Manual

Gossen Metrawatt GmbH 35
8.5 Polarization Index Measurement – PI
(METRAHIT IM XTRA BT and METRAHIT IM E-DRIVE BT only)
Polarization index testing is recommended for electric machines.
This procedure involves expanded testing of insulation resistance
R
ISO
. DC measuring voltage from the multimeter is applied to the
insulation for a duration of 10 minutes. The measured R
ISO
value
is documented after one minute, and after ten minutes. If the insu-
lation is good, the value measured after ten minutes is higher than
the value measured after one minute. The relationship between
the two measured values is the polarization index. Charged mate-
rial within the insulation is aligned due to the application of mea-
suring voltage over a long period of time, resulting in polarization.
The polarization index indicates whether or not the charged mate-
rial contained in the insulation can still be moved, i.e. whether or
not polarization is possible at all. This, in turn, is an indication of
the condition of the insulation.
The following rules apply in general:
PI values < 1: Troubleshooting is required.
PI values = 1  2 Maintenance is recommended
PI values = 2  4 DUT is OK,
no immediate action is required
Preventive maintenance can be planned
according to workload.
PI values > 4 DUT in flawless condition
Applications: Determination of moisture and contamination levels
The following applies to the insulation of electric drive units:
Intact insulation PI  2
Very good insulation PI > 4
➭ Set the rotary switch to “ RISO ”.
➭ Repeatedly press the Func softkey until the PI measurement
view appears at the display.
➭ Contact the device under test with alligator clips which have
been plugged onto the test probes.
➭ Interference voltage measurement (V AC+DC TRMS) is con-
ducted in this switch position.
➭ Measurement may only be started when the device under test
is voltage-free (U
ext
= 0000 V).
➭ Activate the measurement by pressing the Start softkey. The
high-voltage symbol next to the battery level display blinks
during measurement.
➭ You can interrupt the measurement at any time by pressing
the Stop softkey.
The measurement is recorded and you can observe its progress
which is displayed as a curve over the time axis.
Results for R
60
are displayed after reaching the 60 s mark. Mea-
surement is stopped automatically and results for R
600
and PI are
displayed after reaching the 600 s mark.
The following measured values are displayed digitally next to the
graphic representation:
PI Polarization index after 600 s
Riso Momentary measured value in M/G
U
ext
Momentary measuring voltage (actual
value)
U
set
Test voltage (target value)
t
act
Measured time since beginning of mea-
surement
R
60
Measured value after 60 s in M/G
R
600
Measured value after 600 s in M/G
After measurement has been completed, any
remaining residual voltage U
ext
is displayed which
may result from cable capacitance and a capaci-
tive device under test. The instrument’s internal
1M resistor causes rapid discharging.
Contact to the insulation resistance must be retained to
this end.
Error, “OL“ (overload)
During polarization index testing, insulation resistance R
ISO
is
measured after one and after ten minutes, and the second value
must be higher (see beginning of chapter). “OL” (overload) is dis-
played if the upper range limit is exceeded. In the case of R
ISO
,
the upper range limit is 31,000 digits or 3.1 G. If this value is
already exceeded at the beginning of the measurement, a PI
value cannot be determined because the second value would
have to be higher.
+
–
0V!
Discharge
Func
Func
Func
Zero:
RISO
COIL
RISO
Coil
DAR
PI
The above symbol
blinks when test volt-
age is active.
Func