[ Spotlight: Power over ethernet ]
Some people think the main problem with heating cables is that they will catch fire, but although this is a possibility, it is not the main problem
Increased noise When electrical noise in a data cable increases, the signal to noise ratio (SNR) falls. As the temperature increases further, it can reach a point where it starts to cause detrimental SNR and the bit error rate increases, thus slowing down the operational speed of the link. At a certain temperature, the noise will be so great that the bit error rate makes the link unworkable. However, power will usually still manage to get through. Another problem is encountered when PoE
systems are permanently forced to their ‘on state’. This can lead to arcing and subsequent burning of connections when connecting and/or disconnecting and, sometimes in severe cases, fire. In the UK, as in many other parts of the world, we
are very keen on using dual compartment plastic dado trunking, whereby the data cables are often run in the bottom compartment and the electrical cables are run in the top compartment. When this happens and PoE causes heating of the bottom compartment to 60°C, it may cause serious problems to the electrical design, as electrical cables are generally only sized for an ambient temperature of 30°C. BS 7671 Table 52.1 gives the maximum conductor
data cables with larger conductors (Category 6A and 7A), smaller cable bundles, routes which have better ventilation, avoiding grouping cables carrying higher power together, and so on. Some people think the main problem with
heating cables is that they will catch fire, but although this is a possibility, it is not the main problem. Data cables manufactured to EN 50288 have insulation capable of withstanding 60°C maximum, and it should be noted that copper does not melt till over 1,000°C – hence, there usually should be no problem there either. When copper is at minus 273°C, there is no
electrical noise generated in the conductors. As the cable heats up to room temperature, the electrical noise increases, and as the cable continues to heat up due to the transmission of power, the electrical noise increases further. It often does not take much for the temperature
to reach the cable maximum of 60°C within a cable bundle in badly ventilated spaces or poorly designed equipment rooms. Sometimes, high temperatures only affect just a short part of a cable run where full consideration has not been given in the design/ installation phases of a project. With global warming and subsequent ‘hot days’ it is easy for an office to reach 40°C for short durations; 40°C is also sometimes quite close to the ambient temperature in machinery rooms and data centres.
temperature of thermoplastic cables as 70°C and thermosetting cable at 90°C, respectively. This temperature is based on the maximum cable current carrying capacity, given in Appendix 4, which is based on a maximum ambient temperature of 30°C. This figure also allows for fault level currents and disconnection times. Regulation 522 in BS 7671 gives guidance on ambient temperatures (522.1) and external heat sources (522.2). All of these issues have to be considered when designing the electrical installation, in addition to bunching and grouping de- rating factors, and so on.
Advice So, what should happen? In a perfect world, the data designer or installer should advise the electrical designer that their data cables will run
Top 10 tips for reducing heat due to PoE
1) Use low power devices (PDs). 2) Use high energy efficient PSUs. 3) Use 4Pair solutions. 4) Use data cables with large conductors (23AWG or better).
5) Keep cable runs as short as possible. 6) Use smaller cable bundles. 7) Run cables in well ventilated spaces. 8) Use third party certified cables. 9) Do not use CCA data cables. 10)Periodically check the temperature of cable bundles.
November 2012 ECA Today 59
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