Planet POE-1200G User Manual page 64

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.