panel.
  • Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 MΩ.

 (8) IR Value for Substation Equipment:

Generally meggering Values of Substation Equipments are.

.Typical IR Value of S/S Equipments

Equipment   Megger Size IR Value(Min)

Circuit Breaker


5KV,10 KV

1000 MΩ


5KV,10 KV

1000 MΩ

Control Circuit


50 MΩ



5KV,10 KV

1000 MΩ


5KV,10 KV

50 MΩ

Control Circuit


50 MΩ



5KV,10 KV

1000 MΩ


5KV,10 KV

1000 MΩ

Control Circuit


50 MΩ



5KV,10 KV

1000 MΩ

Electrical Motor



50 MΩ

LT Switchgear



100 MΩ

LT Transformer



100 MΩ

IR Value of S/S Equipments As per DEP Standard



IR Value at Commissioning Time (MΩ)

IR Value at Maintenance Time(MΩ)


HV Bus

200 MΩ

100 MΩ

LV Bus

20 MΩ

10 MΩ

LV wiring

5 MΩ

0.5 MΩ

Cable(min 100 Meter)


(10XKV) / KM

(KV) / KM

Motor & Generator




Transformer Oil immersed


75 MΩ

30 MΩ

Transformer Dry Type


100 MΩ

25 MΩ


10 MΩ

2 MΩ

Fixed Equipments/Tools


5KΩ / Volt

1KΩ / Volt

Movable Equipments


5 MΩ


Distribution Equipments


5 MΩ


Circuit Breaker

Main Circuit

2 MΩ / KV


Control Circuit




D.C Circuit-Earth



LT Circuit-Earth



LT-D.C Circuit






 (9) IR Value for Domestic /Industrial Wiring:

  • A low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakage current. This cause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the installation.
  • The resistance between Phase-Phase-Neutral-Earth must never be less than 0.5 M Ohms for the usual supply voltages.
  • In addition to the leakage current due to insulation resistance, there is a further current leakage in the reactance of the insulation, because it acts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but we wish to measure the resistance of the insulation, so DC Voltage is used to prevent reactance from being included in the measurement.

 1 Phase Wiring:

  • The IR test between Phase-Natural to earth must be carried out on the complete installation with the main switch off, with phase and neutral connected together, with lamps and other equipment disconnected, but with fuses in, circuit breakers closed and all circuit switches closed.
  • Where two-way switching is wired, only one of the two stripper wires will be tested. To test the other, both two-way switches should be operated and the system retested. If desired, the installation can be tested as a whole, when a value of at least 0.5 M Ohms should be achieved.

3 Phase Wiring:

  • In the case of a very large installation where there are many earth paths in parallel, the reading would be expected to be lower. If this happens, the installation should be subdivided and retested, when each part must meet the minimum requirement.

  • The IR tests must be carried out between Phase-Phase-Neutral-Earth with a minimum acceptable value for each test of 0.5 M Ohms.

IR Testing for Low voltage

circuit voltage Test voltage IR Value(Min)
Extra Low Voltage 250V DC 0.25MΩ
Up to 500 V except for above 500 V DC 0.5MΩ
500 V To 1KV 1000 V DC 1.0MΩ
  •  Min IR Value = 50 MΩ / No of Electrical outlet. (All Electrical Points with  fitting & Plugs).
  • Min IR Value = 100 MΩ / No of Electrical outlet. (All Electrical Points without fitting & Plugs).

 Required Precautions:

  • Electronic equipment like electronic fluorescent starter switches, touch switches, dimmer switches, power controllers, delay timers could be damaged by the application of the high test voltage should be disconnected.
  • Capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will result.
  • Where any equipment is disconnected for testing purposes, it must be subjected to its own insulation test, using a voltage which is not likely to result in damage. The result must conform with that specified in the British Standard concerned, or be at least 0.5 M Ohms if there is no Standard.

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

174 Responses to Insulation Resistance (IR) Values

  1. modupalli says:

    Outstanding Stuff…. Thanks For Sharing..

