Power over Ethernet (PoE)

What is POE?

PoE is a mechanism that enables power to be delivered to connected devices using the installed data cables, thus eliminating the requirement to provide power at the device location. This has numerous benefits, including a reduction in the amount of cable that needs to be installed and the ability to power devices that are not close to any existing power source.

PoE Components

There are two key components for delivering PoE
1. Power Sourcing Equipment (PSE)
There are two types of Power Sourcing Equipment
                - End span
                               - power is injected by the switch

End span PoE diagram

                - Mid span
                                - power is injected by a standalone device that sits between the switch and the
                                  powered device

Mid span PoE diagram

PD Detection

The PSE initially applies a low voltage on the wires and a valid PD will present a specific signature to the PSE.  If no valid signature is detected the PSE will NOT send power but will still pass data. If a valid signature is detected then the PD may optionally present a classification signature. Upon completion of the detection process, power is supplied by the PSE.

Types of PoE

There are various different levels of power that can be provided by PoE, a summary of which is provided below:

PoE standards table

Potential Issues

  1. High power increases heat in cable bundles
              - This can be mitigated by reducing the size of the cable bundles or loose laying cables as
                well as the use of shielded cables.
  2. Arcing when disconnecting under load
                    - This is unavoidable when disconnecting a powered device.
                    - It can be mitigated through the design of the contacts of the jacks and
                      plugs, i.e. ensure that the initial connection points are different from those
                      where the data is passed.

                Mated Plug Jack Contact Image                  Unmating Plug Jack Contact Image

                - TSB-184-A contains guidance on contact design and IEC 60512-99-001 is referenced as a
                  suitable test regime

Conclusions

The demand for high power PoE and high data transmission rates means that Cat 6A cables should be installed to Wireless Access Point locations.
When procuring cabling components look for independent certification to IEC 60512-99-001.