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Why compliance to PoE safety standards is crucial when moving beyond 60W

Posted: 31 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Power over Ethernet? CAT5? HDBaseT Alliance?

Convenience, flexibility, and enhanced management capabilities are some of the features offered by Power over Ethernet (PoE) as it enables 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 100V-ampere (VA) C or 100W C per port without the need for special overcurrent 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 3.05m (10 feet).

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.

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.

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