Meggar equipment.pdf

Insulation resistance measurement

1. Insulation Resistance Measurement and
IR value Tester
J Niranjan

2. J NiranjanTRAINING OBJECTIVES
Upon successful completion of this module, the
participantwillbeableto:
Recognizefaultyoperation
Locatepropertestarea
Selecttestequipment
Understandsafetyconsiderations
Usepropertestingtechniques
Interpretthereadings

3. J NiranjanIntroduction to IR Measurement
Electrical conductors in any equipment or cable are covered with
someformof
non-conductivematerialcalledinsulation.
The insulation must be the
opposite of a good conductor
—a
material that conducts no current, thus keeping
the electricity in
the path of the conductor
. Insulation can be solid, liquid, or
gaseous dielectric materials that
prevent the flow of electricity
betweenpointsofdifferentpotential. Insulation testing is done to determine the
integrity of the
insulating medium
.
This usually consists of applying a high
potential voltage to the sample under test and
determining the
leakage current
that may flow under test conditions.
Excessive
leakage current flows may indicate a deteriorated conditio n or
impendingfailureoftheinsulation

4. J NiranjanIntroduction to IR Measurement
A typical 1000V IR Value tester
An IR Value tester from AVO International – Trade named “Megger”

5. J NiranjanIntroduction to IR Measurement
Defects in the insulation around a conductor can occur in the form
of cracks, nicks, pits from the factory, pinholes or embedded
foreignobjectslikemetalshavings.
Voids in the insulation readily absorb moisture & dirt, crea ting a
conductivepathfromtheconductortoanotherconductor/ground
The higher the potential across the insulation, the more cur rent
willflowthroughtheinsulation(AsperOhm’sLaw)
Leakage current over time will cause a pinhole to become much
worse resulting in a
total failure
of the conductor. Hence lower the
leakage the better is the insulation.
So a “good” insulation would
have“theabilitytokeepahighresistance.”

6. J NiranjanIntroduction to IR Measurement
When the facility is new, the electrical insulation will be in top-
notch shape. Nevertheless
insulation is subject to many effects
which cause it to fail
—physical damage, excessive heat or cold,
dirt, oil, corrosive substances, moisture from processes o r just
humidity. Over time, these factors coupled with the electri cal
stresses wear down the insulation and the resistance value s tarts
decreasing
By
measuring this resistance periodically
, we can get ample
warning, which allows planned re-conditioning and avoid a m ajor
failure
Testing insulation after installation, modification or ma intenance
canalsopinpointproblemsbeforethesystemisre-energized

7. J NiranjanHow is Insulation Resistance Measured?
The Insulation tester is a portable instrument that provide d a
direct reading of the insulation resistance in Ohms, Mega-o hms,
Giga-Ohms, Tera-Ohms etc.
For good insulation, the resistance
usuallyreadsintheMeg-Ohmorhighervalues
The Insulation tester is essentially a high range resistanc e meter
(Ohmmeter) with a built-in DC generator. The generator can be
hand-cranked, line operated orpowered from a battery which then
produces a high DC voltage which when applied to an insulatio n
produces a current. From this the resistance is directly rea d from
themeter.

8. J NiranjanHow is Insulation Resistance Measured?
There are three different currents that flow through an
insulatorwhenavoltagepotentialisapplied.
First, the "charging current“ starts out high and drops to
nearly zero after the insulation has been charged to full tes t
voltage.Thisisnormallynegligibleafterthefirstfewseconds
ofthetest.
Second the "absorption current" also starts out high and
drops off. The majority of this current dissipated after one
minutes, but continues to decay for up to 5 to 10 minutes.
This is for the polarization of the molecules to the applied
electricfield
Finally the "conduction" or "leakage current" is a small,
mostly steady current which becomes a factor after the
absorption current drops to a negligible value. This curren t
shouldremainsteadyfortheremainderofthetest.

