R-Dar-Pi-Dd / R(T) Button - AEMC 5050 User Manual

2.6.3 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 aging
insulation on electrical machines.
On this material, measurement is adversely effected at the beginning due to
interfering currents (from capacitive charges, dielectric absorption), which
eventually cancel each other out. In order to get an accurate measurement of the
leakage current, it is necessary to perform the measurement over a long period
of time to reduce the influence 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
< 1.25
< 1.6
> 1.6
*The 10 and 1 minute times can be modified in the SET-UP menu, to adapt to any changes
or specific applications.
If one layer is defective but all the others show strong resistance for multi-layer
insulation, the calculation of the PI and PAR ratios is insufficient to show this type
of problem.
Therefore, a dielectric discharge test (DD) is necessary to supplement the PI and
DAR indications.
This test measures the dielectric absorption of heterogeneous or multi-layer
insulation while ignoring the parallel-surface leak currents.
It consists of applying a test voltage for a period sufficient to electrically "charge"
the insulation to be measured (a typical value is the application of a 500V voltage
for 30 minutes).
The test voltage is chosen in the same way as the insulation measurement in the
SET-UP menu. The device will cause a fast discharge during which the insulation
capacity is measured and measured again 1 minute after the residual current is
circulating in the insulation.
Megohmmeter Models 5050 & 5060
PI
< 1
Inadequate, even dangerous
< 2
> 2 to < 4
> 4
State of the insulation
Good
Excellent
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