DCRU testing of installed cabling is not mandatory… yet.

There has been some confusing information put out in the industry recently regarding how the standards view the requirement for DC Resistance Unbalance (DCRU) testing.  We have received a lot of questions, so we wanted to take a moment to clear things up.

DC Resistance Unbalance (DCRU) testing of installed cabling is highly recommended, especially if the cable is to be used for Power Over Ethernet (PoE), BUT…this test is still optional, despite some recent confusion.

DC Resistance Unbalance is an important measurement when network cabling links are to be used for PoE, which is more and more the case these days. The measurement compares the resistance of each pair to the other pairs in the cable and compares the resistance of the two conductors within a given pair. Balance is critical for power delivery and a reliable network connection.

Recent changes included in the ANSI/TIA-568.2-E standard made DC Resistance Unbalance a required measurement for twisted pair cable, so manufacturers must produce the cable to meet these specifications (most of them have done so for years). However, the relevant standard for field testing of the installed cables is ANSI/TIA-1152-A which prescribes which tests are to be performed on the installed cabling system, using the measurement specifications found in the ANSI/TIA-568. Looking at ANSI/TIA-1152-A sec 4.2.1.1 (Measurements of resistance parameters of cabling) it states that field testers may optionally be capable of measuring and reporting these parameters.

I would not be surprised if in the near future we see an update to the ANSI/TIA-1152-A to align it with ANSI/TIA-568.2-E with respect to DCRU measurements, but for now this is still an optional test, which I always recommend for all cabling being used for PoE deployment.

At AEM Precision Cable Test we’ve included this important measurement as long as I can remember. The TestPro Multifunction Certification tester always measures DCRU in pair and pair-to-pair and it is provided as an Informational parameter (not pass/fail). To turn on Pass/Fail, users simply select the limit with the (+) after it.  So instead of using the “TIA-568.2-E Cat6A Permanent Link” test limit, the user would select the “TIA-568.2-E Cat6A Permanent Link (+)” limit to provide the pass/fail for DCRU. This does not add any time, still takes only 6 seconds for Cat6A. Additionally, if you originally tested using the regular Permanent Link limit you can recertify to the (+) limit in our TestDataPro PC software, as the data is already in the test file, so no need to re-test.

If you want to find out more about DC Resistance Unbalance, please check out Tech Talk with Steve episode 72 | Testing PoE Performance Over Installed Cabling Systems. Ep. 72 | Testing PoE Performance over Installed Cabling Systems.

Steve Cowles, RCDD/NTS Product Line and Technical Services Manager.  Steve is also a member of the ICT Installation Workgroup under the BICSI Standards Subcommittee.