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22 Device Functions
6.1.3 Vcommon point and Guarding
This measuring system is able to measure capacitances with highest accuracy to determine
trending analysis of insulating materials. In the range of normal insulation capacitances the always
existent stray capacitances - measured together wit the DUT
– can influence the measuring values
significant. So these unwanted stray capacitance effects have to be eliminated.
This is realized by the so called “guarding” of the relevant elements. That means that the complete
high voltage source, the supply and measuring cables have to be shielded with the so called
“V
(Common)
” which is the low voltage point (reference) of the high voltage supply. All capacitances
connected to this reference point are bypassed and are therefore not influencing the measuring
value. Several parts have to be double shielded (Guard and Ground) to compensate other side
effects and to ensure the specified measuring accuracy. Due to this guarding concept the supplied
shielded coax measuring cables (for High Voltage Supply, Input Cn and Input Cx and input a) have
to be used always. If the system is connected with normal unshielded cables the measuring values
will be incorrect.
To keep in mind for the user of the system is that capacitances related to the V
(Common)
-point are
bypassed. Make sure that all unwanted capacitances are related to the V
(Common)
point and their
current is flowing directly into the V
(Common)
-point and not through the measuring shunt R
X.
This has to be evaluated for every measuring setup. The most common ones are described in
this manual
– for the other ones the user has to make sure that only the desired capacitances are
measured with the chosen test setup.
The V
(Common)
point is accessible over 4 mm plugs on the instruments back panel where the user
can connect external parts of his test setup.
6.2 Standard test circuits
In this chapter the standard measuring methods are explained. It is important to understand how
the current is flowing in the specific measuring application to avoid leakage currents which lead to
inaccurate measurement results. These circuits are also applicable to Inductances.