scope:

This PAS is a technical supplement to IEC 61156-1, edition 3 (2007): Multicore and symmetrical pair/quad cables for digital communications - Part 1: Generic specification.

This PAS, covering exclusively four-pair data grade cables, is intended to provide a test method for the determination of the maximum attained conductor temperatures which occur due to the deployment of the IEEE protocol for PoE /PoEP.

It gives as well the required background information about the thermodynamic behaviour of such bundled cables, if they are located in areas with restricted heat dissipation, a reality which occurs in every installation situation. However, only the basic principles are given, as the rigorous application and solution of these problems fall into the relevant cabling standards.

NOTE 1 The restriction to four-pair data grade cables is very important, as the heating of a multiple pair cable, especially if it has a protective screen, is much worse, since the ratio of the heat generation within the cross-section versus the overall circumferential surface to dissipate the heat is dramatically decreased, thus yielding substantially higher conductor temperatures. Additionally, the screen acts as a near perfect IR-reflector, thus increasing additionally only the excess heat within the cable.

This restriction is of importance considering the installed base, where individual four-pair cables in a loose bundle arrangement may need to replace multiple pair cables.

Hence, the main objective is

a) the indication of a suitable measuring method to assess the heating gradient across bundled data grade cables subject to d.c. power transmission, using for the incident and return conductors the common mode circuits of either two or four pairs;

b) to provide, toward this end, the worst case assessment of the conductor and cable heating in bundled cable configurations, where the densest hexagonal packing configuration is required. This assessment of the heating is anticipated to be carried out under the extremely lenient condition of freely suspended cable bundles in an air-conditioned environment free of any air draft, the heat dissipation thus being achieved by undisturbed convection into the surrounding environment;

c) to provide some explanatory background information on the heat dissipation of heated conductors, insulated conductors, pair and cables, both screened or unscreened;

d) to provide means to assess the installed base of data grade cables with a view to their compliance with the requirements of either PoE or PoEP, if required in a comparative way, but based on the resistance assessment of at least one short cable length withdrawn from the installed base by replacement;

e) to indicate the basic physical assessment proceedure, based upon the testing of a cable bundle according to item b). A comparable heating trial on the same cable bundle, but under restricted heat dissipation conditions, yields then some indication of how to assess the maximum occurring temperatures under these conditions;

f) towards this end, the densest hexagonal packing configuration has to be simplified, using an equivalence in order to allow a consecutive evaluation of the heating under any heat dissipation restriction using a layered structure of the cables and the interstitial air spaces within the bundled structure.

NOTE 2 The scope of this PAS exclusively covers the cable performances. The variable heat insulating properties of the cables resulting out of the installation practices for channels (for instance feeding bundled cables through insulating materials) is outside the scope of this PAS. This has to be initiated and be taken care of in ISO/IEC JTC1/SC25 WG3 in a suitable technical report or installation guide. This is the reason that here only general guidelines are given.

The test method described lends itself also to cable testing if higher currents than those resulting out of the basic specified d.c. resistances and the specified currents for the IEEE 802.3 PoE / PoEP protocol are required. This would eventually allow the transmission of higher powers at the same maximum ambient temperature of 60 ºC, without exceeding the maximum permissible conductor temperatures in the cable. This may be applicable to higher performing cable categories in cases where the user really needs the transmission of higher power levels than anticipated in the IEEE 802.3 PoE / PoEP protocol.

In these cases, a verification of their conductor heating properties has to be assessed, and the cable performance has to be guaranteed by the manufacturer.

The PAS is written in a general way, thus covering not only horizontal cables. Stranded cord cables will have to be evaluated as well, and this very carefully, as they are so far installed in the equipment rooms in higher cable count bundles as well. This PAS establishes some basic guidelines to deal with these problems.

The heating in this PAS is the result of the resistance which is specified in IEC 61156-5 and IEC 61156-6 as 19 [ohm / 100 m] and 29 [ohm / 100 m].