The development of the pending PoE Plus requirements has brought to light a significant new challenge in the specification of power delivery over structured cabling.
July 1, 2009
The higher power delivered by PoE Plus devices causes a temperature rise within the cabling which can negatively impact system performance. The information in this article will allow readers to be better equipped to make PoE Plus-ready cabling choices that will support reduced current-induced temperature rise and minimize the risk of degraded physical and electrical performance due to elevated temperature.
It is well known that insertion loss increases (signals attenuate more) as the ambient temperature in the cabling environment increases. To address this issue, both TIA and ISO specify a temperature dependent de-rating factor for use in determining the length that the maximum horizontal cable distance should be reduced by to ensure compliance with specified channel insertion loss limits at temperatures above ambient (20 C).
What is not well known is that the de-rating adjustment that is made for UTP cabling allows for a much greater increase in insertion loss (0.4% increase perC from 20C to 40C and 0.6% increase per C from 40C to 60C) than the de-rating adjustment that is specified for F/UTP and S/FTP systems (0.2% increase perC from 20C to 60C).
This means that F/UTP and S/FTP cabling systems have more stable transmission performance at elevated temperatures and are more suited to support applications such as PoE Plus than UTP cabling systems.
PoE Plus Challenges: The development of the pending PoE Plus requirements brought to light a significant new challenge in the specification of power delivery over structured cabling.
For the first time, due to the higher power delivered by Type 2 PSE devices, IEEE needed to understand the temperature rise within the cabling caused by applied currents and subsequently specify the PoE Plus application operating environment to ensure that proper cabling system transmission performance is main- tained. In order to move forward, IEEE enlisted the assistance of the TIA and ISO cabling standards development bodies to characterize the current carrying capacity of various categories of twisted-pair cables.
After extensive study and significant data collection, TIA was able to develop profiles of temperature rise versus applied current per pair for category 5e, 6, and 6A cables configured in 100-cable bundles as shown in Figure 1. Interestingly, these profiles were created primarily based upon analysis of the performance of unshielded twisted-pair (UTP) cables.
They were later corroborated by data submitted to the ISO committee. As expected, since category 5e cables have the smallest conductor diameter, they also have the worst heat dissipation performance and exhibit the greatest temperature rise due to applied current.
Note that category 5 cables were excluded from the study since category 5 cabling is no longer recommended by TIA for new installations. IEEE adopted the baseline profile for category 5e cables as representative of the worst-case current carrying capacity for cables supporting the PoE Plus application.
Additional TIA guidance recommended that a maximum temperature increase of 10C, up to an absolute maximum temperature of 60C, would be an acceptable operating environment for cabling supporting PoE Plus applied current levels. In consideration of this input, IEEE chose to reduce the maximum temperature for Type 2 operation to 50C, which eliminated the need for complicated power de-rating at elevated temperatures. Next, IEEE had to identify a maximum DC cable current that would not create a temperature rise in excess of 10C. An analysis of the worst case category 5e current carrying capacity profile led IEEE PoE Plus system specifiers to target 600 mA as the maximum DC cable current for Type 2 devices, which, according to the TIA profile, results in a 7.2C rise in cable temperature.
Although this temperature rise is less than the maximum 10C value recommended, it provides valuable system headroom that helps to offset additional increases in insertion loss due to elevated temperatures and minimize the risk of premature aging of the jacketing materials. Operating margin against excessive temperature rise is especially critical because this condition cannot be ascertained in the field. An additional outcome of the TIA investigation was the understanding that higher temperature rises occur as cable bundle size increases. Analysis of the worst case category 5e profile resulted in TIA providing general guidance that the maximum power injected into any cable bundle shall not exceed 5kW up to 45C. This constraint is likely to be further tightened to accommodate temperatures up the IEEE maximum of 50C. Interestingly, while IEEE acknowledges that cable current carrying capacity is a function of cable type and installation practices (e. g. bundling), it is outside the scope of the proposed 802.3at Standard to address these considerations.
It is anticipated that TIA will address these issues in a future Telecommunications Systems Bulletin (TSB). This TSB is anticipated to describe environmental conditions of installed cabling (including bundled cabling and cabling in conduit), heat
dissipation profiles of different categories and types of cables, and how these conditions might impact the capability of telecommunications cabling to support the PoE Plus application.
Keeping it Cool: For the first time, managing heat build-up in structured cabling must be taken into consideration in new and retrofit installations. The main challenges in designing PoE Plus-ready cabling plants are ensuring that operating temperatures do not exceed 50C and specifying cabling types and installation practices, such as bundling, that support minimal temperature rise due to applied current.
Since there is no easy method to cool down hot pathways or for monitoring PoE Plus induced temperature rise in the installed cabling, the recommended approach to minimize the risks associated with excessive temperature rise is to select cabling media that has superior heat dissipation performance to start with. While the TIA current carrying capacity profiles are helpful in that they clearly demonstrate relative advantages between select media types (e. g. category 6A UTP cables have better heat dissipation performance than category 5e UTP and category 6 UTP cables), the story that they tell is incomplete. Specifically, they do not address the performance of category 6A screened (F/UTP) and category 7A fully-shielded (S/FTP) cables.
Siemon Labs investigated the current- carrying capacity of riser (CMR) and plenum (CMP) category 6A F/ UTP and category 7A S/FTP cables, in addition to the new slim-profile category 6A UTP cables that are just starting to emerge in the market.
Test cables were arranged in accordance with the TIA 100-bundle cable configuration and the worst case temperature rise for each media type was profiled. Reference category 6A UTP measurements were collected and used to normalize Siemon data to the TIA category 6A data. The resulting heat dissipation profiles are shown in Figure 3.
The current carrying capacity of category 7 cables is expected to be equivalent to category 7A cables since their physical construction is so similar. The worst case temperature rise for each media type with 600 mA applied current is shown in Figure 2 on p 15.
Dispelling the Heat Dissipation Myth: Since metal has a higher conductivity than thermoplastic jacketing materials, a thermal model can be used to predict that screened and fully-shielded cables have better heat dissipation than UTP cables. Siemon’s data substantiates the model and clearly demonstrates that screened cables exhibit better heat dissipation than UTP cables and fully-screened cables have the best heat dissipation properties of all copper twisted-pair media types. Unfortunately, the misconception that screened and fully-shielded systems will “trap” the heat generated by PoE and PoE Plus applications still exists in
the industry today. Th
is notion is completely false and easily dispelled by models and laboratory data.
Media Selection: Interestingly, the PoE Plus application is targeted to be compatible with 10BASE-T, 100BASE-T, and 1000BASE-T, while compatibility with 10GBASE-T is noted as not being precluded by the new Standard. Thus, in an attempt to operate over the largest percentage of the installed cabling base possible, the pending 802.3at Standard specifies ISO ‘11801 class D:19955 and TIA ‘568-B. 2 category 56 compliant cabling systems having DC loop resistances less than or equal to 25 ohms as the minimum grade of cabling capable of supporting PoE Plus. CNS
Valerie Maguire is the Global Sales Engineer for The Siemon Company.