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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Commercial PSE’s Interpretation and Implementation of AC Disconnect
Many commercial PSEs, however, use a probe frequency of around 100 Hz. As per their interpretation, AC disconnect should be able to detect even the port capacitor of the PD (typically 0.1 μF). So with 100 Hz, the impedance is Z = 1/(2π × C × f ) = 1/(2π × 0.1 × 10−6 × 100) = 16 k. Since this is well below 27 k, it will be detected by the AC-disconnect circuitry. But note that this is not the way the UNH-IOL test is written above.
There are other commercial PSEs that use a much lower frequency of about 25 Hz. They are counting on the fact that there is a 0.1-μF cap on the output of the PSE too. Plus some cable capacitance (typically 10 to 50 nF for 100-m length), and so on. ...

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