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Home / Technical Articles / Measurement of insulation resistance (IR) – 2
Fluke insulation resistance tester up to 10kV
Fluke insulation resistance tester up to 10kV

Continued from first part: Measurement of insulation resistance (IR) – Part 1

Insulation Resistance (IR) Values – Index

1. IR Values For Electrical Apparatus & Systems
2. IR Value for Transformer
3. IR Value for Tap Changer
4. IR Value for Electric motor
5. IR Value for Electrical cable and wiring
6. IR Value for Transmission / Distribution Line
7. IR Value for Panel Bus
8. IR Value for Substation Equipment
9. IR Value for Domestic /Industrial Wiring
0. Required Precautions

1. IR Values For Electrical Apparatus & Systems

(PEARL Standard / NETA MTS-1997 Table 10.1)

Max.Voltage Rating Of Equipment Megger SizeMin.IR Value
250 Volts500 Volts25 MΩ
600 Volts1,000 Volts100 MΩ
5 KV2,500 Volts1,000 MΩ
8 KV2,500 Volts2,000 MΩ
15 KV2,500 Volts5,000 MΩ
25 KV5,000 Volts20,000 MΩ
35 KV15,000 Volts100,000 MΩ
46 KV15,000 Volts100,000 MΩ
69 KV15,000 Volts100,000 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.

Insulation resistance testing: HV - Earth and HV - LV
Insulation resistance testing: HV – Earth and HV – LV

Transformer 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)
TransformerFormula
1 Phase TransformerIR 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
OCOFCorrection Factor
0320.25
5410.36
10500.50
15590.720
20681.00
30861.98
401043.95
501227.85

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

Transformer Coil  VoltageMegger SizeMin.IR Value Liquid Filled T/CMin.IR Value Dry Type T/C
0 – 600 V1KV100 MΩ500 MΩ
600 V To 5KV2.5KV1,000 MΩ5,000 MΩ
5KV To 15KV5KV5,000 MΩ25,000 MΩ
15KV To 69KV5KV10,000 MΩ50,000 MΩ

IR Value of Transformers

VoltageTest Voltage (DC)  LV sideTest  Voltage (DC) HV sideMin IR Value
415V500V2.5KV100MΩ
Up to 6.6KV500V2.5KV200MΩ
6.6KV to 11KV500V2.5KV400MΩ
11KV to 33KV1000V5KV500MΩ
33KV to 66KV1000V5KV600MΩ
66KV to 132KV1000V5KV600MΩ
132KV to 220KV1000V5KV650MΩ

 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 windings)
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
Insulation resistance (IR) value for electric motor
Insulation resistance (IR) value for electric motor

As per IEEE 43 Standard 1974, 2000
IR Value in MΩ
IR (Min) = kV+1For most windings made before about 1970, all field windings, and others not described below
IR (Min) = 100 MΩFor most dc armature and ac windings built after about 1970 (form wound coils)
IR (Min) = 5 MΩFor 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 VoltageMin IR Value
250V500V DC25 MΩ
600V1000V DC100MΩ
1000V1000V DC100MΩ
2500V1000V DC500MΩ
5000V2500V DC1000MΩ
8000V2500V DC2000MΩ
15000V2500V DC5000MΩ
25000V5000V DC20000MΩ
34500V15000V DC100000MΩ

IR Value of Submersible Motor:

IR Value of Submersible Motor
Motor Out off Well (Without Cable)IR Value
New Motor20 MΩ
A used motor which can be reinstalled10 MΩ
Motor  Installed in Well (With Cable)
New Motor2 MΩ
A used motor which can be reinstalled0.5 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)

ApplicationTest VoltageMin IR Value
New cables – Sheath1KV DC100 MΩ
New cables – Insulation10KV DC1000 MΩ
After repairs – Sheath1KV DC10 MΩ
After repairs – Insulation5KV DC1000MΩ

