Error Indications; Dar (Dielectric Absorption Ratio) And Pi (Polarization Index) - AEMC 6550 User Manual

Record the measurement and compare it to earlier measurements in order to assess the evolution of its value.
Also record the temperature and the ambient relative humidity.
If, at equivalent temperature and humidity, the insulation resistance has fallen significantly, the insulation is
deteriorated and maintenance must be carried out on the device tested.
The result remains displayed until another measurement is made, the rotary switch is turned to a different posi-
tion, or the measurement configuration is changed.
After a test with programmed duration:
■
Pressing the GRAPH button displays the graphical representation of the results (see § 2.4.5).
■
Only for V-FIXED and V-VAR: Pressing the TEMP button opens the temperature menu (see § 2.4.1).
■ Pressing the MEM button opens the recording menu (see § 5.1).
■ At any time, you can press the HELP button for a reminder of the functions of the buttons.

4.5 Error Indications

The most common error in the case of an insulation measurement is the presence of a voltage on the terminals.
The instrument can make the measurement if the peak value of this voltage is less than 0.4V
Above this value, it is necessary to eliminate the voltage and repeat the measurement.
If an external voltage appears on the terminals during the measurement, and its peak value is greater than
1.1V , the measurement is stopped and the error is indicated.
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4.6 DAR (dielectric absorption ratio) and PI (polarization index)

For V-FIXED and V-VAR: In addition to the quantitative value of the insulation resistance, it is very useful to cal-
culate the quality ratios of the insulation (the DAR and the PI) because they can eliminate the influence of certain
parameters likely to invalidate the "absolute" insulation measurement. They also serve to predict the evolution
of insulation quality over time.
The most important of the parameters influencing the measurement results are:
■
temperature and relative humidity, with which insulation resistance varies according to a quasi-
exponential law.
■
the disturbance currents (capacitive charging current, dielectric absorption current) created by the
application of the test voltage. Even if they gradually fade, they interfere with the measurement at the
start, for a more or less long time depending on whether the insulation is sound or degraded.
These ratios therefore complete the "absolute" insulation value and reliably reflect the condition, good or bad,
of the insulation.
In addition, long-term observation of the evolution of these ratios is a way to monitor the aging of the insulation.
For example, that of a revolving machine or of a long cable.
The values of DAR and PI are calculated as follows:
■
DAR = R /R 30
1 min
■
PI = R 10 /R 1
min
The times of 1 and 10 minutes for the calculation of the PI and the times of 30 seconds and 1 minute for the
calculation of the DAR can be modified in the CONFIG menu or the SET-UP function (see § 3.4), to adapt to
particular applications.
Megohmmeter Models 6550/6555
Shop for AEMC products online at:
(2 values to be taken during a 1-min measurement)
s
(2 values to be taken during a 10-min measurement)
min
ShopAEMC
www. .ca 1.877.766.5412
or 1000V max.
AC
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