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Industrial Ethernet Book Issue 69 / 38
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PoE+ networks: how to keep a hot application running cool

The advent of PoE Plus (IEEE 802.3at) brought to light a significant new problem in delivering power over structured cabling systems. The higher supply currents delivered to, and drawn by, 802.3at devices has the potential to create a significant temperature rise within the cabling. This may adversely affect system performance. The following article, based on a white paper by Siemon, explores both the problem of currentinduced temperature rise and the measures which may be taken to minimise its effects.

THE ALLURE OF deploying power concurrent with data over telecommunications cabling is undeniable. The benefits of IEEE 802.3af Power over Ethernet (PoE) equipment include simplified infrastructure management, lowered power consumption, reduced operational costs, and better safety through separation from the building's main AC power ring.

With its 13W average input power to the powered device (PD) at a safe nominal 48V DC over TIA category 3/ISO class C and higher rated structured cabling, IEEE 802.3af PoE, ('Type 1') systems can easily support IP-based voice, video transmission equipment and network security cameras, wireless APs, RFID tag readers, many building automation systems, print servers, and bar code scanners.

UTP vs F/UTP and S/FTP

Insertion loss increases (signals attenuate more) as the ambient temperature around cabling increases. Both TIA and ISO, therefore, specify a temperature dependent de-rating factor to determine 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 (20C).

Less well known is that the de-rating adjustment made for UTP cabling allows for a much greater increase in insertion loss (0.4%/C from 20-40C, and 0.6%/C from 40- 60C) than the de-rating adjustment that is specified for F/UTP and S/FTP systems (0.2%/C from 20-60C). So F/UTP and S/FTP cabling systems have more stable transmission performance at higher temperatures and better support PoE Plus compared with UTP cabling.

PoE Plus

In 2005, IEEE enhanced the capabilities of power sourcing equipment (PSE) to deliver more power potentially to support devices such as laptops, thin clients (typically running web browsers or remote desktop software applications), pan/tilt/zoom security cameras, internet protocol television (IPTV), biometric sensors and WiMAX transceivers providing wireless data over long distances (e.g., point-to-point links and mobile cellular access), plus many other devices that need extra power.

To support these needs, IEEE 802.3at specifies a PoE Plus ('Type 2') system that can deliver 25.5W average input power to the PD at a safe nominal 53VDC over legacy TIA category 5/ISO class D:1995 and higher rated structured cabling. For new installations, cabling should meet or exceed TIA category 5e/ISO class D:2002 requirements. Table 1 compares PoE with PoE Plus systems.

Table 1: Overview of PoE and PoE Plus system specifications.

The power challenge

The development of PoE Plus requirements brought to light a significant challenge in the specification of power delivery over structured cabling. Because of the higher power, the IEEE needed to understand the temperature rise within the cabling caused by applied currents. It then had to specify the PoE Plus application operating environment to ensure that proper cabling system transmission performance be maintained. The IEEE enlisted the help of the TIA and ISO cabling standards development bodies to characterise the current carrying capacity of various categories of twisted-pair cables. TIA developed profiles of temperature rise versus applied current/pair for Cat5e, 6, and 6A cables configured in 100-cable bundles (See Figure 1 right).

Fig. 1: PoE Plus cable temperature rise. TIA developed profiles of temperature rise versus applied current/pair for Cat5e, 6, and 6A cables configured in 100-cable bundles.

These profiles were primarily based upon analysis of unshielded twisted-pair (UTP) cable performance - later corroborated by data submitted to the ISO committee. Since Cat5e cables have the smallest conductor diameter, they also have the worst heat dissipation and thus exhibit the greatest temperature rise. (Cat5 cables were excluded because this cabling is no longer recommended by TIA for new installations). The IEEE adopted the baseline profile for Cat5e cables as representative of the worst-case current carrying capacity for cables supporting PoE Plus.

Following further TIA guidance, the IEEE reduced the maximum temperature for Type 2 operation to 50C, which eliminated complicated power de-rating at raised temperatures. Next, a maximum DC cable current that would not create a temperature rise above 10C had to be found.

Analysis of the worst case Cat5e current carrying capacity profile led IEEE PoE Plus system specifiers to target 600mA 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. This is less than the maximum 10C value recommended, but provides headroom helping to offset further increases in insertion loss because of higher temperatures. It also minimises risk of premature jacketing materials aging. This operating margin is critical because the condition cannot be field ascertained.

Also, higher temperature rises occur as cable bundle size increases. Analysis of the worst case Cat5e profile resulted in TIA providing general guidance that the maximum power injected into any cable bundle should not exceed 5kW up to 45C. However, this is outside the scope of 802.3at.

Keeping it cool

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 (e.g., bundling) that support minimal temperature rise through applied current. The recommended approach is to select cabling media that has superior heat dissipation performance to start with. While the TIA current carrying capacity profiles are helpful by demonstrating relative advantages between select media types, they don't address the performance of Cat6A screened (F/UTP) and Cat7A fully-shielded (S/FTP) cables.

