Electronic Design

Industrial Power-Over-Ethernet

The IEEE 802.3af Power over Ethernet (PoE) standard employs two of the four pairs in a CAT5 cable to deliver 48 V dc to a device. The device can use up to 15 W, which is more than sufficient for a wide range of peripherals (e.g., Voice-over-IP (VoIP) telephones and a variety of control and monitoring devices).

The existing PoE standard suits industrial Ethernet use, but it isn't optimal for a number of reasons. Most industrial automation systems run at 24 V, and most of the fieldbus implementations provide power to downstream devices. This can easily exceed the 15 W from PoE sources.

Researchers are investigating a number of alternatives. Unfortunately, the current power limitations tend to be physical. Pumping more current through the line, changing voltages, and other options run into compatibility issues, as well as electrical issues, such as excessive wire wear, overheating, or shorting possibilities. None of these are welcome in general, and especially not in an industrial setting. Options include examining ac power that would be more efficient over longer distances, plus different cabling and connections.

The biggest problem in developing a different standard will be the lack of compatibility with commercial solutions. One huge benefit of industrial Ethernet is the low cost and high volume of commercial solutions that could be adapted to industrial use. Still, a new standard would have significant advantages on the plant floor, and a single standard would be greatly appreciated.

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