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# Measurement of insulation resistance (IR) – 2

Home / Technical Articles / Measurement of insulation resistance (IR) – 2

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

## Insulation Resistance (IR) Values – Index

### 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.

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

 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 (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.

#### 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.

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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.

1. Deivakumar Annamalai Gounder
Nov 06, 2022

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

2. 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.

3. Rex Gutierrez
May 27, 2022

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

4. 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?

5. OSHORO MICHAEL
Apr 07, 2022

JUST TO THANK JIGNEH P. FOR SHEARING HIS KNOWLEDGE

6. Md. Eyasin
Feb 13, 2022

Low Voltage Motor 5.5kw 400vac.How much is the standard Megger value.

7. 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

8. Ajay Sonker
Oct 13, 2021

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

9. 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 .

Aug 19, 2021

Good knowledge for site works.

11. muzaffar
Jul 11, 2021

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

12. 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 ?

13. Dinesh chandra
Apr 22, 2021

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