TESTING RESISTANCE UNBALANCE

The original PoE standard supported up to 12.95W and PoE+ supported up to 25.5W. PoE++, which was ratified in 2018 and is available in Type 3 and Type 4, offers higher power. Type 4 can supply up to 90W on each PoE port. Recent experiences have brought to light that differences in resistance between wires in pairs are widespread in LAN cabling, which can have a negative effect on the delivery of PoE. Fortunately, a smart approach to testing provides a solution, writes

Andrew Sedman RCDD, Head of Technical Support and Training at R&M

Unbalanced resistance in a cable pair, or between cable pairs, may be caused by substandard cable and connector manufacturing, poor installation practices at the point of termination, or unreliable termination technology that degrades over time. In the worst case, unbalanced pairs can introduce signal transmission failure due to induced insertion loss in transceivers.

The signal transformers of active equipment can easily be saturated by the DC current and fail to receive data sent along lines if the power is mismatched on that same pair. Another issue (albeit less serious) is the heating of cables and the Power Sourcing Equipment (PSE) which supply DC power and data connectivity. This can reduce equipment lifetimes, waste power, and lead to unreliable power provision.

Before explaining the remedy, let’s first take a look at how data and power travel through structured cabling. Data is transmitted through a cable pair by sending differential signals down that pair’s dual conductors. This is done to help reduce crosstalk. Power, however, is sent through the cable pair as common-mode voltage – the same voltage is sent down both conductors. The data signals and power reach a balun, which is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing their impedance – in the active equipment. The power is tapped off, leaving the data to pass through the inductors.

If the resistance between the two conductors of a pair is different, the DC current in these conductors will not be the same. This, in turn, could lead to heating differences in the two conductors and to magnetic saturation effects in the baluns. If saturation occurs in the balun, it won’t be able to transmit data signals (see Fig. 1). For PoE operating on all four pairs, identical positive voltage is sent down two of the pairs and the negative voltage is sent down the opposing pairs. If resistance between those pairs is not identical, an additional heating effect arises through differing current split.

In short, PoE equipment will not send/receive data on cabling that has excess resistance unbalance.

This type of testing verifies the deviation in resistance between each of the cores in a twisted pair. However, although ISO 11801-1 (6.3.3.7. Direct current resistance unbalance) defines maximum resistance unbalance figures, it specifies this element to be tested as optional.

Testing offers insight into pair resistance unbalance, which expresses how much the DC resistances of the conductors of a pair differ from each other.

Previously, the advice was to test the permanent link and then attach standards compliant patch cords. However, this may present problems for PoE if cords and their connectors present resistance unbalance errors that are not tested as part of the complete channel. Therefore, R&M recommends testing the complete channel in any system, with the patch cords in place.

According to ISO/IEC 14763-2, the remote power category must always be specified (RP1, RP2 or RP3) for every installation. Planning, installation practices and product selection must support the specified category. Specifically, with RP3, the assurance that the cable bundle will not overheat is essential for category compliance. An RP3 category must be applied for all newly built systems for offices, homes, industry and smart building systems. Every cable in a bundle must be able to fully support PoE of the highest level. Every cable in that bundle must be able to carry that burden simultaneously. However, undue resistance unbalance will increase the RMS current of a cable and therefore invalidate the RP3 calculation and planning. All that power being transmitted along bundled cables will generate heat, affecting data transmission. The thermal loads can only remain in check if resistance unbalance is minimised.

Fortunately, test equipment manufacturers are currently making it possible to test beyond the basic requirements of standards such as ISO/IEC 11801. Selected devices now provide options to test for additional parameters, whereas some devices already test for this parameter as part of the basic test.

With PoE becoming increasingly widespread, supported by developments in systems convergence and ‘all over IP’, field testing is more important than ever. Resistance unbalance testing ensures installations support PoE without introducing data transmission issues or excess heat, making it vital to warranty agreements and avoiding costly equipment replacement and rework.

R&M, rdm.com