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 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.

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)
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
OC OF Correction Factor
0 32 0.25
5 41 0.36
10 50 0.50
15 59 0.720
20 68 1.00
30 86 1.98
40 104 3.95
50 122 7.85

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  Voltage Megger Size Min.IR Value Liquid Filled T/C Min.IR Value Dry Type T/C
0 – 600 V 1KV 100 MΩ 500 MΩ
600 V To 5KV 2.5KV 1,000 MΩ 5,000 MΩ
5KV To 15KV 5KV 5,000 MΩ 25,000 MΩ
15KV To 69KV 5KV 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 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+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

 

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

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 (Phase-Earth) 5KV,10 KV 1000 MΩ
(Phase-Phase) 5KV,10 KV 1000 MΩ

Control Circuit

0.5KV 50 MΩ

CT/PT

(Pri-Earth) 5KV,10 KV

1000 MΩ

(Sec-Phase) 5KV,10 KV 50 MΩ
Control Circuit

0.5KV

50 MΩ
Isolator (Phase-Earth) 5KV,10 KV 1000 MΩ
(Phase-Phase) 5KV,10 KV 1000 MΩ
Control Circuit 0.5KV 50 MΩ
L.A (Phase-Earth) 5KV,10 KV 1000 MΩ
Electrical Motor (Phase-Earth) 0.5KV 50 MΩ
LT Switchgear (Phase-Earth) 0.5KV 100 MΩ
LT Transformer (Phase-Earth) 0.5KV 100 MΩ

IR Value of S/S Equipments As per DEP Standard
Equipment Meggering IR Value at Commissioning Time (MΩ) IR Value at Maintenance Time
Switchgear HV Bus 200 MΩ 100 MΩ
LV Bus 20 MΩ 10 MΩ
LV wiring 5 MΩ 0.5 MΩ
Cable(min 100 Meter) HV & LV (10XKV) / KM (KV) / KM
Motor & Generator Phase-Earth 10(KV+1) 2(KV+1)
Transformer Oil immersed HV & LV 75 MΩ 30 MΩ
Transformer Dry Type HV 100 MΩ 25 MΩ
LV 10 MΩ 2 MΩ
Fixed Equipments/Tools Phase-Earth 5KΩ / Volt 1KΩ / Volt
Movable Equipments Phase-Earth 5 MΩ 1MΩ
Distribution Equipments Phase-Earth 5 MΩ 1MΩ
Circuit Breaker Main Circuit 2 MΩ / KV
Control Circuit 5MΩ
Relay D.C Circuit-Earth 40MΩ
LT Circuit-Earth 50MΩ
LT-D.C Circuit 40MΩ
LT-LT 70MΩ

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 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 Author //

author-pic

Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. 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.

33 Comments


  1. A meenashankar
    Jan 07, 2016

    This is very useful


  2. 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.


  3. Ronal Manuel
    Nov 09, 2015

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


  4. 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


  5. 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


  6. Virendra Nathe Tiwari
    Jun 12, 2015

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


  7. Vilas Pathak
    Mar 18, 2015

    Very Informative Article.


  8. meenu
    Mar 18, 2015

    really very useful


  9. deshmukh V.
    Jan 30, 2015

    thank you for sharing good information. Keep it up.


  10. gk
    Dec 18, 2014

    GREAT SITE FOR ALL ELECTRICAL INDUSTRY PERSONNEL. BUG UP!


  11. RIYAS
    Nov 29, 2014

    Thank u so much sir…..


  12. jose lintao
    Nov 04, 2014

    Useful guide.thanks


  13. LALO EL HIPPIE
    Nov 03, 2014

    Excelente información, gracias


  14. FELIX Jr.
    Oct 02, 2014

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


  15. 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 : enggsh@gmail.com


  16. Adam Edmont Mbulawa
    Jun 17, 2014

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


  17. 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)


  18. 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


  19. murugesan
    Dec 14, 2013

    useful for me.Thanks

  20. […] the substationBusworkTransformersAir core reactorsSwitchgear and cablingLine trapsCircuit breakersGround gridCapacitorsBattery chargersComputers Electric FieldsSubstation grounding system (photo by […]


  21. zakk85
    Jul 16, 2013

    useful for me.Thanks

  22. […] a transformer temperature as close as possible or at 20 °C. Test conducted […] Log in to Reply Measurement of insulation resistance (IR) – Part 2 | EEPApr 29, 2012[…] and industrial companies for predictive and preventive maintenance. Continued from […]


  23. 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.

  24. […] of the connections at the remote ends, should be done; this should include measuring the insulation resistance and continuity of all cables and wiring including internal and auxiliary connections.Where […]

  25. […] A. Earthing resistance and number of rods for isolated earth pit (without buried earthing strip)The earth resistance of single rod or pipe electrode is calculated as per BS 7430:R=ρ/2×3.14xL (loge […]


  26. shreyansh
    Mar 20, 2013

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

  27. […] DecreaseResistance = IncreaseResistance = Increase If the flow rate were to stay the same while the resistance to flow decreased, the required pressure from the pump would necessarily decrease:Pressure = […]


  28. asheith
    Jul 18, 2012

    THANKS ,GOOD WORK …i LUV this siteeee


  29. 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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