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

ApplicationTest VoltageMin IR Value
11KV New cables – Sheath5KV DC1000 MΩ
11KV After repairs – Sheath5KV DC100 MΩ
33KV no TF’s connected5KV DC1000 MΩ
33KV with TF’s connected.5KV DC15MΩ

 

11kV and 33kV Cables between Cores and Earth
11kV and 33kV Cables between Cores and Earth

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

EquipmentMegger SizeMin IR Value
S/S .Equipments5 KV5000MΩ
EHVLines.5 KV10MΩ
H.T. Lines.1 KV5MΩ
LT / Service Lines.0.5 KV5MΩ

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 SizeIR Value(Min)
Circuit Breaker(Phase-Earth)5KV,10 KV1000 MΩ
(Phase-Phase)5KV,10 KV1000 MΩ

Control Circuit

0.5KV50 MΩ

CT/PT

(Pri-Earth)5KV,10 KV

1000 MΩ

(Sec-Phase)5KV,10 KV50 MΩ
Control Circuit

0.5KV

50 MΩ
Isolator(Phase-Earth)5KV,10 KV1000 MΩ
(Phase-Phase)5KV,10 KV1000 MΩ
Control Circuit0.5KV50 MΩ
L.A(Phase-Earth)5KV,10 KV1000 MΩ
Electrical Motor(Phase-Earth)0.5KV50 MΩ
LT Switchgear(Phase-Earth)0.5KV100 MΩ
LT Transformer(Phase-Earth)0.5KV100 MΩ

IR Value of S/S Equipments As per DEP Standard
EquipmentMeggeringIR Value at Commissioning Time (MΩ)IR Value at Maintenance Time
SwitchgearHV Bus200 MΩ100 MΩ
LV Bus20 MΩ10 MΩ
LV wiring5 MΩ0.5 MΩ
Cable(min 100 Meter)HV & LV(10XKV) / KM(KV) / KM
Motor & GeneratorPhase-Earth10(KV+1)2(KV+1)
Transformer Oil immersedHV & LV75 MΩ30 MΩ
Transformer Dry TypeHV100 MΩ25 MΩ
LV10 MΩ2 MΩ
Fixed Equipments/ToolsPhase-Earth5KΩ / Volt1KΩ / Volt
Movable EquipmentsPhase-Earth5 MΩ1MΩ
Distribution EquipmentsPhase-Earth5 MΩ1MΩ
Circuit BreakerMain Circuit2 MΩ / KV
Control Circuit5MΩ
RelayD.C Circuit-Earth40MΩ
LT Circuit-Earth50MΩ
LT-D.C Circuit40MΩ
LT-LT70MΩ

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.

 

1 Phase Wiring
1 Phase Wiring

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.

 

3 Phase Wiring
3 Phase Wiring

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 voltageTest voltageIR Value(Min)
Extra Low Voltage250V DC0.25MΩ
Up to 500 V except for above500 V DC0.5MΩ
500 V To 1KV1000 V DC1.0MΩ

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.

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Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

85 Comments


  1. abderrazak Dabbagui
    Dec 13, 2023

    What is the minimum require insulation resistance for a furnace transformer rated to 50MVA, 27.6 KV/ 600V.
    Ambient temperature is 25 degree C


  2. Mohamed Mounir
    Aug 23, 2023

    hello
    Would you please explain the difference between Mohm/Km & Mohm.Km for insulation resistance of the cables especially


  3. Nabil Aziz
    Jul 21, 2023

    Please advice the best value must be applied during test the insulation resistance of power supply cord with plug (F) length 1.6 meter .
    Thanks / Nabil Aziz


  4. Deivakumar Annamalai Gounder
    Nov 06, 2022

    Can we use 5Kv IR tester for a 250 v domestic wiring?


  5. michael herrera
    Aug 03, 2022

    May i know how, must be the insulation resistance of a generator 681 kva, we are operaing at 3 phase 400 v line to line .we send our generator for repair/ rewind so that we have a base standard requirement for the IR of our generator.


