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manner may prove to be alright for home use, but not when used in cable bundles.
Copper coated aluminium If the problem was not bad enough, recent years have seen the introduction of copper coated aluminium (CCA) data cables, claimed to be manufactured to the TIA Category 5, 5e and 6 specifications. These CCA cables come in a wide array of
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formats, ranging from an aluminium core with a few microns of copper coating on the outside, to a small aluminium core coated with 65 per cent-by-volume copper coating. This has caused consternation among some of
that they expect them to rise to. If unsure, then the dilemma begins. Sizing electrical cables for a 60°C ambient temperature adds expense to the electrical installation – hence, it is important, wherever possible, to make an accurate prediction. However, when it is not possible, or the data designer/installer is unsure of their client’s future PoE aspirations, then they should advise the electrical designer that it is possible that the data cables may run as hot as 60°C, and the electrical designer must then design the electrical installation accordingly. In most cases, this will usually only affect a small part of the electrical installation. It is important to understand what you are buying
and how you are installing it, as many PoE systems do not comply with the IEEE 802.3af or IEEE 802.3at standards or existing good advice on installing PoE systems. Within the British Standards system, its telecommunications committee TCT/7/-/2 is currently reviewing the matter and will give further guidance on the subject as soon as possible. The ECA chairman of the BSI TCT/7/-/2 committee, Tim Oldershaw, has seen an increase in the number of third-party PoE installations that his company, J Brand, has been called to, in order to give advice on how to rectify problems or repair damage done to installations. Last year, we saw a group calling itself the HDBaseT consortium introduce its own standard that delivers 100 watts to the PD. Its standard is intended to pave the way for next generation ‘Energy Star’ certified televisions to be supplied by PoE. It must be said that singular cables in ambient room temperatures are generally not found to be a problem. Hence, this level of power delivered in that
60 ECA Today November 2012
data cable-manufacturing fraternity, as the CCA cables are being sold very cheaply. There are many presentations showing CCA cable failing cable tests, and statistical information on the resistance of aluminium conductors versus copper conductors, none of which usually state what the composition of the CCA cable is. However, this apart, we can safely say that aluminium has a higher resistance than copper and, therefore, when used with PoE they will produce more heat than copper-only conductors, and this will increase with aluminium content. The heat in cables is produced due to the I2R losses, so the higher the resistance of the core, the higher the level of heat produced for the same value of current.
Compounding the problem, some suppliers
and installers have installed the cables without even knowing that the cables were CCA, raising the question: ‘Were the cables tested?’ and, if so, ‘How did they pass their test?’. Regardless of the answers to these questions, the cables present their owners with a real problem if they want to use them for PoE purposes, particularly at high PoE values.
About the author
Eur Ing David Stefanowicz C.Eng. MInstMC.MITE.MIEEE is ECA technical manager – ITEC
Zone cables Another worrying trend when considering PoE is the introduction of so-called ‘zone cables’, which are being produced by some very reputable data cable manufacturers. With all the best of intentions, some data cable manufacturers are producing data cables to their own specifications (or IEC standards intended for other purposes), which are only suitable for 50-metre cable runs. The cables perform adequately to their data transmission requirements, but have reduced-size copper conductors and overall diameters. While this is a very desirable situation from most points of view other than a PoE, it has the tendency to worry users of PoE, as they may be mistaken for normal data cables, causing heat and performance problems at high PoE values – or when used inadvertently for runs longer than 50 metres. They can also create problems on shorter runs, due to the higher resistance of their smaller conductors. The message from this article, therefore, is: ‘Be
sure of what you are buying and how to design an installation that will use PoE, from both a data and electrical perspective.’
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