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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Short-Circuit Range Comparison (AF and AT)
There is a current threshold higher than ICUT in which active/immediate intervention is called for by both AF and AT. This “intervention,” however, is implicitly multi-level. It does not necessarily mean the port is shut down right away. It means:
1.  The current is, first not allowed to go any higher. So the PSE’s pass-FET takes on voltage across itself in an effort to restrain the increase of current. The port voltage droops.
2.  If the current lasts longer than x ms, the port must be shut down to protect the cabling and/or the PD/PSE.
3.  If at any time, even before the timer mentioned previously times out, the port voltage sags below y volts, the port is allowed to be shut down immediately to ...

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