  6. Rex Gutierrez
    May 27, 2022

    Hello.
    In which Standards can we find the IR Value for Transmission / Distribution Line?

    Thank you in advance.


  7. Venkataramanan Srinivasan
    May 05, 2022

    Hello,

    Advise on the recommended Insulation Resistance Value for a new a) 25kv Flexible Overhead Catenary System (Catenary 65sqmm strandeed Cu + Contact-150sqmm Hard drawn Cu) support on steel strcuture and cantilever with 25kv Insulator on Open air (Railway/Metro System) b) Rigid Conductor Rail bar with Hard drawn 150sqmm Cu. This is because while conducting the tests the IR value is as low as 15 Mega Ohms. Is it objectionable as per Standards to 25kv Charge the line?


  8. OSHORO MICHAEL
    Apr 07, 2022

    JUST TO THANK JIGNEH P. FOR SHEARING HIS KNOWLEDGE


  9. Md. Eyasin
    Feb 13, 2022

    Dear sir/ madam,
    Low Voltage Motor 5.5kw 400vac.How much is the standard Megger value.


  10. Amro Widatalla
    Dec 04, 2021

    I got 800 M-OMG when I tested a new cable by the following details:
    1. 11 kv
    2. 12 km
    3. 22 nos of joint

    I need to knew why it is came less the 1G-OMG


  11. Ajay Sonker
    Oct 13, 2021

    Neutral & Sectioned Insulator Rate of 25 kv, 1600 Amp Single Pole


  12. Yuvaraj Rajendran
    Sep 27, 2021

    Kindly suggest, machine operating voltage is 11 KV , for IR value improvement purpose heating process done. Can i measure Megger test at 90- 100 Deg Celsius .


  13. Rajendra Prasad Thapliyal
    Aug 19, 2021

    Good knowledge for site works.


  14. muzaffar
    Jul 11, 2021

    Is there any formula to calculate insulation resistance for cables with respect to temperature?


  15. Minh Bang
    Jun 07, 2021

    When testing IR for transformer, what is the difference between Insulation Resistance of Transformer Coil and IR Value of Transformers ?


  16. Dinesh chandra
    Apr 22, 2021

    Permissible IR Value of 132,33,11 KV Transf,C.T, P.T.&Breaker Meggered by 5 KV.


  17. Muhammad Fawad
    Feb 17, 2021

    Dear Sir,
    I need IR Test Acceptance Value of 630 Sq.mm single-core armoured cable Cu.Pvc/Pvc.


  18. ananad
    Feb 07, 2021

    i need 95sq mm LT UG CABLE insulation valve


  19. ali raza
    Nov 18, 2020

    why ct pt isolate or disconnectec durring generator meggering


  20. Jonson
    Nov 13, 2020

    Hi Mr. Jiguparmar,

    Could you please advise as follow :

    1. How to measure IR OHTL 500KV (line to line and line to earth). Transmission length 52Km. Conductor per phase ACSR quadruple 4 x ACSR Zebra. Is it sufficient 5KV megger or have to use bigger scale such as 20KV?
    2. Is it possible to measure IR transmission line 52Km or have to be divided a few segment?
    3. What is the IEEE, IEC, BS or NETA standard regarding IR test OHTL EHV?

    Thank you so much for your help.

    Cheers,

    Jonson


  21. MK Matook
    Oct 08, 2020

    Good article.
    From where I can get the ETSA standard?


  22. SP MATHUR
    Sep 13, 2020

    Very useful article


  23. Ramesh
    Aug 11, 2020

    Super information


  24. Veeresh Kumar Sharma
    Jul 24, 2020

    Sir, is it must to disconnect diodes & AVR of an alternator 450V 1.5 MW prior measuring IR value. I have seen in Europe, IR measured with diode & AVR connected. Because AVR out put can be upto 500AC. Meggar out put is 500VDC.


