Power over ethernet

Power over Ethernet (PoE) is a technology for hard wired Ethernet LAN’s (Local Area Networks) that allows power to flow along the CAT5e cable (the minimum cable type) that would normally be reserved for data signals only. A standard (802.3af) was established by the IEEE in 2003 that specified a 15.4 W maximum power delivery (12.95 W at the end device) using two pairs of wires in the CAT5e cable. This was then uprated to 30 W (802.3at) but there are propriety offerings that boast 60 W by using all 4 pairs of wires in the Ethernet cable.

The system works by injecting approximately 50 V DC onto the Ethernet cable from a central point which can be a suitable existing network switch or an additional injector. If the end device does not have the inbuilt capability, a splitter is used near the point-of-load to split the power from the data and convert the 50 V to what’s required by the end device. Where a fully configured network is not used a single channel injector can be used to power just one end device using a single Ethernet cable. As with normal Ethernet networks the maximum cable length between injector and end device, which is defined by the data component, is 100 m however this can be extended by various means.

The combination of data and power capability over one cable makes product installation easier and potentially safer through avoiding the need for 230 V at the end device. Energy efficiency may also be improved by removing local power adapters but this depends on the relative efficiencies of these compared to the power supply in the injector. By interrogating the injector via the network it is also possible to read the current supplied by each injector port and to switch the port on and off thereby saving energy. Power over Ethernet can be used to power almost any device that falls within the power limits whether it has the inbuilt capability or via a suitable external splitter. This includes things such as Voice Over Internet Protocol (VoIP) phones, access cameras, terminals, Visual Display Units (VDU’s), thin clients (computers) and so on. Recently, with the growing popularity of low-power Light-Emitting Diode (LED) lighting, Power over Ethernet has been used to power luminaires where its data capabilities also allow additional functionality such as occupancy and lighting level sensors to be included.

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This article was created by --BRE. It was taken from The future of electricity in domestic buildings, a review, by Andrew Williams, published in November 2014.

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