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Is IEEE 802.3bt a game changer for PoE?

Originally written for Inside Networks: December 2018 Issue


Given the importance of power over Ethernet in the development of intelligent buildings, how much of a ‘game changer’ is IEEE 802.3bt in terms of creating a single network infrastructure for building services technologies? How will this standard affect the specification of structured cabling within buildings and what do you think the future holds for PoE?

There is little doubt that the ability to transmit power over all four pairs of a data cable represents perhaps the final step in the evolution of PoE. However, IEEE 802.3bt has to be seen in context with other standards that have been amended this year that also impact on adoption. In combination with EN 50173-2:2018 which no longer recognises Class D / Cat5e for new office spaces, it becomes a more compelling proposition. Whether these new standards will accelerate the implementation of connected buildings or IoT remains to be seen but this will ensure that office cabling infrastructure in future is fit for purpose for the adoption of some form of PoE.

95W PSE as ushered in by IEEE 802.3bt Type 4 /  PoE++ brings into scope more remote powered device types, including lighting, signage & monitors as well as a higher power, wave 3 AP’s and turret PTZ CCTV.  Logic would indicate greater implementation of zone architectures to minimise cable run lengths and support ELV office desks that will power a laptop or a low wattage PC. If there can be a reduction in the number of 13A sockets in an office there could be a tipping point when deploying PoE makes economic sense all round.

The cable length and type will be more critical than previously due to potential losses. With the introduction of MPTL (Modular Plug Terminated Link) in the standards e.g. ANSI/ TIA-568.2-D, there comes a simplification of cabling to remote devices and consequently fewer points of failure/power loss. However, installers will need to become familiar with the test methodology for MPTL links.

Readers will be fully aware of the potential for power loss over copper cables and the restrictions that will apply to using legacy Cat5e and Cat6 24AWG cables for PoE + and now PoE ++. With the introduction of 802.3bt and use of all 4 pairs, it will be necessary to confirm the suitability of existing cabling. However, it is not just the type of cable and CSA of the conductor but it is also advisable to check the resistance unbalance between the pairs which are defined in IEEE 802.3bt. If this is unequal (e.g. due to uneven twisting of cable) this could cause performance issues if significant. In addition, mating RJ45 plugs and sockets will ideally need to be selected to minimise arcing at the connectors when subjected to powered connection/disconnection. There are several manufacturers that have designed connector contacts with this in mind and new testers to check PoE availability are also being introduced.

Overall there is a lot to take in but everything points to the widespread future adoption of PoE in new buildings. 

27 August 2019