    • Deja jacob says:


    • mahesh chander gupta says:

      i wish to contact mr parmar by e mail

    • SIVA NAIK says:

      Thanks for your information

      How to measure the IR value across the (XLPE, 1.1 kV grade) straight through joint?


      • MATHEW KK says:

        Thanks for sharing information and knowledge on the topic. These information is useful to even technician level for understanding the theory of testing using Megger. I like to understand the information and knowledge thoroughly. Thanks for the dedication exhibited.

  2. vilas K. says:

    Dear Sir Hats Off !! & many thanx for your invaluable contribution in enriching the experience of Elect. Engineers & making them available the fabulous data at a glance.. Pl keep posted. If U can throw some light on selection of MCB/MCCB & contactors in relation to their diff characteristics.

  3. varghese says:

    dear sir… is a great work…. thanks

    could u pls explain the effects of rise in temperature on insulation resistance of transformer.

  4. Barun Majumder says:

    Dear sir many many thanks for sharing valuable article of electrical engineering . I want to know more about electrical design (Distribution, industrial & commercial ) and protection system from your end point to point . If you have any book/site for those design please let me know the name.
    I am working as a design engineer but i am not enough confident about my design and protection system. i want your valuable assistance in this field .

    Thanking you
    Yours faithfully
    Barun Majumder


  5. kumereshan P S says:

    great work,i want to know the IS standards for IR value of Sub station equipment

    With regards


  6. Honestly i am not quite good on electric thing and this one is very good for me to study so well and learn a lot many thanks for this article.

  7. keyur says:

    Thanks for sharing your valuable exp. and it is very helpful for me because I am diploma holder and I am working in gulf country as electrical supervisor so it is useful for me and

  8. MANOHAR says:

    Dear sir

    I need calculation formula of IR Values of an elect drives

  9. Madanmohan says:

    Wonderful explanation. Really very much useful document. Thnak you very much

  10. sathesh says:

    Very nice sir.. Thanku..

  11. rehan says:

    good information, thanks for your time.

  12. custom milagros says:

    2 thumbs up.

  13. Gurmeet Singh says:

    Very -2 Thanks sir

  14. preeti says:

    thanks, very good information

  15. osarenkhoe nosa glory says:

    this is absolutely nice. a well written document. thanks.

  16. Naveen says:

    Thank you Sir, much appreciated

  17. A. Hasasn says:

    Dear Sir;
    Really your this article is very very important for electrical related people.Your this contribution is excellant. A lot of thanks sir.
    Kind Regardrs

  18. K.M.Pandey says:

    Respected Sir.

    This article very important and useful for all those person who are related with Electrical field. Thanks a lot for sharing your knowledge with all people. Really you are nice and gentleman.
    With regards,

  19. ahmed nawaz says:

    Ch.Ahmed Nawaz

    thanks sir for giving very usefull information.

  20. radhakishan pawar says:

    thanks sir i have no word to explain how much it is useful to student.
    thanks again……

  21. THAKOR SANJAY says:


  22. Vilas G. says:

    Thanks . valuable note for electrical engineers.

  23. Steve Arreola says:

    Thank you much for taking the time to share this information

  24. Mr. Judy Diano says:

    Thank you so much,for sharing Sir, I learned a lot of your link GOD BLESS

  25. arshad says:

    thanks for beautiful notes

  26. Jet Paliangayan says:

    how to do IR test for single core pvc/xlpe/pvc cables

  27. jamestryre says:

    great info, thx

  28. Thiyaga says:

    Incredible work. Hats off for your awesome articles. pl keep posting more and more

  29. sambasivarao chagantipati says:

    Very useful information

  30. rohtash verma( Himachal pradesh) says:

    Thanks parmar ji very use full information regarding IR value of electric motors

  31. M.Zaman says:

    thanks for valuable information.