9. J NiranjanHow is Insulation Resistance Measured?
Connections for IR value testing

10. J NiranjanSelecting the right Insulation Resistance Tester
The following table gives the “Voltage” that is to be selecte d
dependingontheratedvoltageoftheequipmentbeingtested

11. J NiranjanSelecting the right Insulation Resistance Tester
IR testers can be battery powered, line powered or hand cranked. Advantages of the battery-powered tester are (1) constant
voltage,and(2)thearmdoesn’tgettired
.
The line-powered set is less portable, but there are no
batteriestogodead
.
The hand cranked one is still preferred by a number of
usersbecauseof(1)itisportable,(2)therearenobatteries,
and (3) if the user gets shocked, he or she will likely stop
turningthecrank,thusde-energizingthespecimen

12. J Niranjan
PROPER TESTING TECHNIQUES
(INCLUDING SAFETY CONSIDERATIONS)
WARNING
Any insulation resistance tester packs a lethal voltage, an d equipment
undertestcanstorethatvoltage.
1. Always ensure the equipment has been de-energized and
disconnectedfromassociatedpowersourcesorotherequipment.
2. Never use the IR tester in a flammable atmosphere. Some min or
sparking may occur when the IR tester is disconnected from a
specimen,orfromflashover,whichcanigniteexplosivefumes.
3. Ensure that no person can accidentally contact terminals , wires or
devices under test. The test voltage of the IR tester is letha l and a
long cable or busduct with good insulation can store deadly
charge.

13. J Niranjan
PROPER TESTING TECHNIQUES
Discharge any stored capacitance in busducts or cables by
groundingthembeforetesting,thenagainaftertesting
Use the proper personal protective equipment (PPE) which
includes safety glasses, long sleeve cotton or Nomex, and
gloves
Connect both leads of the IR Values Tester to ground to
checkthetester.Itshouldreadzero

14. J Niranjan
PROPER TESTING TECHNIQUES
Discharge any stored capacitance in busducts or cables by
groundingthembeforetesting,thenagainaftertesting
Use the proper personal protective equipment (PPE) which
includes safety glasses, long sleeve cotton or Nomex, and
gloves
Connect bothleads ofthe IRValuesTestertoground
tocheckthetester.Itshouldreadzero
Connect the line terminal of the tester to the conductor or
device to be tested. Connect the earth (E) terminal to the
frameoradjacentconductor
.
Make sure the specimen is clean and dry. Separate and
isolateallconductorsfromtheframeandeachother.

15. J Niranjan
PROPER TESTING TECHNIQUES
Clear the immediate area.
NotifyothersthataIRvalue testisabouttotakeplace.
Set the correct tester voltage,
dependingontheratedvoltageoftheequipment

16. J Niranjan
PROPER TESTING TECHNIQUES
Connect the tester, take the hands away from the
specimen under test and the wires, and then initiate
the test by pushing the TEST button or turning the
crank.Continuethetestfor1minute
Caution:
The leads and specimen are now energized

17. J Niranjan
PROPER TESTING TECHNIQUES
Release the TEST button, or stop turning the crank.
MostIRtestershaveabuiltindischargecircuit,butit
isstillagoodpracticetodischargethewinding.
Recordthereadingsandcontinuetotesteachwinding
to the frame, then test each winding against the
adjacentwinding
Ifthetestresultsaretoolow,investigatetheproblem

18. J Niranjan
Specific Guidelines for Motor IR value testing
•Openmaindisconnect(CircuitBreaker)tothemotor •Disconnect the motor leads from the feeder in the
motorterminalbox
•Testallcircuitstobesuretheyarede-energized •Connectfromtheloadsideofthecontactor(ifpresent)
ordisconnectswitch(ifcontactornotpresent)andhold
thetestvoltageforoneminute,thendischarge

19. J Niranjan
Specific Guidelines for Cables IR value testing •
Isolatebothendsofthecableoropendisconnects
toisolatethecablefromotherequipment
•Perform the insulation resistance test on each
cable or conductor with respect to ground and
withrespecttoadjacentcables

20. J NiranjanSpecific Guidelines for Panels &Associated Bus test ing
•Open all circuit breakers in panel-board including
maincircuitbreaker
•Remove all metering and control power fuses to
prevent feedback, oraccidentally applying test voltage
tosensitiveelectronicequipment
•Perform the insulation resistance test phase-to-phase
and phase- to-ground—Hold the test voltage for one
minute

