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Industry Insights

PoE, Topics That Impact Connectivity and Cabling - "Spark Gap" - Part 2

Tuesday, July 19, 2016 | by Gregg Lafontaine

“Power over Ethernet or 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 closet or communication cabinet, using the same transmission cabling path at the same time as Ethernet data transmissions.
 
IEEE PoE applications were designed based on specifications & understandings of structured cabling defined by TIA & ISO/IEC.
 
Current IEEE approved PoE applications have been operating on 4 pair structured cabling specified by TIA and ISO category 5e, cat6 and cat6a cabling standards since 2003.  First IEEE 802.3af PoE supplied 15.4W at the PSE (Power Supplying Equipment), with a min of 12.95W available to the PD (Powered Device)*. In 2009 a PoE+ version was introduced to the market as IEEE 802.3at supplying  up to 30W available from the PSE and a min of 25.5W delivered to the PD. Both 802.3af & at applications only use 2 of the 4 pairs to supply power.
 
While PoE has been successfully deployed for over 13 years, one connectivity related concern, recently receiving attention, is a condition referred to as Spark Gap Erosion. This condition can be caused by un-mating the plug-jack connection under PoE load. This concern has gained more interest in light of the proposed higher power versions of PoE on the horizon. See the table below:

To understand this concern further, it is important to understand when DC power is actually introduced on to the cabling. Before PoE power is supplied to the PD, a valid “PD signature” must be first returned to the PSE in response to a polling voltage sent by the PSE. If a valid signature response is not detected, the PSE will not supply power to that connected cable port.  This ensures PoE power is not introduced on to the cabling until after a circuit is completed and confirmed with a valid PD signature.  Therefore, there is no opportunity for a spark to jump between plug and jack contacts when the channels are being connected together or when a PD is attached to cabling, because PoE power has not yet been injected on to the cabling. *

Disconnecting a powered PD from a POE enabled cabling channel, or unplugging an element of an active PoE channel, is a different situation.  In this case, disconnecting a jack-plug connection while transmitting PoE power can produce the potential for a small arc to jump between the plug and jack contacts. This occurs internally to the mated plug- jack and creates no safety issues. However, the concern is that arcing could damage the plug and jack, specifically the gold plating on the contacts, potentially impacting the future data or power transmission capabilities of a connection. Arcing can cause eroding and damage at the very last point of connection of the plug and jack.  Any arcing could impact both the plated jack and plug contact surfaces at this final point of connection.  Severe enough erosion of the contact’s plating surfaces might cause some performance degradation over the life of the connection.

 

The concern regarding contact plating damage from arcing can be successfully addressed by having any arcing take place outside the fully mated contact area of both the plug and jack.  This approach to the spark gap concerns has also been confirmed in other position papers on this topic.

The jack contact design, developed by Ortronics in 2002 and used initially on category 5e & 6 jack ports, separates this last area of contact where any arcing might take place during disconnect, away from the fully mated position of the plug and jacks. This protects the 50 micro-inch gold plating of the fully mated position ensuring a long -lasting, high quality contact connection for the life of these products.   This is an effective way to protect against degradation that could compromise network performance or increased bit error rates.
 

In Summary: 
 
PoE power is not on the cabling until after a circuit is completed, and only after confirmation from a valid PD.
 
Un-mating a jack-plug connection while transmitting PoE power can produce the potential for a small arc between the plug and jack. This all occurs internally to the mated plug-jack and is not a safety issue.
 
Arcing can cause erosion damage at the very last point of connection and can damage the gold plating on the plugs and jack contacts. Severe enough erosion of the contact plating might cause some performance degradation over the life of the connection. This arcing concern may increase as the pending PoE standards introduce higher power level solutions to the marketplace.
 
There are jack contact designs that separate the last area of contact, where any arcing might take place, away from the fully mated position of the plug and jacks.  This will ensure a long lasting / high quality contact connection for the life of these products. 
 
Therefore, for PoE applications, specifying jacks with an arc-tolerant modular contact design that prevents any arcing damage, especially those applications brought forth by the pending higher wattage PoE standards, will help to assure the continued performance of the mated connection.
 
 
Note:
Please refer to Part 1 of Connectivity and Cabling CTD for more information.