  32. FELIX Jr. says:

    Please, you must also include here the Polarization Index (PI) as well for it’s significance and explain why it is required with respect to the IR test being done on the electrical equipment and the electrical cables and installations, as an additional information for the new commissioning engineers and enthusiasts. Supressing other comments are not at all interesting and helpfull.

    • Yes You are right PI value is as important as IR Value specially HV Equipments.
      Here I gave only Overview of IR Testing and it’s value.
      To rise comment is totally depend upon the reader.
      In near future I will definitely cover the Topic of PI.

      • deepak parmar says:

        I want to know the acceptable leakage current values in the HV testing as per IEC standards.

        And thanks for the info provided above

  33. sateesh says:

    excellent explanation about IR test
    thanks a lot.

  34. vincent says:

    Good knowledge.thank you.

  35. Anderson says:

    Excelente trabajo, quiero saber cuales son las normas que rigen estas pruebas y si existe alguna estandarización de resultados. Gracias

  36. Thanks for such a great article. Very interesting and informative. Thanks Buddy

  37. Diganta ku. Ray says:

    Thank you so much sir,this help me a lot in my field.its very very useful for those are in electrical line.

  38. JAYESH PATEL says:

    how many Insulation Resistance of the Whole Installation permissible ?
    how many Insulation Resistance Between Conductors ?
    how many Insulation Resistance Between Each Conductor & Earth ?

  39. Hello very nice blog!! Man .. Excellent .. Amazing .. I will bookmark your web site and take the feeds additionally?

    I am glad to find numerous helpful info here in the put up, we want work out more strategies on this regard, thank you for sharing.

    . . . . .

  40. fayyaz says:

    thks good information can you mail me how to check insulation of dc 24 volt system.
    also i want to know how insulation monitor works.

  41. sonu kumar says:

    thanks for this valueable data.

  42. Excellent information. Really appreciate the knowledge you have shared with readers.
    Sir can you pls tell me what should be minimum value of IR for PVC make busboot used for 11 KV , 22 KV , 33 KV and 66 KV.

    Debashish Bhattacharya

  43. Gerald says:

    Thanks for sharing…

  44. CPR vittal says:

    excellent article.
    the formula for insulation resistance of a cable per 1000 ft.
    how to calculate it per meter or per feet.
    pl explain

  45. what precautions i should take when CT is connected in HT board and i have do megger.?

  46. ibnu says:

    lightning cable for crane – there are 2 point earth terminal, each has individual ground cable, i meggar 1kV both cable, gives different reading 200Mohm & almost infinity for another one. why? can accept the value?

  47. ibnu says:

    can 11kv cable, meggar at 1kv only, during before & after hipot test? (hipot at 30.5kv)

  48. parveen kumar says:

    dear sir i want to know why used in transformer secondary side very thin wire

  49. FELIX Jr. says:

    Yes, you can use 1kV DC- MEGGER TESTER prior to HIPOT Testing if your electrical cable is rated 11kV and HIGHER and if it has 130 % rated insulation. After HIPOT test, if successful, you can megger it again with 1kV only at no more than 1 minute just to make sure no damage has occurred during and after HIPOT TEST.

    By FMSJr. / Abu Dhabi, UAE

  50. Nasurudheen Kamal Basha says:

    Useful stuff.. Thanks for sharing!!

  51. MOHAMMAD says:


  52. Pushkar Singh says:

    Thanks for sharing excellent information.
    Pushkar Singh ONGC

  53. A V N Phanindra says:

    thank u so much sir its very brief and simple to understand clearly.

  54. ASMA says:


    • FELIX Jr. says:

      GOOD DAY ! Your posted question about the MEGGER result for a good condition for 25kVA Transformer is very general as you did not specify the input and output voltages of this electrical equipment and if it is oil immersed or dry type. At anyrate, assuming it’s oil filled step-up transformer, outdoor pole mounted, for a good IR result for primary winding, normally it should not be less than 500Megaohm and the secondary winding’s IR reading should be not less than 300Megaohm with respect to earth and higher between windings. Again, these are assumptions only and the higher the voltage rating of the transformer the higher IR reading should result. Defective insulations will read low values of IR readings and contaminated oil insulation, for oil immersed type transformers. For dry type Transfomers, the good IR readings of the windings are also in that vicinity but lower than oil immersed type.

