HT instruments HT7052 - Manual (Page 19)

HT7052
EN - 17
6.3 USE OF INTERNAL FILTERS
Filters are built in to reduce the influence of noise on measurement results. This option
enables more stable results especially when dealing with high Insulation Resistances
(Insulation Resistance, Diagnostic Test, Step Voltage). In these functions, the status of the
filter option is shown in the top right corner of the LCD screen. The below table contains a
definition of the individual filter options
Filter options Description
Fil0 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
the signal line
Fil2 Fil1 with increased integrating time (4 s)
Fil3 Fil2 with additional cyclic averaging of 5 results
Table 2: Filter options
6.3.1 The purpose of filtering
The internal 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 cross talk 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 to filter the measured current
CAUTION
 Any of the selected filter options increases the settling time with Fil1 to 60
s, Fil2 to 70 s, and Fil3 to 120s
 It is necessary to pay close attention to the selection of time intervals when
using the filters
 The recommended minimum measuring times when using filters are the
settling times of the selected filter option
Example:
A noise current of 1mA/50Hz adds approximately
15% distribution to the measured result
when measuring 1G .

By selecting FIL1 option the distribution will reduce to less than 2 %
 In general using FIL2 and FIL3 will further improve the noise reduction