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DC Loop Resistance
DC loop resistance is the resistance of a wire pair. It is one of several parameters which are important for remotely powering devices via Power over Ethernet. Read on to find out more about it.
DC loop resistance is the electrical resistance of a wire pair. The pair is shorted on the remote end for measuring the resistance of this wire loop. Naturally the loop resistance is twice the resistance of the link as in the loop the current runs to the remote end and back again. Other terms for DC loop resistance are loop resistance or just resistance.
DC loop resistance should be as small as possible.
Caption: DC loop resistance is the resistance of one pair of wires.
When devices are powered via the data cable, it is done using direct current (DC). A high resistance means high losses. With DC loop resistance, the resistance of the whole wire pair is measured as the electric current runs to the remote end of the pair and back again. The resistance of an individual wire is not measured. The resistance of the two wires in a pair might differ. This can lead to an uneven load and to uneven powering. Because of this the test parameter “Resistance Unbalance within a pair” was introduced which we will discuss in the next issue.
Troubleshooting tips if DC loop resistance tests fail
| Typical cause of the problem | This might help (without engagement) |
| Horizontal cable too long | Cut the cable (if possible) |
| Horizontal cable was repaired | Re-work the repaired part, use appropriate connection modules |
| Contacts in patch panel, outlet or plug are dirty | Clean contacts |
| Contacts in patch panel, outlet or plug are worn | Replace patch panel/outlet/plug |
| Contacts in patch panel, outlet or plug are corroded | Replace patch panel/outlet/plug |
| Wires in patch panel, outlet or plug are connected badly | Reconnect the wires |
| Wires in patch panel, outlet or plug have become loose | Reconnect the wires |
| Contacts of the test cord are dirty | Clean contacts |
| Contacts of the test cord are worn | Replace plug/test cord |
| Contacts of the test cord are corroded | Replace plug/test cord |
| Wire diameter of the horizontal cable is too small (AWG number is too big) | Replace cable |
| Wire diameter of the patch cord is too small (AWG number is too big) | Replace patch cord |
Typical causes of DC loop resistance problems and what might help to solve them.
Contact
Dirk Traeger