  55. J.R Bohra says:

    very very helpful
    J,R Bohra
    Unique Engineer Noida

  56. Diptiranjan Nayak says:

    Thank u sir,
    I am clear about IR value.

  57. raghav says:

    dear sir,
    i need more information about how to calculate ir values and what principle it requires to calculate ir values?

  58. Musa says:

    Anybody can answer my query?

    We have 25kV AC system for Overhead Line Equipment (OHE). This system is installed for Electrified Train system (Railway project).

    We have conduct IR testing before charging the lines for each electrical subsection (the longest section about 20km). The result we achieved 0.5MOhm – 5MOhm (taken during night time) and 200MOhm – 700MOhm (taken during night time). The temperature during night time is around 27 – 28 degree Celsius (in Malaysia).

    We injecting megger size 5kV and result achieved as above. As mentioned in the article (i think it is similar to our system – IR value for transmission/ distribution line) which is stated Min. IR values shall be 10MOhm.

    Is it the result we achieved okay, especially the result taken during night time?

  59. sampath says:

    thank for more information now i harry to check insulation resistance long 132Kv transmission line & will have called you after testing

  60. SYAM says:

    Dear Jignesh,

    Just for curiosity can you please tell me what is the full form of “DEP” standards which you have mentioned under the section “IR Value of S/S Equipments As per DEP Standard”.

    Hope you will reply



  61. Sekar Ram says:

    Mr Jignesh
    This is one of the greatest selfless efforts I have seen. This is pure social service. Hats off!!

  62. Stephen says:

    Hello. Question, if we have 5 conductors per phase on a lugs and we want to megger the groups, ie: phase to phase and phase to ground and the readings are above 1 M ohm, is that OK? We had previously meggered all the cabels individually prior to installing. The job was shut down for a while, and this is for a precaution, prior to re-energizing.

  63. vijay says:

    dear jignesh sir,

    Can i have the entire notes in site in form of book? How much does it cost?

  64. very very useful and very thanks for made me doubt clear..

  65. Michel says:

    thanks Bro that is too goooooooooooooood for us

  66. ESAKKIPANDI says:

    the explanation is excellent

  67. rahul jangme says:

    thank you sir

  68. ali says:

    thank you so much Dr

  69. Doma Hudu says:

    thank sir. I need a copy via my e-mail. please am wait for your response Sir.

  70. Srinivasan says:

    Thank you sir. Useful detail information.

  71. aby says:

    appreciate your ideas and procedures.
    May i know ,whether there is any necessity or mandatory rules saying that the speaker cables to be meggered?
    if yes,then what should be the applied voltage?

  72. roop says:

    dear mr Parmar first of all thanks for sharing the information

  73. roop says:

    we are building a 9 km road tunnel, we usually facing problem with ht cable, whenever it is damaged it took 3 to4 hour for jointing, lot of production loss please advise any alternate means of transmission of ht cable or any ht joint available around the world to do the jointing quickly.

  74. Felix Jr San Buenaventura says:


  75. SB says:

    Thank you ever so much! For sharing such good info, extremely relevant.

  76. rd says:

    What is the minimum acceptable insulation resistance for 1.8 kv DC cables? temperature is around 50 deg C. My understanding is that IR decreases when there is increase in temperature. So how do i calculate Insulation resistance considering the temperature?

  77. jake says:

    what can we expect for transformer being energize not meeting the minimum standard for insulation resistance?the future effect of it?

  78. golfziimo says:

    thank you for good information

  79. Aen Bali says:

    Plz tell me CTPT value in HT & LT side proper load

    50 Hp

  80. reshi says:

    Dear sir ,
    we have hayward tyler 480V, 60HZ , 100HP submersible pump motor in out facility used for boiler water circulation. The demin water is used for the stator cooling in the closed loop means water is directly entering the stator winding.

