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3.1.3 Capacitive symmetry measurement (C
With the aid of capacitive symmetry measurement, it is possible to detect irregularities in
the wiring that can result in signal distortions or transmission errors. The line must be open.
In a real line, a network of serial and parallel capacitances is formed between the individual
reference points, as illustrated in the following diagram.
The total capacitance Cab of the series connection of CaE and CbE is actually measured
between a (tip) and b (ring).
The measured value shows the mutual capacitance (Cm) value. This applies correspond-
ingly for measurements between a (tip) and ground (E) and between b (ring) and ground.
These measurements for the basis for calculating the actual values of Cab, CaE and CbE.
The calculated values are then used in determining symmetry.
Additionally, the absolute deviation of the two capacitances CaE and CbE with respect to
ground and the relative deviation are determined.
Relative deviation (in %) = 2x
Absolute deviation (in nF) = absolute difference (CaE-CbE)
The calculated values represent the actual values CaE, CbE and Cab for the individual
capacitances. As the capacitances with respect to ground CaE and CbE are particularly
important for the symmetry of a line, the absolute and percentage values are also given
here for the sake of clarity. The relative value should not exceed 1 % (recommendation).
The absolute deviation is of particular importance on extremely short lines, as even small
differences here can result in a higher-percentage deviation. It must be reviewed on a
case-by-case basis whether this is acceptable.
The line must be open. This can be achieved e.g. using the instrument kit
function "open circuit". A voltage check is performed before the measure-
ment in which the voltage limits given in the table (see page 7) may not be
exceeded.
Always make sure that no instrument kit is connected when measuring
capacitance on extremely short lines (< 100 m). If this occurs, the decou-
pling capacitance of the instrument kit may also be measured, which will
significantly falsify the measuring results. On the TS916, for example, an
additional capacitance of 3.3 nF is measured at a measuring frequency of
8 Hz.
ARGUS Copper Test Manual
)
Sym
3 Copper tests
15
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