Siemon Labs investigated the current-carrying capacity of riser (CMR) and plenum (CMP) Cat6A F/UTP and Cat7A S/FTP cables, in addition to slim-profile Cat6A UTP cables. Test cables were arranged in accordance with the TIA 100- bundle cable configuration (see Box 100-bundle tests), and the worst case temperature rise for each media type was profiled. Reference Cat6A UTP measurements were collected and used to normalise Siemon data to the TIA Cat6A data. The resulting heat dissipation profiles are shown in Fig. 2.

Fig. 2: Heat dissipation profiles. Siemon Labs investigated the currentcarrying capacity of riser (CMR) and plenum (CMP) Cat6A F/UTP and Cat7A S/FTP cables, in addition to slim-profile Cat6A UTP cables. These curves compare the heat dissipation profiles for each cable type.

The current carrying capacity of Cat7 cables is expected to be equivalent to Cat7A cables because they are so similar in construction. The worst case temperature rise for each media type with 600mA applied current is shown in Fig. 3.

Fig. 3: Worst case temperature rise. This is for each media type with 600mA applied current The current carrying capacity of Cat7 cables is expected to be equivalent to Cat7A cables.

Dissipation myth

As metal has a higher conductivity than thermoplastic jacketing materials, thermal models predict that screened and fully-shielded cables have better heat dissipation than UTP cables. Siemon's data substantiates the model and clearly shows that screened cables dissipate heat better than UTP cables. Fully-screened cables have the best heat dissipation of all copper twisted-pair media types. Unfortunately, the misconception that screened and fullyshielded systems will trap the heat generated by PoE and PoE Plus applications still exists. This notion is completely false.

Media selection

Interestingly, PoE Plus is compatible with 10BASE-T, 100BASE-T, and 1000BASE-T, while compatibility with 10GBASE-T is not precluded by the latest standard. Therefore, in an attempt to operate over the largest percentage of the installed cabling base possible, 802.3at specifies ISO '11801 class D:1995 and TIA '568- B.2 Cat5 compliant cabling systems having DC loop resistances less than or equal to 25ohms as the minimum grade of cabling supporting PoE Plus.

These are legacy grades of 100MHz cabling; TIA recognises '568-C.2 Cat5e cabling, and ISO recognises class D:20028 cabling for new installations. While these objectives represent good news for end-users with an installed base of Cat5/Cat5e or class D:1995/class D:2002 cabling, such cabling systems typically have poor heat dissipation properties - much better choices now exist.

Specifying cabling with better heat dissipation characteristics means that:

Operating temperatures are less likely to exceed 50C;

Common installation practices, such as bundling, are less likely to affect temperature rise;

Undesirable increases in insertion loss though higher temperatures will be minimised;

Cabling jacket material premature aging is reduced.

Good heat dissipation performance by cabling plant is critical, since no methods exist today for monitoring temperature rise in an installation. Historically, a comfortable performance margin is considered to be 50% headroom to standards-specified limits (equivalent to 6dB headroom for a transmission performance parameter).

Following these guidelines, the solutions that offer the most desirable levels of heat dissipation headroom in support of PoE Plus are Cat6A F/UTP and Cat7A S/FTP cabling systems. Indeed, Cat7A S/FTP cabling systems dissipate at least 60% more heat than Cat5e cables.

100-bundle tests

An overview of 100-Bundle temperature rise v applied current test configurations is (in brief): beginning with a core comprising a 1.2m length of cable, layers of 1.2m long cable lengths were carefully applied around the core to create a symmetrical 6- around-1 bundle. This was secured with electrical tape and a thermocouple was embedded into the cable jacket. Extra 1.2m cable lengths were applied, taped, and embedded with thermocouples to grow the bundle size incrementally to a 100-around-1 bundle.

Careful testing at the applied 2.88A current showed that the highest temperature rise was recorded at the thermocouple closest to the core of the bundle. The thermal resistance of the cable bundle was determined from measurements, and a heat dissipation profile, including performance at 600mA, was derived. The measurement accuracy is around +/- 1C.

Beyond PoE Plus

With the many functional and cost-savings advantages associated with PoE Plus, the need to supply even more power to the PD is becoming more pronounced. Fortunately, an element of improved heat dissipation is also the ability to support more current delivery within the IEEE maximum 10C temperature rise constraint.

Based upon their vastly superior current carrying ability, it is a safe bet that Cat6A and higher-rated cabling will be the targeted media for the support of tomorrow's high performance telecommunications powering applications.

From the Siemon Company paper IEEE 802.3at PoE Plus Operating Efficiency: How to Keep a Hot Application Running Cool

Source: Industrial Ethernet Book Issue 69 / 38
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