Updated PoE specs let significant amounts of power flow down networking cables, so much that Ethernet wiring now can double as a power delivery system.
Leland Teschler | Executive Editor
Back in 2003, you could read articles about a new networking standard called Power over Ethernet (PoE). It allowed network devices to get dc power directly over Ethernet cabling. Promoters of PoE gushed that it would banish the wall-wart transformers otherwise necessary to power such appliances as wireless access points, docking stations for personal digital assistants, and IP phones. (Not mentioned: smartphones. The first iPhone didn’t come out until 2007.)
The problem was that the 2003 PoE spec only let networks deliver 13 W of dc power from each Ethernet port. This was long before the invention of LED bulbs drawing less than 10 W but able to put out as much light as 60-W incandescents. All in all, the low power capability of the early PoE spec kept it from shrinking the market for wall-wart transformers by much.
Fast forward to today. LED technology has reduced the power necessary for lighting applications, while advances in technology have yielded substantial increases in how much power can be delivered to a device over a network cable. A revision of the PoE spec in 2009 boosted the available power to 25.5 W per port. A draft standard now on the drawing boards will increase that figure to 72 W normally, 95 W under special circumstances.
One consequence of this rising power capacity is a proliferation of apps that use Ethernet wiring as a source of electrical power. Video cameras, point-of-sale machines, access control card readers, LED luminaires, and other industrial and building automation applications now avoid the need for a wall socket by using PoE connections.
Some vendors of networking equipment aren’t waiting for the new standards. Cisco, for example, developed its own PoE variant called Universal Power over Ethernet that delivers 60 W to each Ethernet port. It works with existing Ethernet cabling and handles devices with higher power requirements such as telepresence systems, Virtual Desktop Infrastructure (VDI) clients, nurse call systems, LED lighting fixtures, and digital signage.
Instead of delivering power over two of the twisted pairs in Ethernet cable, UPoE uses all four of them for power. It does this by using two power sourcing equipment controllers to power both the signal pairs and the spare pairs.
One advantage of using PoE as a primary source of power is that the network cable (usually Cat 5 twisted pair) needn’t be put in metal conduit like regular ac lines. This drastically reduces installation time and effort. It also facilitates IoT schemes that involve building automation. For example, fire alarm systems can also trigger blinking and a color change of network-connected lighting over emergency exit paths; lighting moods in individual offices might be set with a few keystrokes on an IP phone.
One appliance segment that is making ample use of PoE capabilities is that for smart LED lighting. Today several suppliers make PoE-powered luminaires targeting uses in open offices and meeting rooms. Employees can personalize the lighting around their desks from luminaires that each have their own IP address so they can be individually monitored, managed, and controlled. Sensors in the luminaires sense whether office occupants are around and kill some of the lighting to save energy in empty areas.
Devices that send power to PoE loads such as luminaires are either PoE network switches, a midspan (also known as a PoE injector), or a PoE splitter. PoE network switches have the
ability to send out both Ethernet signals and PoE power. The switch can detect whether a given Ethernet connection needs PoE or not through a handshaking arrangement.
PoE switches resemble ordinary Ethernet switches in that they can range from inexpensive unmanaged devices with a few ports to complex multiport versions with sophisticated management functions.
A midspan is used to add PoE to ordinary Ethernet connections. It basically accepts an Ethernet line from a network switch or other source and outputs a PoE version that goes to PoE loads.
Finally, a device called a PoE splitter accepts a PoE line and splits out power for use by the Ethernet load. It has two output cables, one for Ethernet data, the other strictly a dc power cable that runs the load.
One complication that arises out of using Ethernet lines for power is that the cabling must be treated more like conventional power-carrying wire than like data lines. That means the cable dc resistance factors into the installation design more so than if the wires handled just ordinary 5 Gbit/sec Ethernet.
Concerns about PoE wiring resistance prompted the Dept. of Energy to investigate how much power is likely to be dissipated in PoE cable. Researchers at the Pacific Northwest National Laboratory (PNNL) measured the amount of power dissipated in the cable itself rather than in the PoE load. After evaluating nine different brands of Cat5e and Cat6 cable, DoE researchers concluded that energy lost in the cable stays below 5% of the total load in PoE lighting applications so long as the average cable length doesn’t exceed 50 m.
Cat5e twisted-pair cable has been the standard Ethernet cable for some time. But Cat5e has trouble handling the climbing data rates specified in next-generation Wi-Fi devices and Ethernet links. Specifically, Wi-Fi equipment on the drawing boards will transmit at data rates greater than 1 Gbps, possibly reaching up to 7 Gbps. Ethernet lines will hit 10 Gbps and higher. Cat5e can’t handle 10 Gbps speeds up to the 100-m distances called out in Ethernet specs. Cat6A cables with their larger 23 AWG wires will probably handle these high-speed applications.
Consequently, the PNNL group tested both Cat5e and Cat6 cables. As you might expect, the PNNL group found cables with bigger conductors exhibited less dc resistance. In general, Cat6 cables carry larger-gauge wire than Cat5e versions, though there are exceptions. ANSI C137.3, the standard for PoE lighting systems, recommends at least 24 AWG wire for PoE loads drawing less than 55 W and at least 23 AWG wire for higher-power four-pair PoE applications. In addition, several manufacturers market even bigger 22 AWG cables, both in Cat5e and Cat6, specifically for PoE applications.
The Cat5e cables in the DoE test carrying 24-AWG wire all had a rated dc resistance, called DCR, of less than 9.38Ω/100 m. Most Cat6 wires with 23-AWG had a similar rating though a few of them were rated to have less resistance, as low at 7Ω/100 m in one case. The 22-AWG Cat6 cable carried a 6.5Ω/100 m rating while the Cat5e cable with 22-AWG wire didn’t have a rating for dc resistance.
One concern PNNL researchers noted was that DCR ratings are maximum ratings. A few cable makers put out nominal resistance ratings, as well as a DCR, for their products. The nominal ratings can be significantly lower than the DCR figure and may be more helpful for calculating expected power losses.
Another concern the PNNL researchers voiced was that they found PoE luminaire designs appeared to differ in how they treat cable shields and drain wires connected to shielded (metallic) RJ45 plugs. The problem emerged when researchers tried to hook a shielded version of a PoE cable to a luminaire; that action apparently fried the PoE network switch providing the power. It emerged that some PoE luminaires energize the shield in the RJ45 plugs while others do not. The lesson: At least today, not all PoE loads exactly follow the IEEE 802.3 PoE spec.
This realization led the PNNL group to suggest buyers and specifiers only use products independently certified (e.g., by the Ethernet Alliance) as IEEE PoE compliant. Researchers further suggested that manufacturers of Ethernet cables and connectors (RJ45 plugs) publish lists of compatible cabling products or parameters relevant to compatibility (e.g., tolerances for overall diameters of cable and insulated conductors).
Difficulties with shielded PoE cable are worth noting because some cables marketed as UTP (unshielded twisted pair) contain “isolation wrap” that resembles the foil used in F/UTP cables, those which have an overall shield containing all eight conductors. Shielded cable definitions vary, and several manufacturers market or recommend shielded cables for use in PoE applications. Some PoE luminaire makes claim their products are compatible with shielded cables. Both others are silent on this point.