Power Over Ethernet Interoperability Guide

Worst-Case Imbalances and DC Bias

The TIA-568B cable spec we had mentioned in Chap. 2 specifies up to 5 percent imbalance between conductors of a given pair. Note that between pairs, the measured imbalance was around 8 percent as seen on the bench. But clearly, interpair imbalance does not affect the DC bias of transformers, only intrapair imbalance does. We also know that as per the IEC method, the resistance ratios stated above need to be halved.

There are other contributions to total imbalance that we now try to consider. For example, a typical male-female connector can have a contact resistance typically 20 mΩ (but can be as high as 40 mΩ after aging, though we will ignore the aging aspect here). We can have a case where the contact of ...