    The motor was send to workshop for mechanical repair. After installing back The I.R value is 0.3MOhm , can we test the motor and run it.

    For your suggestion Thanks in anticipation.

  81. praneeth says:

    thank U for the post,very helpful
    very few people share their knowledge.
    hats off.

  82. Abdul Hameed says:

    good information. appreciate it very much. thanks to all

  83. nacykoe says:

    IR instrument from IEC or IEEE standard?

  84. Jagadish says:

    What should be the minimum value of Insulation resistance for Low Voltage Switchgear panels?

  85. V V G says:

    Dear Sir
    In our one of the applications we need to check the High Voltage test in our product,(Automotive part).where customer is asking ask to check the HV test with 1.5KV

    As per customer demand,now we need to make a HV instrument to detect the current leakage when applyinh High volt(1.5KV) between two terminals.(open circuit)
    Customer requirement is that when the distance between the terminals if it is less than 1.2mm our instrument must detect it.
    Now we are facing the problem as how to derive the resistance between two terminals(in air) when we applying high volt.Kindly help us


  86. dipti says:

    very good stuff,so usefull…………

  87. nikesh srst says:

    thanks for help us. to write about its

  88. Sam says:

    Hello Jignesh Parmar,

    Many thanks for your above useful article.

    I have a quick question and will appreciate it if you can help me to find the answer please. It is about IR Value for Electrical 11kV and 22kV cables with XLPE insulation. As far as I know, there is a min value for IR in ambient temperature, but my question is : “is there any high limit for this value in ambient temperature?”

    I am asking this question because I have got a test result from a factory with very very high values more than usual for IR in ambient temperature.

    Thanks and looking forward to receive your answer.

  89. ashish says:

    132/33kv transformer the LV resistance is.02 per unit resistance referred to HV

  90. himanshu patel says:

    such a knowledgeable thing about our work for reference to introduce us in smart work

  91. Asif says:

    Dear all,

    A question is that what is the IR acceptance criteria for bus bars which are installed in MCC panels?

  92. K.Raghuveer says:

    Dear sir,

    Thanks for the information,


  93. kushru says:

    I would like to know if a wire rated for 1100v ac can withstand a breakdown voltage of 4 kv

  94. karan jani says:

    sir Thank you for sharing your Knowledge i just have one query related to the H.v Cable test. Is is mandatory to have PI Test on H.v Cables along with the Ir value?

  95. Akhtar K says:


    Very nice and good information of electrical- IR.

    Thanks for sharing the all necessary technical information.

    With all respect;
    Akhtar Khatib

  96. Sujeet Dubey says:

    Thanks Sir
    Its very complicated share.

  97. lalit Kalihari says:

    Hello Mr. Jignesh Parmar,

    Please let me know the documentary evidence of acceptable IR limits refering to IEC 61439 standard.

    Thanks & Regards,
    Lalit KK

  98. Owais says:

    Please give IR value information of Core-Earth, Frame-Earth, Core-Frame.Also give their correction factor as per temperature. Also please indicate whether the Temperature difference is of Insulation or Ambient.

  99. E. Eng. Ghanim says:

    I want to know the value measurements of resistance for motor it is capacity is 1MW & the applied voltage is 600V because I have a re-winding motor and I want to comparing the results with slandered values

  100. NAEEM says:

    dear what should be the minimum insulation Resistance (IR) of AC Generator in accordance to IEC standards both at the time of Commissioning and at the time of Maintenance

  101. very valuable writeup with lot of experience behind it

  102. jangchu Sinhgye says:

    Sir what is the minimum megger value of IR for 120kN disc insulator ?

  103. Ksalvi says:

    Can we do meggering of Power cables in Sand storm. If not then why??

