Cable Heating is Limiting Factor for PoEPlus Solutions

February 09, 2006 by Jeff Shepard

Phihong USA was among the companies that presented proposals on achievable limits of 2-pair and 4-pair solutions at the January 12th meeting of the IEEE 802.3at Task Force. Based on these presentations, the task force concluded that the standard's objective of providing enough power for multiband access points and controllable security cameras can be met with a 2-pair solution, which is lower in cost and simpler to implement compared to 4-pair solutions. Designated PoEPlus, the new standard will more than double the wattage available to powered devices.

"The specification is far from finalized but most members believe a source voltage from the PSE of 51-57V with up to 840mA is a reasonable expectation," said Keith Hopwood, vice president of marketing for Phihong USA. "The drop per pair total in a worst case scenario of 100m cable and connectors is 12.5 ohms, which would provide over 30W at the powered device. The limiting factor is the temperature rise in bundled cabling," concluded Hopwood.

The cable working group is testing 196 cables bundled in a conduit and measuring the temperature rise in all the conductors when current runs through them. The testing does not reflect a safety concern or fire hazard, but the cables will permanently deform above certain temperatures, causing data attenuation.

"At Phihong, we offer both 2-pair and 4-pair solutions, so we know the 2-pair option is feasible," said Hopwood.

In addition, at the meeting, a group was formed to define a detailed classification proposal. The new classification will not only give a much higher resolution of current required by the powered device but will also signal the powered device of the source power capability of the power sourcing equipment.

If implemented as a standard, PoEPlus will allow at least 30W of power and potentially up to 45W. Products that deliver high-power PoE have already been developed, but most customers would prefer a common industry standard for all devices on the market.

Applications that will benefit from the IEEE 802.3at initiative include wireless multi-radio access points, Pan Tilt Zoom security cameras, and IP phones with streaming video displays. Other emerging applications, including emergency lighting, security system sensors and even medical monitoring, will greatly benefit from combining data and centralized backup power.