Fluke CableAnalyzer DSX-5000, Technical Reference Handbook, Page: 176 / 700

Versiv Series Cabling Certification Product Family
Technical Reference Handbook
140
Resistance Unbalance Between Pairs (P2P UBL)
The IEEE 802.3bt standard gives limits for resistance unbalance
between pairs. These limits are used for Type 3 and 4 PoE systems,
which deliver power over 4 wire pairs.
In 4-pair systems, resistance unbalance between pairs reduces the
current in one pair, and increases the current in another. The
increased current can cause the power source to limit the current
on the pair, which can cause a powered device to operate
incorrectly.
Resistance unbalance between pairs is affected mainly by cable
design. For example, for adequate NEXT performance in Cat 6A
cable, it is necessary to use different twist rates among the pairs.
This causes the pairs to be different lengths, which causes a
difference in the resistance of the pairs.
Figure 63 shows test results for resistance unbalance between
pairs.