21. J Niranjan
Specific Guidelines for Transformer testing
•Open primary and secondary disconnects (circuit
breakers)tothetransformer
•If the Neutral of the transformer is earthed, remove
theearthingconductorfortestpurposes
•Performthreeseparatetests:
1. Highwindingtolowwinding
2. Highwindingtogroundedlowwinding
3. Lowwindingtogroundedhighwinding

22. J Niranjan
Effect of Temperature on IR Value
The resistance of insulation decreases significantly
with an increase in temperature. To ensure precise
comparisons, readings must be taken on specimens at
the same temperature or corrected to the base
temperature of 20°C (68°F), which is standard in the
industry
Agoodruleofthumbforthiscompensationis
:
For each 10°C increase (above 20°C) the insulation
resistancewillbeone-halfthevaluemeasuredat20°C
.
For each 10°C decrease (below 20°C) the insulation
resistancewillbedoublethevaluemeasuredat20°C.

23. J Niranjan
Effect of Temperature on IR Value
The resistance of insulation decreases significantly
with an increase in temperature. To ensure precise
comparisons, readings must be taken on specimens at
the same temperature or corrected to the base
temperature of 20°C (68°F), which is standard in the
industry
Agoodruleofthumbforthiscompensationis
:
For each 10°C increase (above 20°C) the insulation
resistancewillbeone-halfthevaluemeasuredat20°C
.
For each 10°C decrease (below 20°C) the insulation
resistancewillbedoublethevaluemeasuredat20°C.

24. J Niranjan
Effect of Temperature on IR Value
Example Acablemeasuring8meghomsat30degCwillcorrect
to16megohmsat20°C
Atransformermeasuring20megohms at 10°C will be
correctedto10megohmsat20°C

25. J Niranjan
Effect of Temperature on IR Value

26. J Niranjan
Effect of Temperature on IR Value

27. J Niranjan
Effect of Humidity on IR Value
It shouldbe kept inmindthat humidityalso
decreases
insulationresistance. If the specimen is kept fairly warm, moisture will be
not collected in the insulation. However if the
equipment is allowed to become cool, particularly
below the dew point, then humidity can drastically
reducetheinsulationresistancereadings
.
Equipment subjected to excessive humidity must be
driedoutpriortotesting.

28. J Niranjan
Interpreting the readings
The best guideline of what insulation resistance readings
are acceptable are the manufacturers specifications.
However, these are rarely available in the field, so other
meanshavebeenused.
IECrecommendsthefollowingvalues

29. J Niranjan
Interpreting the readings of IR
The best guideline of what insulation resistance readings
are acceptable are the manufacturers specifications.
However, these are rarely available in the field, so other
meanshavebeenused.
AVOrecommendsthefollowingvaluesandtheseareusually
accepted

30. J Niranjan
Interpreting the readings of IR
The recommended minimum IR values after one minute at
40degCisgiveninthepreviousslide.
The Minimum resistance of one phase of a three phase
armature winding tested with the other two grounded
should be approximately twice that of the entire winding. If
each phase is tested separately, the observed minimum
resistance should be three times the entire winding
approximately

31. J Niranjan
Polarization Index (PI)

32. J Niranjan
Polarization Index (PI)
Why is this important?
Resistance-to-ground readings involve three different current
components:charging,absorptionandleakage.
The PI test allows the charging and absorption currents to
decay so that only actual leakage current is measured.
As a
voltage is continuously applied, healthy insulation slowly
polarizesandtheabsorptioncurrentdiminishes
.
Thiscausesa
steady rise in resistance until the majority of the current is
from the small amount leaking to ground
. In poor insulation,
leakage current is high enough to overshadow the lowering
absorptioncurrentandprovidelittleincreaseintheresistance
over time. Polarization Index is thus an indicator of the
cleanlinessofthewindings.

33. J Niranjan
Interpreting PI Values
The following Table may be used to interpret the results of
PI

34. J Niranjan
Interpreting PI Values
As per IEC, the PI values is dependent on the ‘Thermal
Class” of Insulation used. For various thermal classes,
various insulation materials are used and hence PI will
change. The following table gives these values

35. J Niranjan
Using both IR and PI
The following table can be used to evaluate the condition
of the machine using both PI and IR