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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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The Input Voltage Source and Corresponding Power Levels
In PoE, the first requirement is a power source at one end of the Ethernet cable to inject power into it. Historically, this DC source used to be a battery of nominal value “48V,” but now it is almost invariably the output of a (“silver-box”) AC-DC switching power supply. To supplement the AC-DC power supply, there may be an uninterruptible power supply (UPS) present somewhere. Its purpose is to maintain the DC source in the event of a power outage.
Why do we say “48V”? Because safety agencies have decided that 60 V is the maximum voltage safe for a user to inadvertently touch without fear of electrocution. So to keep a little headroom (a safety margin of 5%), the IEEE 802.3af/at standard ...

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