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POE safety standard-compliance is critical beyond 60W

19 Jun 2012  | Daniel Feldman, Microsemi Corp.

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The power-over-Ethernet standard offers convenience, flexibility, and enhanced management capabilities by enabling power delivery over the same Category 5 cabling that delivers the data. The current generation of standards-based technology enables the delivery of as much as 60W of power over four pairs of cabling, which also offers better efficiency than that of earlier two-pair approaches.

As the industry moves toward delivering even more power over the Category 5-or-better cabling infrastructure, system designers and network administrators must understand various emerging-technology options, including those developed under the auspices of the IEEE, and others that bring expensive and cumbersome deployment complications and, potentially, safety risks. For instance, some manufacturers have touted 100W-per-port approaches that do not perform detection before power-on-a dangerous omission.

Others offer 200W/port approaches that are even more dangerous unless they use a hard resettable fuse at every PSE (power-sourcing-equipment) unit or a certified electrician deploys the cables. The only safe approach for powering devices over Ethernet cabling is to follow IEEE802.3at-2009 specifications.

The HDBaseT Alliance is developing 100W power specifications for products that transport uncompressed, high-bandwidth multimedia content, 100BaseT Ethernet, power, and various control signals through a single LAN cable. As a cross-industry organization to promote and standardize whole-home high-definition-multimedia-content distribution, the HDBaseT Alliance has created a "five-play" feature that converges uncompressed, full-HD digital video, audio, 100BaseT Ethernet, IEEE802.3at-compliant power, and various control signals for transmission over a Category 5 cable.

Although independent manufacturers are pursuing approaches to higher power levels, the HDBaseT-powering approach offers other features, including compliance with Section 33.7.1 of the IEEE802.3at-2009 standard, which mandates that all PSEs conform with International Electrotechnical Commission 60950-1:2001 specifications, including classification as a limited power source carrying no more than 100VA or 100W, per port without the need for special overcurrent-protection devices. The HDBaseT also performs powered-device detection and classification to determine a device's power consumption before start-up.

Other standards also come into play as the industry moves to higher power. For instance, HDBaseT-powered TV and audio equipment must comply with Underwriters Laboratories 60065, which requires the use of a fire enclosure for loads greater than 15W. Consequently, even if a TV load meets the less-than-100W/port limited-power-source requirement of IEC 60950-1:2011, it still requires a fire enclosure.

Moving beyond the limited-power-source requirement to greater-than-100W/port implementations requires that the cables have a special flame-resistant conduit that attaches to the PSD and power-device inputs through metal boxes that are enclosed in brackets. This requirement applies to cables longer than 10ft (3.05m).

For cables shorter than 10ft, designers can use the same four-pair POE/HDBaseT cabling system comprising a data cable and RJ-45 connector. The cable must, however, have features exceeding the requirements of typical Category 5/E cable (Figure 1). Some manufacturers try to circumvent the 100W ceiling and associated protection requirements by suggesting that the ceiling can apply to each of two PSEs running on two circuits within one cable. Using this logic, they believe that a compliant PSE can support 95W and that a PSE pair can support roughly double that amount on the same cable. This idea is incorrect-and dangerous-however. The IEEE802.3at-2009 standard specifies that the 100W limited-power-source power ceiling be measured at the physical connector.

Tables 2B and 2C of the UL CAN/CSA-C22.2 60950-1-07 and UL 60950-1 documents clearly define restrictions for power sources with and without overcurrent devices (Tables 1 and 2). The existence of two PSEs on a circuit is irrelevant from a UL point of view because it is an inside-the-box power arrangement that ends at a single connector, which is the only output-power connector and must not exceed 100W.

The only ways of implementing powering at 250W or 250VA are either to use a circuit breaker or a fuse at each port or to have a certified electrician install the cabling, which effectively negates the deployment benefits of POE technology. Figure 2 shows the proper configurations for delivering greater than 100W over all wires-for example, 200W-in two single-port scenarios.

Both scenarios require a metal enclosure if the total powered device's load is greater than 100W for information-data equipment or greater than 15W for TV and audio equipment. The scenario in Figure 2a requires a nonstandard cabling infrastructure to ensure safety. The approach cannot use RJ-45 connectors directly at PSE and powered-device inputs and requires a certified electrician for installation. The scenario in Figure 2b can use a typical POE/HDBaseT cabling system but also requires special cabling. In summary, the use of a standard Ethernet-cabling infrastructure for a single port delivering greater than 100W is simply unsafe under the National Electric Code standard.

