Electronic Measurement - Insulation Resistance Measurement - Megger

This article speaks about Insulation Resistance (IR) and its measurement using Megger.

1. Electrical & Electronic Measurement
Part –X
Megger
ER. FARUK BIN POYEN
[email protected]
AEIE, UIT, BU

2. Contents:
Definition
Construction
Working Principle
Reading Inference
Types
Electronic Type
Handheld Type
Applications
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3. Megger: Definition
Meggeristheinstrumentusedformeasuringtheresistanceofthe
insulation.
Meggerworkingprincipleisbasedontheworkingprincipleofmoving
coilinstruments,whichstatesthatwhenacurrentcarryingconductoris
placedinamagneticfield,amechanicalforceisexperiencedbyit.
Themagnitudeanddirectionofthisforcedependuponthestrengthand
directionofthecurrentandmagneticfield.
Itworksontheprincipleofcomparison,i.e.,theresistanceofthe
insulationiscomparedwiththeknownvalueofresistance.
Iftheresistanceoftheinsulationishigh,thepointerofthemovingcoil
deflectstowardstheinfinity,andifitislow,thenthepointerindicates
zeroresistance.
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4. Megger:
Insulationresistance(IR)qualitydegradesovertimedueto
temperature,humidity,moistureanddusteffects.
InsulationResistance(IR)isacriticalparameterasitdirectlyaffects
personalandequipmentsafety.
Itismainlyusedtomeasureelectricalleakageinwires.
Itiscapableofshowingtheamountofleakagecurrentandlevelof
moisturewithintheequipmentorwindingsystem.
Itcanmeasurehighresistancesoftheorderofmegaohms.
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5. Megger: Construction
DeflectingandControlcoilV1&V2:Connectedparalleltothe
generator,mountedatrightangletoeachotherandmaintainpolaritiesin
suchawaytoproducedtorqueinoppositedirection.
PermanentMagnets:Producemagneticfieldtodeflectpointerwith
North-Southpolemagnet.
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6. Megger: Construction
Pointer:Oneendofthepointerconnectedwithcoilanotherenddeflects
onscalefrominfinitytozero.
Scale:Ascaleisprovidedinfront-topofthemeggerfromrange‘zero’
to‘infinity’,enableustoreadthevalue.
D.CgeneratororBatteryconnection:Testingvoltageisproducedby
handoperatedDCgeneratorformanualoperatedMegger.Battery/
electronicvoltagechargerisprovidedforautomatictypeMeggerfor
samepurpose.
PressureCoilResistanceandCurrentCoilResistance:Protect
instrumentfromanydamagebecauseoflowexternalelectrical
resistanceundertest.
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7. Megger: Working Principle
A test voltage of 500 V, 1000 V or 2500 V is generated by handheld
generator.
A constant voltage source is maintained to test the insulation having low
resistance.
Two pressure coils are rotated in the anticlockwise direction, whereas
the current coil is rotated in the clockwise direction.
With the unknown resistance connected in the circuit, the moving coil
pointer becomes stable.
The pressure coil and current coil balance the pointer and set it in the
middle of the scale.
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8. Megger: Working Principle
Thedeflectionofpointerisdirectlyproportionaltothevoltageapplied
totheexternalcircuitandtherationofcurrentsinthetwocoilsi.e.
unknownresistance.
Withanincreaseofpotentialvoltageacrosstheexternalcircuit,the
deflectionofthepointerincreases;andwithanincreaseofcurrent,the
deflectionofpointerdecreasesotheresultanttorqueonthemovementis
directlyproportionaltothepotentialdifferenceandinversely
proportionaltotheresistance.
Whentheexternalcircuitisopen,torqueduetovoltagecoilwillbe
maximumandthepointerwillread“infinity”.Whenthereisshort
circuitthepointerwillread“0”.
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9. Megger: Reading Inference
Producedtorqueshallbeinoppositedirectionifcurrentsuppliedtothe
coil.
1.HighResistance=NoCurrent:-Nocurrentshallflowthrough
deflectingcoil,ifresistanceisveryhighi.e.infinitypositionofpointer.
2.SmallResistance=HighCurrent:-Ifcircuitmeasuressmallresistance
allowsahighelectriccurrenttopassthroughdeflectingcoil,i.e.
producedtorquemakethepointertosetat‘ZERO’.
3.IntermediateResistance=VariedCurrent:-Ifmeasuredresistanceis
intermediate,producedtorquealignorsetthepointerbetweenthe
rangeof‘ZEROtoINIFINITY’.
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10. Megger: Types
There are two types of megger
1.Electronic Type (Battery Operated)
2.Manual Type (Hand Operated)
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11. Megger: Electronic Type
Theimportantpartsofthismeggertestequipmentare:
1.DigitalDisplay:-Toshowtheinsulationresistancevalueindigitalform
2.TestingWires:-Twowireleadsforconnectingmeggerwithan
electricalexternalsystemfortestingthelater.
3.SelectionSwitches:-Differentparameterrangesareprovidedonthe
meter,whichcanbeselectedbyusingtheselectionswitches.
4.Indicators:-Differentindicatorsareincorporatedintotheinstrumentto
giveavisualandaudibleindicationwhentheinstrumentisON,fora
warning,parameterstatusetc.
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12. Megger: Electronic Type –Pros & Cons
Pros:
1.Level of accuracy is very high.
2.IR value is digital type, easy to read.
3.One person can operate very easily.
4.Works perfectly even at very congested space.
5.Very handy and safe to use.
Cons:
1.Requires an external source of energy to energize i.e. Dry cell
2.High initial cost
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13. Megger: Handheld Type
Themainpartsofsuchmeggertestingunitcompriseof:
1.Display:-AnAnalogdisplayisprovided,whichisapointerandascale,
toshowtheIRvaluerecording.
2.HandCrank:Asthisamanuallyoperatedmegger,ahandcrankis
providedwhichcanberotatedtogeneratetherequiredvoltage,which
runsthroughtheelectricalsystemforaninsulationresistancetest.
3.WireLeads:-Twowireleadsareprovidedwhichcanbeconnectedto
theelectricalsystemwhichneedstobechecked.
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14. Megger: Handheld Type –Pros and Cons
Pros
1.No external source required to operate
2.Excellent choice for emergency use
3.Cheaper than the electrical megger
Cons
1.Moremanpowerrequired.
2.Accuracyisnotuptothelevelasit’svarieswithrotationofcrank.
3.Requireverystableplacementforoperationwhichisalittlehardto
findatworkingsites.
4.Unstableplacementoftestermayimpacttheresultoftester.
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15. Megger: Application
Circuit Breaker Testing
Cable Fault Testing
Power Quality Analysis
Power Factor Testing
Low Resistance Testing
Motor winding testing
Alternator winding testing
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16. Reference:
https://www.electrical4u.com/megger-working-principle-types-history-uses-of-megger/
https://circuitglobe.com/megger.html
https://www.yourelectricalguide.com/2017/01/megger-insulation-tester.html
https://www.marineinsight.com/marine-electrical/construction-and-operation-of-
megger-explained/
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