AEMC 5050 - R-DAR-PI-DD; R(T) Button

12 Megohmmeter Models 5050/5060
2.5.3 R-DAR-PI-DD / R(t) Button
First Function - R-DAR-PI-DD
TheR-DAR-PI-DDbuttonallowsautomaticmeasurementof:
• DielectricAbsorptionRatio(DAR)
• PolarizationIndex(PI)
• DielectricDischargeTest(DD)
ThePIandDARvaluesareusefulonlongcables,orwhenmonitoringtheagingof
insulationonelectricalmachines.
Onthismaterial,measurementisadverselya󰀨ectedatthebeginning,duetointer-
fering currents (from capacitive charges, dielectric absorption) which eventually
canceleachotherout.Therefore,inordertogetanaccuratemeasurementofthe
leakagecurrent,itisnecessarytocarryoutthemeasurementoveralongperiod
oftime,toreducetheinuenceofanyinterferingcurrentsatthebeginningofthe
measurement.
ThenthePIortheDAR ratiosarecalculated:
PI*=R10min/R1 min(2valuesrecordedduringa10minutemeasurement)
DAR=R1 min/R30s(2valuesrecordedduringa1minutemeasurement)
Thequalityofinsulationdependsontheresultsfound.
DAR PI State of the insulation
< 1.25
< 1
Inadequate, even dangerous
< 2
< 1.6 > 2 to < 4 Good
> 1.6 > 4 Excellent
*The 10 and 1 minute times can be modied in the SET-UP menu, to adapt to any changes or specic
applications.
Formulti-layerinsulation,ifoneofthelayersisdefective,butalltheothersshow
strongresistance,thecalculationofthePIandPARratiosisnotsu󰀩cienttoshow
thistypeofproblem.
Therefore,itisnecessarytosupplementthePIandDARindicationsbyadielectric
dischargetest(DD).
Thistestmeasuresthedielectricabsorptionofheterogeneousormulti-layerinsu-
lationwhileignoringtheparallel-surfaceleakcurrents.
Itconsistsofapplyingatestvoltageforaperiodsu󰀩cienttoelectrically“charge”
theinsulationtobemeasured(atypicalvalueistheapplicationofa500Vvoltage
for30minutes).
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