measurement of insulation resistance

7/22/2019 Measurement of Insulation Resistance

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Measurement of insulation resistance (IR)Part 2PostedAP R 29 20 12 by JIGUPARMAR in HIGH VOLTAGE,MAINTENANCE,MOTOR,TRANSFORMERS,TRANSMISSION

AN D DI ST RI BU TI ON with3 COMMENTS

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Fluke insulation resistance tester up to 10kV - Allows testing of high voltage systems such as control gears, engines,

generators and cables. It can be adjusted to all testing voltages that are specified in IEEE 43-2000. Ideal for Electricity

Board and industrial companies for predictive and preventive maintenance.


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Continued from first part:Measurement of insu lat ion resis tance (IR) Part 1

Insulation Resistance (IR) ValuesIndex

1.IR Values For Electrical Apparatus & Systems

2.IR Value for Transformer3.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

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 M

600 Volts 1,000 Volts 100 M

5 KV 2,500 Volts 1,000 M

8 KV 2,500 Volts 2,000 M

15 KV 2,500 Volts 5,000 M

25 KV 5,000 Volts 20,000 M

35 KV 15,000 Volts 100,000 M

46 KV 15,000 Volts 100,000 M

69 KV 15,000 Volts 100,000 M

One Meg ohm Rule for IR Value for Equipment

Based upon equipment rating:

< 1K V = 1 M minimum

>1KV = 1 M /1KV

As per IE Rules-1956
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At a pressure of 1000 V applied between each live conductor and earth for a period of one minute

theinsulation resistanceof 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

Index

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

Transformer windingsare 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 w ith solidly
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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 (M) = C X E / (KVA)

3 Phase Transformer (Star) IR Value (M) = C X E (P-n) / (KVA)

3 Phase Transformer (Delta) IR Value (M) = 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 orDry Type T/C.

Temperature correction Factor (Base 20C):

Temperature correction FactorOC 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 M at 20 0C IR Value at LV Side = (1.5 x 400 ) / 1600= 320 / 40 = 15 M at 20 0C IR Value at 300C =15X1.98= 29.7 M

Insulation Resistance of Transformer Coil


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Transformer Coil Voltage Megger Size Min.IR Value Liquid Filled T/C Min.IR Value Dry Type T/C

0600 V 1KV 100 M 500 M

600 V To 5KV 2.5KV 1,000 M 5,000 M

5KV To 15KV 5KV 5,000 M 25,000 M

15KV To 69KV 5KV 10,000 M 50,000 M

IR Value of Transformers

Voltage Test Voltage (DC) LV side Test Voltage (DC) HV side Min IR Value

415V 500V 2.5KV 100M

Up to 6.6KV 500V 2.5KV 200M

6.6KV to 11KV 500V 2.5KV 400M

11KV to 33KV 1000V 5KV 500M

33KV to 66KV 1000V 5KV 600M

66KV to 132KV 1000V 5KV 600M

132KV to 220KV 1000V 5KV 650M

Steps for measuring the IR of Transformer:

Shut down the transformer and disconnect the jumpers andlightning 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 M)

Two winding transformer1. (HV + LV)GND
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2. HV(LV + GND)

3. LV(HV + GND)

Three winding transformer1. 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:

HVEarth 200 M LVEarth 100 M HVLV 200 M

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

Index

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

Index

4. IR Value for Electric motor

Forelectric motor,we used a insulation tester to measure the resistance of motor winding withearthing (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
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Insulation resistance (IR) value for electric motor

As per IEEE 43 Standard 1974, 2000

IR Value in M

IR (Min) = kV+1 For most windings made before about 1970, all field

windings, and others not described below

IR (Min) = 100 M For most dc armature and ac windings built after about1970 (form wound coils)

IR (Min) = 5 M For most machines with random -wound stator coilsand form-wound coils rated below 1kV

Example-1:For 11KV, Three Phase Motor.

