Andrew Sedman RCDD, head of technical support & training, R&M looks at PoE implementation.

When the first version of the PoE standard appeared, it supported up to 12.95 watts. Its successor, PoE+ supported up to 25.5 watts. PoE++, available in Type 3 and Type 4, offers higher power. Type 4 is capable of supplying up to 90W on each PoE port (with a minimum of 71W at the powered device). However, differences in resistance between wires in pairs in LAN cabling occur frequently and can negatively affect PoE delivery.

Fortunately, a smart approach to testing provides a solution. Unbalanced resistance in a cable pair, or between cable pairs, can be the result of poor installation practices at the point of termination, substandard cable and connector manufacturing, or unreliable termination technology degrading 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 (less serious) issue is the heating of cables and the Power Sourcing Equipment (PSE) which supply DC power and data connectivity. This can result in shorter equipment lifetimes, wasted power, and unreliable power provision.

Fortunately, resistance unbalance testing offers a solution. Before we look into that, let’s examine 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. In the active equipment, data signals and power reach a balun - an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing their impedance. Power is tapped off leaving data to pass through the inductors.

If the resistance between the two conductors of a pair is not the same, the DC current in these conductors will also be different. This, in turn, may 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. 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.

Resistance unbalance testing offers a solution. This type of testing verifies the deviation in resistance between each of the cores in a twisted pair. However, although ISO 11801-1 ( 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: how much the DC resistances of the conductors of a pair differ from each other. (Not the same as the "Resistance" or "DC Loop Resistance" commonly seen in the measurements.)

The advice has always been to test the permanent link and then attach standards compliant patch cords. However, this approach can result in problems for PoE if cords and their connectors present resistance unbalance errors that are not tested as part of the complete channel. We strongly recommend testing the complete channel in any system, with the patch cords in place. To use tested permanent links without surprises, one would need to use specified patch cords designed to prevent resistance unbalance. Patch cords with IDC termination, for example, ensure a reliable, low resistance wire termination over the entire service life of the cord.

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. The assurance that the cable bundle will not overheat is essential for RP3 category compliance. An RP3 category MUST be applied for all newly built systems for offices, homes, industry and smart building system. Every cable in a bundle MUST be able to fully support PoE of the highest level and carry that burden simultaneously. However, undue resistance unbalance will increase the RMS current of a cable and therefore invalidate RP3 calculation and planning. All that power being transmitted along bundled cables will generate heat, affecting data transmission. 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. Certain devices already test for this parameter as part of the basic test. Field testing is more important than ever as PoE becomes widespread, supported by developments in systems convergence and ‘all over IP’. Resistance unbalance testing ensures installations support PoE without introducing data transmission issues or excess heat. That makes it vital to warranty agreements and avoiding costly equipment replacement and rework.