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 ...
Get Power Over Ethernet Interoperability Guide now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.
