Interlogix NS3502-8P-2S User Manual - Manual (Page 544)

IFS NS3502-8P-2S User Manual
544
Power Disconnection Scenarios
The IEEE 802.3af standard requires that devices powered over Ethernet be disconnected safely (i.e.
power needs be shut down within a short period of time following disconnection of a PD from an active
port).
When a PD is disconnected, there is a danger that it will be replaced by a non-PoE-ready device while
power is still on. Imagine disconnecting a powered IP phone utilizing 48 VDC, and then inadvertently
plugging the powered Ethernet cable into a non-PoE notebook computer. What’s sure to follow is not a
pretty picture.
The standard defines two means of disconnection, DC Disconnect and AC Disconnect, both of which
provide the same functionality - the PSE shutdowns power to a disconnected port within 300 to 400ms.
The upper boundary is a physical human limit for disconnecting one PD and reconnecting another.
DC Disconnect
DC Disconnect detection involves measurement of current. Naturally, a disconnected PD stops
consuming current, which can be inspected by the PSE. The PSE must therefore disconnect power
within 300 to 400 ms from the current flow stop. The lower time boundary is important to prevent
shutdown due to random fluctuations.
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 48 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.