Compliance to Power-over-Ethernet safety standards is critical when moving beyond 60-W

Technology News | January 23, 2012

By eeNews Europe

The basic situation

Power over Ethernet (PoE) offers convenience, flexibility, and enhanced management capabilities by enabling power to be delivered over the same CAT5 cabling as data. The current generation of standards-based technology enables up to 60 watts of power to be delivered over four pairs of cabling, which also improves efficiency when compared to earlier two-pair solutions.

As the industry moves toward delivering even more power over the CAT5-or-better cabling infrastructure, system designers and network administrators alike, need to 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 solutions that do not perform detection before power-on, which can be very dangerous. Others offer 200W/port solutions that are even more dangerous unless they use a hard re-settable fuse at every Power Sourcing Equipment (PSE) unit, or if a certified electrician deploys the cables. The only safe approach for powering devices over Ethernet cabling is to follow IEEE802.3at-2009 specifications.

This is what the HDBaseT Alliance is doing as it develops 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 formed to promote and standardize whole-home HD multimedia content distribution, the HDBaseT Alliance has created a 5Play™ feature set that converges uncompressed full HD digital video, audio, 100BaseT Ethernet, IEEE802.3at-compliant power and various control signals so they can be transported over a CAT5 cable.

The key differences between the HDBaseT-powering approach and those from other independent manufacturers pursuing higher power levels are that it:

•Complies with the section 33.7.1 of the IEEE802.3at-2009 standard, which mandates that all PSEs conform to International Electrotechnical Commission (IEC) 60950-1:2001 specifications including classification as a Limited Power Source (LPS) carrying no more than 100 volt-ampere (VA) – or 100W – per port without the need for special over-current protection devices, and

•Performs Powered Device (PD) detection followed by PD classification to determine a PD’s consumed power level prior to its ignition.

There are other standards to consider as the industry moves to higher powering. For instance, HDBaseT-powered TV and audio equipment will be expected to comply with UL 60065, which requires that a fire enclosure be used for loads above 15W. Consequently, even if the TV load meets the sub-100W/port LPS requirement of IEC 60950-1:2011, it will still need a fire enclosure.

Moving beyond the LPS requirement to greater-than-100W/port implementations requires that the cables be protected with special flame-resistant conduit that is attached to the PSD and PD inputs through metal boxes that are enclosed in brackets. This requirement applies for cable lengths greater than 10 feet (3.05m).

Below 10-foot cable lengths, it is possible to use the same four-pair PoE/HDBaseT cabling system (i.e., data cable plus RJ45 connector); however, the cable will have specific feature requirements beyond those of typical CAT5/E cable. Figure 1 shows a circuit meeting these requirements.

Figure 1: Example of a circuit featuring two separate ports, each carrying 100W. Power is calculated at the PD side, and LPS requirements apply to each port. NOTE: According to the National Electric Code (NEC), multiport midspans that power PDs using this approach must carry a nameplate with the power rating for each port. (Click here for enlarged image.)

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 separate circuits within the same cable. Using this logic, they believe that a compliant PSE can support either 1 x 95W, alone, or roughly double that when two PSEs are paired on the same cable.

This is incorrect—and dangerous—thinking. As specified in the IEEE802.3at-2009 standard, the 100W LPS power ceiling is measured at the physical connector. Restrictions for power sources with and without overcurrent devices are clearly defined in Tables 2B and 2C of the Underwriters Laboratories, Inc. (UL) CAN/CSA-C22.2 NO. 60950-1-07 + UL 60950-1 document (see Tables below). The fact that there may be two PSEs on the circuit is irrelevant from a UL point of view, since 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.

(Click here for enlarged image.)

The only way to implement powering at 250W (or 250VA) is to use a circuit breaker or fuse at each port, or have a certified electrician install the cabling, which effectively negates the deployment benefits of PoE technology. Figures 2a and 2b show the proper configurations for delivering greater than 100W over all wires (e.g., 200W) in two different single-port scenarios.

In both cases, a metal enclosure is required if the total PD load is greater than 100W for information data equipment, or greater than 15W for TV and audio equipment. In Figure 2a, a non-standard cabling infrastructure must be used to ensure safety, RJ45 connectors cannot be used directly at PSE and PD inputs, and a certified electrician will likely be required for installation. In Figure 2b, a typical PoE/HDBaseT cabling system can be used, but special cabling must also be specified. In summary, the use of a standard Ethernet-cabling infrastructure for a single port delivering greater than 100W is simply not safe under the NEC standard.

Figures 2a and 2b: Proper circuit implementations for powering at 250W over cable lengths greater than 3.05m (Fig. 2a) and less than 3.05m (Fig. 2b). (Click here for the complete image.)

Four-Pair Delivery: optimal path to higher power

HDBaseT achieves its higher-powering capabilities while maintaining full standards compliance by using the IEEE802.3at-2009 specification’s mechanism for delivering power over all four pairs. Four-pair powering enables more power to be delivered with greater efficiency. It gives PDs two power interfaces so they can receive twice the power of earlier two-pair solutions by using all four pairs of Ethernet cable (see Figure 3).

