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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Reasons to Avoid AC Disconnect
AC disconnect can perhaps skirt some lingering IP issues if required to do so, but it has serious drawbacks, as listed here, and should be avoided.
1.  Very careful design of AC disconnect circuitry is required and can cause interoperability issues with some PDs because it is just not precise enough, nor is it very well-defined in the standard itself. It is misinterpreted a good deal too.
2.  AC disconnect causes significant waste of energy and overheating.
3.  Presence of diode can cause detection and disconnect failures too, especially at elevated temperatures.
4.  Because of this blocking diode, when a surge strikes the line, this diode prevents the incoming surge energy from flowing backward into the output ...

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