  25. Veeresh Kumar Sharma
    Jul 24, 2020

    IR measured without disconnection of diode & AVR to be added to MY previous comment.


  26. Ahmed yaqoob
    Jul 20, 2020

    dear sir
    please send me this pdf file and help me how to download this file


  27. Berwin Lazer Poulin
    May 25, 2020

    as the temp increases, the insulation strength decreases. For every 10 degree increase, as a rule of thumb, megger value should be halved.


  28. PHISTUS
    Apr 14, 2020

    What is the normal heating temperature for generator to read 2.0 PI?


    • PHISTUS
      Apr 14, 2020

      How long does it take to heat the generator to reach the temperature level where we can be able to test IR and get PI value of between 1.5 to 2.5?

      Is it possible to measure the IR and get accurate value for PI of 2.0 at high temperature?


  29. RAEES ANSARI
    Dec 14, 2019

    Please can you help me to know the maximiumvalue of How do I get out of 3 phase 415 VOLT IR Megger Test in mega OHMS value?
    cable size copper 4 core 70 mm@ distance 30mtr.room temp.28-30C


  30. Amit vyas
    Oct 03, 2019

    HT isolator 22 KV during off condtion megger value is 2000M ohm but in ON condition megger value is zero. Why?


    • Santha kumar
      Nov 16, 2019

      Check to connected load


  31. Naresh
    Sep 10, 2019

    Hi…
    Great it is,
    I have aquestion:
    I have spare LT panel (operating voltage-420VAC) and I have to use it somewhere in my factory, but when I am checking its pahse to earth IR value with 1000V IR tester, then it is showing 20 MOhms, is it sufficient to use the panel.


  32. jagjit
    Sep 01, 2019

    10 hp 3 phase indication motor how to check resistance


  33. izzath
    Jul 11, 2019

    Very Useful article. I need an information regarding this.
    Why IR testing is done for 1 minute? Is it a standard procedure which is followed everywhere or any technical logic behind it ??


    • Real
      Apr 05, 2020

      Always you should wait at least 1m.


  34. Hebouche
    Jan 22, 2019

    hi and happy to join you !
    i want just to confirm if someone can confirm that IR test can stress the cable in case that testesting requeted every 6 weeks.


  35. C.V. Krishnaiah
    Oct 24, 2018

    What are standards or practices to consider K-Factor (Absorption Coefficient) and Polarization Index (P.I) specially in respect of EHV Power Transformers (viz 220 KV/ 11 KV)


  36. Jeevan Ranadive
    Sep 29, 2018

    Dear Sir,

    I need to know the HV and Megger test voltage value and routine tests to be performed for medium voltage 6.6KV cables.

    Please email me to my email.

    Regards
    Jeevan Ranadive


  37. Usha
    Aug 31, 2018

    how frequently should we measure the Insulation resistance value in a motor under service


  38. VITORINO DAVID
    Jul 25, 2018

    Very useful article.


  39. Abdulhameed
    Jul 01, 2018

    Dear Sir
    Thanks for all this good work that you have done. I just have doubt that one of the examples of insulation temperature correction factors is given with a probable mistake from the point I looked to the insulator.
    If an Insulator measures 15 MOhms @ 20C then it must measure less at 30C. The example you stated mention some thing in contrary (See Below).

    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


  40. Bilel Alibi
    Jun 07, 2018

    For the 22 KV cable (3 cores x 300 sq.mm) :
    1- How to Indicate/Define the minimum pass/criteria value, for the Megger (IR) test?
    2- How to Indicate/Define the minimum pass/criteria value and define the test voltage, for the outer sheath test ?


