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

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 affected 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 influence of any interfering currents at the beginning of the
measurement.
Then the PI or the DAR ratios are calculated:
PI* = R
DAR = R
The quality of insulation depends on the results found.
*The 10 and 1 minute times can be modified in the SET-UP menu, to adapt to any changes or specific
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 sufficient 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 sufficient to electrically "charge"
the insulation to be measured (a typical value is the application of a 500V voltage
for 30 minutes).
12
0.561.8187
/ R (2 values recorded during a 10 minute measurement)
10 min 1 min
/ R (2 values recorded during a 1 minute measurement)
1 min 30 s
DAR
< 1
< 1.25
< 2
< 1.6 > 2 to < 4
> 1.6 > 4
www.
PI State of the insulation
Inadequate, even dangerous
.com
Good
Excellent
Megohmmeter Models 5050/5060
information@itm.