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Industry Insights

PoE, Topics That Impact Connectivity and Cabling - "Power Loss & Heat" - Part 3

Tuesday, July 19, 2016 | by Gregg Lafontaine

“Power over Ethernet (PoE) technology is designed to safely deliver DC electrical power, along with data, to remote devices over standard data-com cabling on an Ethernet network.”
 
(PoE) is a low voltage technology that offers the ability to power an end device from a central location, such as a telecom room or communication cabinet, using the same transmission cabling path at the same time as Ethernet data transmissions.
 
While PoE has been successfully deployed for over 13 years, proposed higher power PoE is raising new questions about cabling, connectivity and installation.
 
IEEE 802.3af was the first standards based PoE, supplying 15.4W from the PSE (Power Supplying Equipment) delivering a minimum of 12.95W to the PD (Powered Device).  From the beginning, IEEE designed PoE applications & standards based on the specifications & understandings of 4 pair structured cabling defined by TIA & ISO/IEC standards. Later a PoE+ version was introduced as IEEE 802.3at, supplying up to 30W from the PSE and a min of 25.5W delivered to the PD. The increased power of 802.3at supported multi- band, multi- antenna WAPs and motorized (PZC) cameras.  Both of these PoE standards (802.3af & at) only used 2 of the available 4 pairs to supply power.
 
The growing number of devices utilizing or looking to use PoE continues to increase expectations for the application and the supporting infrastructure. Higher PoE power levels were pursued so new PoE devices that would require more power could be created.  Applications began to surface with some of the following estimated power requirements:

    • Digital Signage >30W
    • Retail Point of sale devices 30-60W
    • Multichannel Wireless APs >30W
    • Pan, tilt and zoom (PTZ) cameras 30-60W
    • Banking/financial IP turrets/ kiosk 45W
    • Building management, air & access controls 40-50W
    • Virtual desktop terminals 50W
    • Healthcare Nurse call systems up to 50W

While some of the above applications are being addressed with proprietary approaches for delivering power, the IEEE has been working to introduce standardsbased models for higher power versions of PoE.
 
The PoE application and supporting infrastructure, including the cabling, must deliver more power while also increasing efficiency.  In addition to higher power needs, some devices, such as IEEE 802.11ac WAPs, will exceed gigabit Ethernet speed so these new standards will also look to accommodate PoE with higher speed Ethernet  (such as 2.5GBASE-T, 5GBASE-T and 10GBASE-T)
 
Pending (higher power) IEEE PoE standards use all 4 pairs and are projected to be ratified in 2017.
These include:

    • A proposed IEEE 802.3bt PoE Type 3, providing 60W at PSE, with a min of 49 watts available to the PD, (double the power of IEEE 802.3at) 
    • A proposed IEEE 802.3bt PoE Type 4 will provide 100W at PSE • with up to 96 watts available at the PD

 
Both of the proposed 802.3bt PoE standards are expected to increase the Max DC current per circuit/pair to 960 mA.
 

“In addition to the above existing and pending IEEE standards other proprietary approaches include Cisco UPOE which also targets 60W and 100W supplied by the PSE.”
 
The impact on connectivity:   While jacks and panels may have little attenuation impact, because circuits within these parts are very short, these connectors can still influence power throughput. Many of today’s jacks and panel connectors utilize printed circuitry to control/cancel noise and elevate readable signal performance. This emphasis on the data transmission when designing printed circuit  board (PCB) traces did not cause concerns when the initial lower power PoE standards were released. But PCB circuit paths (total surface area, trace widths and content) can differ from one design to another, and one manufacture to another. Each variation can produce differences in current carrying capacities. And now we need to be more aware of these current carrying capacities. Connectivity that utilize printed circuit boards and designed to IEC 60512-99 recommendations are already designed to support 1 amp on each circuit path/trace. (Since a complete circuit is a loop, typically containing 2 traces, this requirement can sometimes be listed as 1 amp per pair)  A 1 amp design has been more than enough to support the max 350mA of 802.3af and the max 600mA of 802.3at. But with 960mA projected for 802.3bt types 3 & 4 we are getting quite close to the 1 amp capability and design protection recommended by the IEC.  To ensure some level of margin, connectivity with PCBs would benefit from all circuit traces designed to handle more than 1 amp.  
 

What to do:

  1. If connectivity does not comply with IEC 60512-99, that connectivity should not be described as able to support all levels of PoE.
  1. By depending on the only standards body recommendation to date (IEC 60512-99), compliant connectivity might be very close to its design limits when supporting the high power PoE applications. To eliminate risk connector manufactures are designing beyond a 1 amp capacity. 
  1. Connectivity designed or specified with 1.5 amp or greater will offer larger power paths capacity and margin.

 
Higher PoE current levels can also have an impact on patch cords.  The first impact relates to attenuation (power lost).  The TIA already de-rates 24awg patch cords by a factor of 1.2 when compared to 24 awg solid conductor horizontal cable for the following reasons:
 

    • Stranded conductor is not as efficient a medium as solid conductor
    • The TIA wanted to budget for the anticipated use of stranded conductor patch cords in their Structured Cabling Systems (SCS) channel models.

 
When comparing other gauge patch cords, 26awg conductors lose (attenuate) 25% more power than 24awg stranded patch cords. 28awg conductors lose (attenuate) 37 % more power than 24awg stranded patch cords. So if 26awg or 28awg patch cords are used in longer channels, the actual PoE power available to a PD may be less than what was planned by the IEEE.  
 
A second impact is the heat generated by the above increase in attenuated power caused by a smaller wire gauge size. This heat increase is not considered a safety issue, because cable designs can safely handle significant increases. But the data transmission performance of the channel is more sensitive to heat rise, making this a potential performance issue when using smaller gauge cords in highly powered applications. The issue of increased heat with both cords and cables is further complicated when cords or cables are bundled, which reduces the ability of cables in the center of bundles from dissipating any heat generated by attenuation. 
 
TIA TSB-184 (Technical Service Bulletin) and TIA184A ( currently in draft development ) include guidelines for horizontal cable bundle sizes based on conductor gauge to minimize un dissipated heat increase within cables while also recommending max heat increase limits for cables to ensure proper data transmission performance of cables.  It is recommended that bundle sizes should be limited to ensure no more than a 15 degree C temperature increase from a targeted base of 45 degrees C.  
 
So at a time when many installation specifications have been looking for smaller cords and cables to address density, the trade off can be a counter-productive relative to channel performance.
This leads to considering a recommendation that 802.3BT type 3 & 4 or Cisco UPoE be supported with full size patch cords to reduce power loss and heat generation.

 Summary:

 “Power over Ethernet (PoE) technology is designed to safely deliver DC electrical power, along with data, to remote devices over standard data-com cabling on an Ethernet network.”
 
While PoE has been successfully deployed for over 13 years, proposed higher power PoE is raising new questions about cabling, connectivity and installation.
 
Cabling components that are expected to support the higher power PoE should, at a minimum, meet IEC 60512-99 recommendations of 1 amp capacity while connectivity specified with 1.5 amp trace capacity or greater will offer larger power path capacity and margin.
 
Because cables and cords can be bundled together, the cables/ cords most internal to larger bundles may not be able to dissipate enough heat to stay within the +15 degree max guidelines recommended by TIA TSB-148 (Technical Service Bulletin). Bundle sizes should be reduced or avoided altogether with smaller conductor cords.
 
Consider a recommendation that 802.3BT type 3 & 4 or Cisco UPoE applications should be supported with full size patch cords to reduce power loss and heat generation.