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Crawford Engineer Delves Into PoE

It’s more complicated than you thought

This article has been amended to correct an error.

Art Reis, chief engineer of Crawford Broadcasting’s Chicago operations, has an interesting perspective on power over Ethernet, in May’s Local Oscillator, Crawford’s monthly engineer’s newsletter.

Recounting dealing with a pair of storm-fried wireless STLs, Reis kicked-off an adventure-in-wiring. He recalled that when the IEEE standards for Cat-5 were still new: “Only two pairs were designated for use at the time, the orange and green, for data. The other two, blue and brown, were labeled in that early standard as ‘reserved for future use.’”

Over the last decade, he wrote, he has discovered that things have changed, including the advent of power over Ethernet. However, the standards haven’t been adapted to define power over Ethernet.

As Reis and his Crawford colleagues have discovered, there is no accepted industry standard for which wires will have power or how much. One of the many conclusions he has is, “The one thing I’ve noticed about the PoE wiring so far is that within a pair, the same side of the voltage holds.”

Reis notes that equipment manufacturers say to use only their provided connecting appliances. There’s a reason for that, he determined.

“True, in most instances, especially if the PoE includes both supply and interconnect between their equipment and the real world. And be very careful picking which plug to plug into which jack in a PoE. There seems to be two kinds of folks in this world who have dealt with PoEs in their work. Those who have plugged the wrong plug into the PoE, and those who will,” he says.

Reis goes into great detail on what he found using his home-made Cat-5 cable tester. He used the tester to figure the pin-out voltages (or lack thereof) of each pin of the Cat-5 power over Ethernet cables and ultimately to determine the health of the STLs (dead and essentially dead).

Read the whole thing for juicy power over Ethernet sleuthing.