Nothing precludes the two power interfaces to be connected — one over the 2 pairs using lines 1,2,3 and 6, and the other using the 2 pairs that use lines 4, 5, 7 and 8. This is what makes it possible to increase power delivery while fully complying with the standard. In a typical HDBaseT implementation, the PSE is installed and powered by a 50 to 57-volt DC power supply, and all PDs receive power directly over the HDBaseT link across all four pairs of CAT5-or-better cables.

Figure 3: Four-pair powering (Click here for enlarged image.)

Additionally, core PoE technology has been enhanced for HDBaseT to use a 1 amp current for every two cabling pairs, and also uses 3-event classification to identify compliant PSEs. This enables HDBaseT technology to transfer of up to 100W of continuous DC power, per port, from one side of the HDBaseT link to the other.

Also, The HDBaseT powering standard takes a step beyond the IEEE802.3at-2009 standard by enabling the PD to identify the cable length/resistance and draw more power when required (as long as the overall power consumption does not exceed 100 W), rather than assuming a worst-case cabling infrastructure at all times.

HDBaseT is fully backwards-compatible with the IEEE802.3at-2009 PoE specification, including the section 33.7.1 mandate that all power sourcing equipment (PSE) conform to IEC 60950-1:2001 and be classified as a Limited Power Source (LPS) carrying no more than 100 volt-ampere (VA)per port without the need for special over-current protection devices. HDBaseT also does not infringe on any of the mandated PoE safety requirements.

HDBaseT’s ability to deliver up to 100W of power (over 100m, via a single LAN cable, without any additional power source) is actually very nicely aligned with trends in energy usage and demand, as well as government-led efficiency improvements. The power level is more than adequate for supporting today’s typical 40-inch LED TV, which requires 70W of power.

Beginning in 2012, the latest Energy Star™ 5.3 specifications from the U.S. Environmental Protection Agency (EPA) will restrict all TVs to 108 W of power consumption, regardless of screen size, while EnergyStar™ 6.0 is targeting a cap of 85 W for all screen sizes. The Table shows proposed Version 6.0 power specifications which are targeted to become effective during the summer of 2012. More information on these developments is available at https://www.energystar.gov/index.cfm?c=revisions.television_spec.

It is expected that both LCD and LED TV monitors will soon be averaging approximately one watt of power consumption per inch of screen size. At these levels, even very large displays can be supported by the HDBaseT powering specifications.

While PoE technology can be, and frequently is, embedded directly into today’s Ethernet switches, one of the most power-efficient ways to deploy PoE is via systems called midspans, which are installed between an existing non-PoE switch and the network PDs. Midspans also are the most likely deployment model for HDBaseT, since adding 100 W per port inside an Ethernet/HDBaseT switch would be a particularly daunting engineering task and would be likely to greatly reduce overall system reliability.

Midspans also feature capabilities including remote PD monitoring and configuration, which significantly reduces power consumption. Network administrators can monitor per-port and total power consumption, and configure PDs for instant and scheduled port ON/OFF functions, as well as UPS 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 (SPSs) that are 90-percent efficient at full load. This means that up to 220W of AC power is consumed for 200W of PoE power, or as much as 440W for 400W of PoE power.

Unfortunately, SPS units have high switching-power losses when idle (from 20W to 40W with zero W load for 200W-rated units, and 40W to 80W with zero W load for 400W-rated units). The solution is to exploit PoE’s distributed power architecture. For instance, network administrators can start with a 450W internal power supply to handle all real-time requirements, and only upgrade to full power -per-port with an external 450W to 1KW power supply (such as Microsemi’s PowerDsine RPS1000) when needed.

One other key area of PoE innovation that is worth noting is "reach extension". Four-pair powering has enabled PDs to work with cables up to 100 meters, as specified in the HDBaseT standard. Also, today’s PoE midspans actually measure the cable length and correctly allocate power to a PD, when the cable length and the PD’s maximum power requirements are both known. This ensures that a PD which is connected across a 100-meter cable and requires more than 22.8W will not unexpectedly become disconnected when it reaches maximum load.

Additionally, 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 200 meters, or 100 meters more than IEEE specifications. Extender devices are simply connected to a midspan and powered via the PoE input. The following Table shows the resulting baseline distance extensions. Extenders can also be cascaded to reach even longer distances.

(Click here for enlarged image.)

PoE continues to evolve and offer an even 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 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 100 W solution that fully supports all safe-powering requirements while offering the unprecedented opportunity to merge uncompressed full HD digital video, audio, 100BaseT Ethernet, standards-compliant power delivery and various control signals onto a single 100m CAT5e/6 cable.

About the author

Daniel Feldman is the Vice President of Business Development for Analog and Mixed-Signal Group at Microsemi Corp. and a member of the HDBaseT Alliance PoH technical committee. He is a former chairperson of the Ethernet Alliance PoE Technical Committee and was an active member of the IEEE802.3at Task Force. Previously, Mr. Feldman worked at Microsemi in several Marketing roles, for PowerDsine as Senior Product Manager responsible for Outbound Marketing activities in the Americas, at IC4IC as System Architecture Group Manager, as a VHDL Engineer at NICE Systems and as VLSI Engineer at RAFAEL

Mr. Feldman holds an MBA from UC Berkeley’s Haas School of Business and a B.Sc. (Cum Laude) in Computer Engineering from the Technion Institute of Technology in Haifa, Israel.