After measurement completion, the residual voltage is removed automatically from
the object, its value "Ui" is displayed on the display and accompanied with frequent
intermittent audio signal until safe level of 50 V is reached.
After measurement completion, the results are shown on the display during 20
seconds and can be recorded to the memory (see item 2.3.2). After that, instrument sets to
the mode of voltage measurement.
Note. During measurements on some grounded objects consider the following:
- normally, if one of the contacts of the measured resistance is grounded (Figure
2.6.
а
), it is recommended to connect "-" of the instrument to it. However, in case of unstable
readings the change of connection polarity can provide more stable readings.
- on some objects, it is required to determine the permissible polarity of test voltage
applying in advance to prevent actuation of safety devices. Polarity of the test voltage is
specified at the megohmmeter sockets.
- induced DC voltage can be present on the object. In this case it is recommended to
perform measurements twice
–
with change of applied test voltage polarity. This allows to
determine the true value of insulation resistance as an average value of two measurements.
For short-time measurements of the insulation resistance press and hold button
, the measurement is stopped after the button is released.
2.3.6 Calculation of Dielectric Absorption Ratio and Polarization Index
Dielectric Absorption Ratio (DAR)
is used to evaluate the wetting degree of the
insulation on cable lines, transformers, electric motors, etc.: charge rate of the absorption
capacity is evaluated (capacity due to non-homogenity and contamination of material,
inclusions of air and moisture) for insulation at test voltage applying. Dielectric Absorption
Ratio is automatically calculated by the results of insulation resistance measurement by one
of the following formulas:
DAR=R
60
/
R
30
(default)
(1а)
DAR =R
60
/
R
15
(1b),
where R
15,
R
30,
R
60
–
value of insulation resistance after 15, 30 and 60 seconds from
the beginning of measurement, respectively. On default, Dielectric Absorption Ratio
calculation is set to formula 1а and can be changed by the user during the measurement
parameters setting (see item 2.3.5).
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