  104. mahalingam says:

    very thank you sir, very useful my carrier


    i have a challenge with my megger. I disconnected the cables to be meggered. and set my test voltage to 500V. But when i start my test i get my test voltage decreasing….and eventually an IR reading in Kilo ohms… whilst my expected value is at least 1 Giga Ohm. kindly advice. Thanks

  106. pradeep says:

    what is the difference between insulation resistance and PI test

  107. PLJ says:

    Very useful document. Thank you

  108. zen says:

    not megger – should be insulation resistance meter

  109. S.G.Jayakumar says:

    whict type of wires are used to IR test.PVC,PUR,Teflon insulation wires.Please send the reply I am struugling the one project

  110. Phu Ta Quoc says:

    Ok, Thank you very much.

  111. kesarinath ullengal says:

    Your information regarding allt the test is very much usefull.Is there any app for this for andriod so it wìll help to easy view and information.Thanks once again
    kesari ullengal

  112. Raj Kumar says:

    Thanks for such valuable data shared.

  113. Ronal Manuel says:

    Does anyone know how to compute the minimum recommended value for busbars. let’s say from generator to transformer.

  114. Rajashekar.V says:

    Hello Jignesh Parmar,

    Many thanks for your above useful article.

    I have a quick question and will appreciate it if you can help me to find the answer please. It is about megger instrument which haveing (analog instrument with hand generater) the analog reading needel inthe right and moves to left but in all the instruments it will in left to right Why?

    Thanks and looking forward to receive your answer.

  115. raj says:

    thank you so much

  116. arif says:

    Dear sir
    what is the minimum insulation resistance value is acceptable for 415 volt LV cable

    • ghanim yousif says:

      Dear Sir The minimum insulation resistance value acceptance for 415V should be at least 1KV & not more than 2KV BR Ghanim

      On Thu, Feb 4, 2016 at 4:04 PM, Electrical Notes & Articles wrote:

      > arif commented: “Dear sir what is the minimum insulation resistance value > is acceptable for 415 volt LV cable” >

  117. Nitin vishwakarma says:

    thank you sir vary information.

  118. bharat says:

    explanation good

  119. Kiran Ufade says:

    useful information sir thanks

  120. Saurabh Gupta says:

    I want to know IR value for 160 KN & 210 KN Composite Polymer Long Rod Insulators using in Transmission line tower

  121. Saurabh Gupta says:

    Jignesh Sir, I want to know IR values of 160 KN and 210 KN composite polymer long rod insulators, which we are using now a days instead of Porcelain types.

  122. ajojacob says:

    what is the IR acceptance criteria for LV to earth in transformer

  123. Magdy souror says:

    very useful information.
    Thanks for the effort done…

  124. Asif Nazir says:

    Great work

  125. Zaine Binti Abdul Karim says:

    Hi Mr.Jignesh Parmar, thank you for your technical write-up it is very useful. However, I would like to ask what is the recommended IR value for a Fuel Gas Superheaters (powered from 400V AC, load is 550kW). Heaters IR is not covered in your article, can you assist me on this, because we do have some issues with our present installation.

    For the rest of readers, kindly chip in too if you guys know the answer to my question above. Thank you all.

  126. Amila De Silva says:

    Hi, one of our drainage pump installed in a dry well caught to flood. now when we megger it including the cable ( now the well is dry) it shows zero. what can be the reason.

    Please advise

  127. Raja pandiyar says:

    Good detail’s for us

  128. Manish Gupta says:

    Hi , what is the hipot test voltage for 24VDC field wire line to secondary side ( for an digital IO card) used in PLC product. I believe it is 500Vrms or 707 VDC, can you please confirm.

  129. sreedevi says:

    How much dc voltage has to be applied in megger for measuring IR in 4 pair cable harness

  130. YEE GUO QUAN says:

    I’ve seen other ppl doing megger test where they short L1,L2,L3,N to E and get the ohm value.
    Is this the correct method? if so how does it work? mind to share? TQ

    Warmest Regards,

    Danny Yee

  131. Tushar Mahajan says:


  132. Nilesh Amrelia says:

    Interesting information shared. Thanks

  133. Aman says:

    how many Ir Value of 120 & 210kn insulator

  134. Martin says:

    Dear Parmar..