Four-pair delivery
HDBaseT achieves its high-powering capabilities and maintains full standards compliance by using the IEEE802.3at-2009 specification's mechanism for delivering power over all four pairs. Four-pair powering enables greater power delivery with greater efficiency. It gives powered devices two power interfaces so that they can receive twice the power of two-pair approaches by using all four pairs of Ethernet cable (Figure 3). Nothing precludes the connection of the two power interfaces-one over the two pairs using lines 1, 2, 3, and 6, and the other using the two pairs that use lines 4, 5, 7, and 8. This arrangement makes it possible to increase power delivery and fully comply with the standard. In a typical HDBaseT implementation, a 50V to 57V-dc power supply installs and powers the PSE, and all powered devices receive power directly over the HDBaseT link across all four pairs of Category 5-or-better cables.

With HDBaseT, core POE technology uses a 1A current for every two cabling pairs and uses three-event classification to identify compliant PSEs. This approach enables HDBaseT technology to transfer as much as 100W of continuous dc power per port from one side of the HDBaseT link to the other. The HDBaseT powering standard takes a step beyond the IEEE802.3at-2009 standard by enabling the powered device to identify the cable's length and resistance and draw more power when necessary, as long as the overall power consumption does not exceed 100W, rather than always assuming a worst-case cabling infrastructure. HDBaseT's ability to deliver as much as 100W of power over 100m using one LAN cable without any additional power source aligns with trends in energy usage and demand as well as government-led efficiency improvements. The power level is more than adequate for supporting typical 40in LED TV, which requires 70W of power.

This year, the latest Energy Star 5.3 specifications from the US Environmental Protection Agency will restrict all TVs to 108W of power consumption, regardless of screen size, whereas Energy Star 6.0 is targeting a cap of 85W for all screen sizes. Table 3 shows proposed Version 6.0 power specifications (Reference 1). Both LCD- and LED-TV monitors should soon be averaging approximately 1W of power consumption per inch of screen size. At these levels, the HDBaseT powering specifications can support even large displays.

Although today's Ethernet switches can and often do embed POE technology, another power-efficient way to deploy POE is through midspans, which reside between a non-POE switch and a network's powered devices. Midspans also are the most likely deployment model for HDBaseT because adding 100W per port in an Ethernet/HDBaseT switch would be a daunting engineering task and would greatly reduce overall system reliability. Midspans also feature remote power-device monitoring and configuration, which significantly reduces power consumption.

Network administrators can monitor per-port and total power consumption and can configure power devices for instant and scheduled port on/off functions as well as uninterruptible-power-supply-status-port on/off functions.

POE technology also can improve energy efficiency by minimizing the effects of idle power consumption. Many POE midspans and switches use switching power supplies, which are 90%-efficient at full load, meaning that they consume as much as 220W of ac power for 200W of POE power or as much as 440W for 400W of POE power.

Unfortunately, switching power supplies when idle have switching-power losses of 20W to 40W with 0W load for 200W-rated units and 40W to 80W with 0W load for 400W-rated units. Address this issue by exploiting POE's distributed-power architecture. For instance, network administrators can start with a 450W internal power supply to handle all real-time requirements and upgrade to full per-port power with an external 450W to 1kW power supply when necessary.

One other key area of POE innovation is reach extension. Four-pair powering has enabled powered devices to work with cables as long as 100m as the HDBaseT standard specifies. POE midspans currently measure the cable length and correctly allocate power to a powered device when the cable length and the device's maximum power requirements are both known. This approach ensures that a powered device that connects across 100m and requires more than 22.8W will not unexpectedly disconnect when it reaches maximum load.

Network administrators can use extender technology to deliver both data and power to network devices, such as WLAN access points and network cameras, at baseline distances of 200m-that is, twice the distance of IEEE specifications. Extender devices simply connect to a midspan and receive power through the POE input. Table 4 shows the resulting baseline distance extensions. Cascade extenders reach even longer distances.

POE continues to evolve and offer a wider variety of high-value power-delivery and -management capabilities. As the technology moves beyond 60W to a new generation of higher-power capabilities, it is critical for designers to maintain compliance with all IEEE802.3at-2009 specifications, including those that ensure safe powering. The HDBaseT Alliance has adapted IEEE802.3at-2009 technology to a 100W technology that fully supports all safe-powering requirements and offers the opportunity to merge uncompressed full-HD digital video, audio, 100BaseT Ethernet, standards-compliant power delivery, and various control signals onto a single 100m Category 5e/6 cable.

Energy Star Television Specification


This article originally appeared on EDN and Power Management Designline.

About the author
Daniel Feldman is the vice president of business development for the analog and mixed-signal group at Microsemi Corp, and a member of the HDBaseT Alliance's POH (power-over-HDBaseT) technical committee. Feldman holds a master's degree in business administration from the University of California-Berkeley's Haas School of Business, and a bachelor's degree in computer engineering from the Technion Institute of Technology in Haifa, Israel.

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