XS series operating manual
XS user manual v0.55 - 137 -
Dielectric strength tests with a dc voltage
BENEFITS:
The power of the HV source can be lower than the one necessary in AC voltage (less weight and
more safety for the operator). The current flows through the specimen only during the charging phase.
DISADVANTAGES:
The charging current can trigger the breakdown detection
The specimen being charged, it must be discharged through the built-in discharge resistance (1.5
Mohm).
WARNING: Wait enough for the discharge of the specimen capacitance before disconnection from the
unit (about 8 seconds per uF).
The specimen is tested in only 1 polarity
The test voltage must be higher than the one provided with AC test voltage. A common rule is to use
a 1.4 correction factor between the DC and the AC voltages (= square root of 2 = ratio between the
rms value of a sinusoidal wave and its crest value) :
Udc = Uac * 1.4
Breakdown detection mode selection
The most common and simple leakage current control mode is the threshold current control mode or
IMAX mode. This mode allows setting a maximum limit of current flowing through the specimen under
test above which the unit detects a breakdown and stops the test by cutting off the HV generation and
memorizing the voltage value on the LCD screen. As described, the HV source has to supply the
leakage current as well as the reactive current coming from its capacitance. Therefore this requires to
adjust the breakdown threshold regarding the reactive current of each specimen, and to follow the
procedure :
Make a test on a good specimen
Collect the total current flowing
Adjust the current threshold to a value greater than the total current.
The XS series units offer the above described detection mode, combined with the -I detection mode
(called sometimes ARC detection mode). The -I mode allows to release from the reactive current
flowing through the capacitive specimens.
To detect a breakdown, the -I mode (ARC detection) monitors only the fast current variations (t>10uS
and amplitude>1mA). This doesn’t require any adjustment regarding the specimen capacitance.
However this mode can not detect a dielectric strength default in case of a specimen in short-circuit
since the high voltage application. That is why the MG series units allow to combine the IMAX and the -
I modes in order to make reliable and without adjustment of dielectric strength tests. The IMAX value is
set to a value close to the unit short-circuit current under the test voltage.
The MG series units allow to inhibit the detection systems (OFF mode) in order to locate visually where
is the dielectric strength fault. WARNING : this mode doesn’t cut off the high voltage and therefore it is
possible to destroy or burn the specimen under test. The power of the HV source being limited, a
continued use of the OFF detection mode can trigger the built-in thermal safety switch. Under those
circumstances (display of the message : INTERLOCK DISABLE) wait between 3 and 5 minutes before
proceeding to the tests.
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