TeraOhm 5kV
17
Filter option Meaning
Off
Low pass filter with cut off frequency of 0.5 Hz in signal line.
Fil1
Additional low pass filter with cut off frequency of 0.05 Hz in signal
line.
Fil2
Fil1 with increased integrating time (4 s).
Fil3
Fil2 with additional cyclic averaging of 5 results.
Table 2. Filter options
THE PURPOSE OF FILTERING
In simple terms the filters smooth the measured currents by means of averaging and
bandwidth reduction. There are various sources of disturbance:
-
AC currents at the mains frequency and its harmonics, switching transients etc,
cause the results to become unstable. These currents are mostly crosstalk through
insulation capacitances close to live systems,
-
Other currents induced or coupled in the electromagnetic environment of the
insulation under test.
-
Ripple current from internal high voltage regulator,
-
Charging effects of high capacitive loads and / or long cables.
Voltage changes are relatively narrow on high resistance insulation
,
so the most
important point is filtering of the measured current.
Note:
Any of the selected filter options increases the settling time with Fil1 to 60 s, Fil2 to 70 s,
and Fil3 to 120 s.
-
It is necessary to pay close attention to the selection of time intervals when using
filters.
-
The recommended minimum measuring times when using filters are the settling
times of the selected filter option.
Example 1:
A noise current of 1 mA / 50 Hz adds approximately
±
15 % distribution to the measured
result when measuring 1 G
Ω
.
By selecting FIL1 option the distribution will reduce to less than
±
2 %.
In general using FIL2 and FIL3 will further improve the noise reduction.
Example 2:
Testing an object with the following parameters:
Utest =250 V, Inoise = 3 mA, R= 2.4 G
Ω
, C=220 nF
Filter options give following results:
OFF: distribution of results 30 %,
Fil1: distribution of results 3.5 %,
Fil2: distribution of results 1.5 %,
Fil3: distribution of results 0.9%.
For this example the setup time is also increased and for the Fil3 option, the time to
capture the first good result (Time1) is approximately 2 min.
