Power Disconnection Scenarios - Planet Switch User Manual

Industrial Renewable Energy 4-Port 10/100/1000T
802.3at PoE+ Managed Ethernet Switch/Router
BSP-360

6.5 Power Disconnection Scenarios

The IEEE 802.3af / IEEE 802.3at standard requires that devices powered over Ethernet be disconnected
safely (i.e. power needs to 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 52 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 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.
77