How to test DC Resistance Unbalance for PoE

Differences in resistance between the pairs in cabling has been found to cause problems with PoE (Power over Ethernet) delivery. From reduced efficiency and service life to insertion loss and packet errors. DC Resistance Unbalance (DCRU) is the measurement that is performed on the cabling after installation to test for differences in DC resistance and, hopefully, avoid these issues.

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Why test DC Resistance Unbalance?

Resistance unbalance can be inherent in the cable or can be a result of cable termination. Pair unbalance, the difference between conductors in the same pair, is unlikely in the cable. It is usually going to be the result of a poor termination of one of the conductors.

The latest TIA, ISO and EN/CENELEC cabling standards do not require field testing DCRU. However, it is important to consider the importance of DCRU in how it impacts the ability of a cabling system to support PoE++. Best practice is to require DCRU testing in addition to the usual certification testing.

Testing DCRU in the field

While all cable certifiers, can measure DC loop resistance, not all cable certifiers can measure DCRU. Because it is a relatively new test, only certifiers designed within the last few years can measure DCRU. This means that you’ll need a fairly new certifier, such as the TREND Networks’ LanTEK IV-S, to be able to test DCRU. Loop resistance should also be measured when certifying for PoE support because it is an important parameter in ensuring the full amount of power can be delivered to the PoE device.

Since none of the certification test standards require DCRU testing, your DCRU enabled certifier will have a setting to add DCRU to the regular Autotest for the category/class being certified. Due to the specific parameters being measured, poor termination would go unnoticed if the DCRU test was not enabled on the certifier.

Get full details on how to conduct a DCRU test in our Whitepaper here

Can your cable certifier test DCRU?

Some manufacturers, such as TREND Networks, offer DCRU adapters for previous generation certifiers that extend their useful life by allowing them to test DCRU with a permanent link and channel adapters that have the precision resistance measurement circuitry built in.

For example, new adapters and a software update allow the LanTEK III certifier to measure DCRU along with the LanTEK IV which has integrated DCRU measurement capability.

As DCRU testing becomes a common requirement and PoE++ capability is increasingly a necessity for new cabling installations, it’s important to check now that your certifier is up to the job. If you have any questions about selecting the right test equipment for your PoE installation or troubleshooting, contact our expert team.

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