IR Value =11+1=12 M but as per IEEE43 It should be 100 M Example-2: For 415V,Three Phase Motor IR Value =0.415+1=1.41 M but as per IEEE43 It should be 5 M. 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 M

600V 1000V DC 100M

1000V 1000V DC 100M

2500V 1000V DC 500M5000V 2500V DC 1000M

8000V 2500V DC 2000M

15000V 2500V DC 5000M

25000V 5000V DC 20000M

34500V 15000V DC 100000M


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IR Value of Submersible Motor:

IR Value of Submersible Motor

Motor Out off Well (Without Cable) IR Value

New Motor 20 M

A used motor which can be reinstalled 10 M

Motor Installed in Well (With Cable)

New Motor 2 M

A used motor which can be reinstalled 0.5 M

Index

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. Thewiring and cablesneed 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 Ms 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 )

dDiameter of conductor

cThickness of conductor insulation

bThickness of jacket insulation

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

Application Test Voltage Min IR Value

New cablesSheath 1KV DC 100 M

New cablesInsulation 10KV DC 1000 M

After repairsSheath 1KV DC 10 M

After repairsInsulation 5KV DC 1000M
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11kV and 33kV Cables between Cores and Earth (As per ETSA Standard)

Application Test Voltage Min IR Value

11KV New cablesSheath 5KV DC 1000 M

11KV After repairsSheath 5KV DC 100 M

33KV no TFs connected 5KV DC 1000 M

33KV with TFs connected. 5KV DC 15M

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 Lineterminal 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 showthe 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 conductorsto 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.


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Rotate the handle of megger or press push button of megger. The reading of meter will show theinsulation 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 M/km atburied temperature, the subject cable should be programmed for replacement.

If insulation resistance of the cable is found between 1000 and 100 K/km, at buriedtemperature, 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 cablemust be replaced immediately on emergency basis.

Index

6. IR Value for Transmission / Distribution Line

Equipment Megger Size Min IR Value

S/S .Equipments 5 KV 5000M

EHVLines. 5 KV 10M

H.T. Lines. 1 KV 5M

LT / Service Lines. 0.5 KV 5M

Index

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 M.

Index

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 M

(Phase-Phase) 5KV,10 KV 1000 M

Control Circuit 0.5KV 50 M

CT/PT (Pri-Earth) 5KV,10 KV 1000 M
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(Sec-Phase) 5KV,10 KV 50 M


Isolator (Phase-Earth) 5KV,10 KV 1000 M

(Phase-Phase) 5KV,10 KV 1000 M


L.A (Phase-Earth) 5KV,10 KV 1000 M

Electrical Motor (Phase-Earth) 0.5KV 50 M

LT Switchgear (Phase-Earth) 0.5KV 100 M

LT Transformer (Phase-Earth) 0.5KV 100 M

IR Value of S/S Equipments As per DEP Standard

Equipment Meggering IR Value atCommissioning

Time (M)

IR Value atMaintenance

Time

Switchgear HV Bus 200 M 100 M

LV Bus 20 M 10 M

LV wiring 5 M 0.5 M

Cable(min 100 Meter) HV & LV (10XKV) / KM (KV) / KM

Motor & Generator Phase-Earth 10(KV+1) 2(KV+1)

Transformer Oilimmersed

HV & LV 75 M 30 M

Transformer Dry Type HV 100 M 25 M

LV 10 M 2 M

Fixed Equipments/Tools Phase-Earth 5K / Volt 1K / Volt

Movable Equipments Phase-Earth 5 M 1M

Distribution Equipments Phase-Earth 5 M 1M

Circuit Breaker Main Circuit 2 M / KV -

Control Circuit 5M -Relay D.C Circuit-Earth 40M -

LT Circuit-Earth 50M -

LT-D.C Circuit 40M -

LT-LT 70M -

Index
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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

3 Phase Wiring


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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

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 0.25M

Up to 500 V except for above 500 V DC 0.5M500 V To 1KV 1000 V DC 1.0M

Min IR Value = 50M / No of Electrical outlet. (All Electrical Points with fitting & Plugs)

Min IR Value = 100M / No of Electrical outlet. (All Electrical Points without fitting & Plugs).

Index

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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High Voltage Maintenance Motor Transformers Transmission and Distribution
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measurement of insulation resistance

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