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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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PoE Rails, Ethernet/Telecom Systems
In PoE, as per the governing IEEE 802.3at standard, we typically use a “48V” rail coming from a silver-box AC-DC power supply positioned at the input of the PoE circuitry. Note that the PoE voltage rail does not exceed 57 V, so it is by choice an SELV-voltage level.
But is it really SELV just because it is less than 60 V? Alternatively we can ask is it really completely safe? Not necessarily so! Because as indicated earlier, a low-voltage level by itself does not constitute an SELV rail right off the bat. A SELV rail is a SELV rail not only because of what it is, but what it will be after a single fault! For example, a sub-60-V rail must also have two levels of protection present, between itself and any hazardous ...

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