Versiv Series Cabling Certification Product Family
Technical Reference Handbook
136
Note
All balance measurements together (+All) add ~7.6
seconds to the Autotest time.
Resistance Unbalance
Resistance unbalance is important in PoE (power over Ethernet)
systems, which supply DC power over twisted pair cabling.
When required by the selected test limit, the tester shows two
types of resistance unbalance: resistance unbalance within pairs
(PAIR UBL in the results) and resistance unbalance between pairs
(P2P UBL in the results). The tester uses the resistance of the
individual wires in the pairs to calculate these values.
Resistance Unbalance within Pairs (PAIR UBL)
Resistance unbalance within a pair is the difference in DC
resistance between the wires in a wire pair. This is different from
the DC loop resistance measurement, which is the total resistance
of the two wires.
Resistance unbalance in pairs is important in PoE systems where
DC power is carried over the same wire pairs as data. In these
systems, the PoE power source applies a DC current to the center
taps of the isolation transformers for two pairs. This causes DC
current through the pairs, which provides DC power across the
center taps of the transformers at the far end. The currents in the
two wires of each pair is common-mode: the flow in the same
direction. So, when the resistance of each wire in the pair is equal,
the DC voltage on each wire is equal and the voltage across the
wires is 0 V, as shown at the top of Figure 61.
At the center tap of the transformer for the pair, the net DC
current from the pair is zero, so the transformer operates at the
mid-point relative to the maximum flux density of its core.
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