  41. MAHADEO JADHAV
    Mar 28, 2018

    thanks for valuable information


  42. Prageeth
    Mar 05, 2018

    To whomever this may concern, whilst the information contained on this website is useful, it is no substitute for the application of competent electrical engineering by experienced practitioners. Electrical engineering practitioners should always read and understand the appropriate guidance, incorrect information is misleading. The primary source of information is described within IEEE, IEC or national standards which must be reviewed before embarking upon electrical engineering works or projects.I wish to thank the author for making this information available to the general public who could be harmed as a consequence. A little knowledge is dangerous.


  43. Arabinda
    Feb 08, 2018

    Sir,one 40MVA Transformer which was in service for 3 years.later moved to 300kms and filteration of oil started.After 30 Days Filteration the IR value of LV to earth is 10 Mega ohms.what is the reason for such low value?


  44. Sagar Vaghasiya
    Feb 03, 2018

    Which Indian Standard is for Electric Insulation resistance measurement/Testing & Evaluate the reading as either acceptable or not. Value below which Insulation is not acceptable etc


  45. p m yadav
    Dec 31, 2017

    Excellent work on single paper.


  46. Vincenzo
    Dec 30, 2017

    Thank you for this very useful article.
    Is there any IR lower-limit for turn-to-turn insulation in random wound (low voltage) electric motors?

    Thank you,
    VM


  47. Arunkumar
    Dec 12, 2017

    Why to avoid meggering when the transformer is under vacuum?


  48. KRISHNAN SUBRAMONIAM
    Dec 05, 2017

    An electrician performed Insulation Resistance test on Low Voltage cable between a Generator and switchboard. Due to site constraints the cable could not be disconnected at both ends of the installation, however the circuit breakers were switched off at both ends leaving the neutral and earth connected. Do you think the neutral and earth cable needs to be disconnected or can remain and we measure the IR value between Phase and Neutral/earth.


  49. Aniruddha Bhattacharjee
    Nov 04, 2017

    Sir, it has been observed that during oil replacement procedure of a power transformer in one of our Substations IR values were taken after draining of oil and after filling with oil. It was found that the reading was less with oil than without oil. What is the reason?


  50. reinyer ocuig
    Jul 30, 2017

    sorry I’m a little bit confused…what formula should I use for IR testing of motor is it

    Min IR Value (For Rotating Machine) =(Rated voltage (v) /1000) + 1
    or
    As per IS 732 Min IR Value of Motor=(20XVoltage(p-p/(1000+2XKW)


  51. Jayant sahoo
    Feb 10, 2017

    How to check d IR value of 765 kv polymer insulator??

    What should b d megger range n what is d value ??


  52. Tashi Tobgay
    Sep 16, 2016

    Value for LT to Earth is coming only 43M-ohm, any one can give me solution


  53. A meenashankar
    Jan 07, 2016

    This is very useful


  54. JOEL COMIA
    Nov 11, 2015

    Thanks for your article. I sent your link to one of overhead crane supplier which was located in some part in Europe, because we were requesting for the IR but they say they do not know such IR. Luckily after searching in the web your article showed up.

    Again thank you.


  55. Ronal Manuel
    Nov 09, 2015

    Sir, do you know how to compute the minimum recommended value for busbar?


    • jaleel
      Feb 24, 2017

      thanks for the valuable tips about insulation resistance and its importance.. ..
      again thanks…


  56. Subhajit Shee
    Sep 17, 2015

    can u plz tell me what will be the practical connection diagram by utilizing the theory given by ur article for transformer insulation resistance measurement such as

    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 windings)
    1. (HV + LV) – GND

    Auto Transformer (three winding)
    1. (HV + LV) – (TV + GND)
    2. (HV + LV + TV) – GND
    3. TV – (HV + LV + GND)

    and is the meaning of TV ; plz rely it as it is very urgent


    • harshal
      Aug 25, 2016

      I think is tertiary winding


  57. pathak satish
    Jun 21, 2015

    how to calculate lekage current of hv test
    exmp.
    i have given power frequency 70kv for vcb panel and this vcb ir value coming 123684 ph to earth and phase to phase 254637megaohum


  58. Virendra Nathe Tiwari
    Jun 12, 2015

    an 11 kw AHU motor how much ohms it should have with body earth .


