PLANET HPOE-1200G - Manual (Page 55)

AC Disconnect
This method is based on the fact that when a valid PD is connected to a port, the AC impedance measured on its terminals
is significantly lower than in the case of an open port (disconnected PD).
AC Disconnect detection involves the induction of low AC signal in addition to the 52 VDC operating voltage. The returned
AC signal amplitude is monitored by the PSE at the port terminals. During normal operation, the PD's relatively low
impedance lowers the returned AC signal while a sudden disconnection of this PD will cause a surge to the full AC signal
level and will indicate PD disconnection.
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User’s Manual of HPOE-1200G / HPOE
2400G