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3 Copper tests
3.1.6 Resistance symmetry (R
This measurement determines the resistance symmetry of a line. Of primary importance for
resistance symmetry are the two resistances Ra and Rb, i.e. the resistances of the two sig-
nal wires. To great a difference in resistance between the two wires can result in signal dis-
tortions and transmission errors. In a real line, a network of serial resistances is formed
between the individual reference points, as illustrated the following diagram.
The measuring loops between Tip-Ring-Ground can be realised
manually or using an electronic instrument kit.
Ideal measuring configuration:
Values Ra, Rb and RE are not measured directly, but instead are calculated from three
individual measurements (Rab, RaE, RbE). The caculated values are then used in
determining symmetry.
RE is dependent on the measuring current here and is an auxiliary quantity. RE is generally
significantly less (RE << Ra reasp. Rb) than Ra or Rb.
The cable shield is generally connected to ground at the main distribution frame resp. at the
cable branch box. When the demarcation point is earthed, an electrical connection exists
via both the cable shield and the ground connection. Always make sure that the measuring
loops are properl connected to ground.
If the Tip-Ring-Ground measuring loops are not earthed, the measurement cannot be con-
ducted (e. g. clamping to painted, non-conducting termination frames with crocodile clips).
24
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Sym
ARGUS Copper Test Manual
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