    Thanks for the Usefull Post..

    Its very Helpfull for the Beginner level of the Megger Tester…

    Keep it Up…

  135. IEC says: provide easy way to get IEC within 3 to 7 working days. if you are looking iec consultant in India, Please call us today +91 9540195222

  136. Gaurav Shah says:

    Thanks Jigneshbhai

  137. Ganesh Nama says:

    dear sir, this information is very use full for us people, about basic knowledge.

    thank you very much.

  138. Rahul Gupta says:

    Can you suggest the suitable Megger value for a 33kV Vacuum Circuit Braker ?

  139. Rahul Gupta says:

    Please tell me the suitable insulation resistance value for interrupter in open condition for a 33kV Vacuum Circuit Breaker

  140. C.Baskar says:

    Dear sir,
    we have 50MW 11KV Generator under preservation what will be the safe minimum IR value to maintain for stator and Rotor also 11KV/110KV 52/68 MVA Transformer under preservation -what will be the safe minimum IR value for both HV/LV windings.

  141. Giorgos says:

    Thank you very much for these information.
    I would like to ask if the Extech 380396 insulation resistance meter is suitable for substation measurements??

  142. Alozie solomon says:

    i need to understand the minimum value for 1kv cable. I got some values but need to understand if am correct.

  143. Arka Nandi says:

    Better for all

  144. shiva prasad says:

    very good for giving fundamentals and standards.

  145. topics are worth reading

  146. DPGupta says:

    Tell me, the insulation resistance per km of a 100meter length of a two core cable sized 2.5mm²

  147. fendi says:

    Can we use 5 kV DC to check the insulation value of inductive voltage transformer? Can we assume the acceptable value must be more than 1000Mohm?

  148. PC YADAV says:

    Dear sir, this information is very useful for me and to increase my knowledge.

    thank you very much.

  149. rajesh Thapa says:

    can any one explain me the different between 1.1kV for 220/ 415 V and 1kV for 220/415V since some suggest 1.1 volt grade cable to select and some said to go for 1.0kV for internal house wiring cable selection. Normally i used to go for 1.1kV for LT cable. Can any one explain & clarifyplease. me

  150. rajesh Thapa says:

    Reasons for using different kV rating for selecting house wiring cables, which to go for 1.1kV or 1.0 kV rating for 200 to 440 V . please leave valuable comments and suggestion .

  151. Dipankar Sharma says:

    I would like to get a Test result of a 2000KG Lift Installation:

    1. Insulation Resistance for the whole installation,
    2. Between Phases,
    3. Between each phase and earth,

    4. Resistance of each electrode for earthing system,
    5. Maximum earthing resistance of installation.

    Can any one please help me what would be the values I suppose to get.

    This required for reference purpose.

  152. Erdinç says:

    Hi to all;
    One question to calculate cable wire, IR, with a unit MOhmxkm
    the formula is: R = K x Log 10 (D/d)
    D and d should be in mm unit, m umit or km unit while calculating.

  153. Sibu Chakrabarti says:

    IR or VR values of Sioplas XLPE insulation is coming higher at 90 deg. C than at Room Temp. Can you explain it?

  154. Souvik Ghosh says:

    Its Souvik Ghosh from Kolkata. Operating KV Rating of Porcelain Wall Through Bushing Insulator is 90 KV. What should be megger rating of It.

  155. Hi sir, this information is very useful for me and to increase my knowledge.

    thank you so much.

  156. hninwintsan says:

    thanks a lot

  157. Highly useful information for all electrical engineers. Appriciate your efforts. Would like to contact through your e mail

  158. Dinesh Subudhi says:

    What are the necessary conditions for a voltage stabilizer terminal to body insulation test?

  159. opv038 says:

    for 6.6 kv generator set , what should be the test voltage (HV). please answer with formula

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