  59. Vilas Pathak
    Mar 18, 2015

    Very Informative Article.


  60. meenu
    Mar 18, 2015

    really very useful


  61. deshmukh V.
    Jan 30, 2015

    thank you for sharing good information. Keep it up.


  62. gk
    Dec 18, 2014

    GREAT SITE FOR ALL ELECTRICAL INDUSTRY PERSONNEL. BUG UP!


  63. RIYAS
    Nov 29, 2014

    Thank u so much sir…..


  64. jose lintao
    Nov 04, 2014

    Useful guide.thanks


  65. LALO EL HIPPIE
    Nov 03, 2014

    Excelente información, gracias


  66. FELIX Jr.
    Oct 02, 2014

    INDEED, THIS IS A GOOD REFERENCE FOR ALL ELECTRICAL ENGINEERS, PRACTICING OR NOT. AND FOR ENGINEERING STUDENTS, TOO.


  67. Ganesh Kumar
    Aug 17, 2014

    any one tell me about insulation measurement by using Impulse tester and Tan Delta Tester.
    kindly help me to complete my project.
    my email id : [email protected]


  68. Adam Edmont Mbulawa
    Jun 17, 2014

    What voltage do u use 5 test a 400v bus bar and what are the expected results.


  69. Qaisar
    Feb 02, 2014

    1. How to perform meggering of a 33/11kv outdoor transformer . What are the permissible IR values for, HV-LV, HV-Earth, & LV-Earth while using digital megger at 5kv.
    2. What is the “normal and maximum” oil & winding temp allowed on 10MVA(33/11kv) transformer ? Its urgent, Plz help


    • Qaisar
      Feb 02, 2014

      the transformer to megger is 10MVA(33/11KV)


  70. Sami Rashed
    Dec 29, 2013

    Please can you help me to know the maximium fire resistance temperture for the follow fire cable with following specifiction:
    2×1.5mm2
    Aluminum mylar 7*0.18mm
    conductor diameter 2.8-3.1mm
    Resistance less than 13.3 ohm/km
    operationg temperature -20C – +85C
    impedence 115 ohm
    test voltage 1500V
    Isolation resistance more than 10 000 ohm/km
    weight per 100m 7.3kg


    • catchy mack fires
      Oct 29, 2014

      load up the cable thill it cats fire and then dial the load back a bit and you should ok


  71. murugesan
    Dec 14, 2013

    useful for me.Thanks


  72. zakk85
    Jul 16, 2013

    useful for me.Thanks


  73. tigerdon
    Jun 05, 2013

    Dear All,

    Megger Value of HV 6.6kV 2 MW Motor after No Load Run (4 Hours Running) are coming Low compared to Before No Load Run Reading of Motor IR??

    This is happening for both different motors with ideal rating.

    Can any one explain the Reason???

    Regards,
    T.D.


    • senthillv
      Jul 10, 2013

      please ensure the values are corrected for standard temperature. ( i.e – 20 degree Celsius). Definitely there will be a temperature rise of 10 degree Celsius from cold to No-load running.


    • v.c sasikumar
      Nov 08, 2015

      before motor running stator and winding cold condition that time test leakage current more reading showing low resistance. because temperature factor after some hours running the motor current flow to the winding and resistance varying stator heat up and all moisture out of from motor. stop motor after test showing high resistance value because leakage current less.


  74. shreyansh
    Mar 20, 2013

    A Very Useful Article
    Request to mentioned the source of the information provided


  75. asheith
    Jul 18, 2012

    THANKS ,GOOD WORK …i LUV this siteeee


  76. Samer Alamouri
    Jun 30, 2012

    Very useful article , I appreciate your efferots to collect all these indexes and values in one paper
    Thank you….

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