GE 82-POE Benutzerhandbuch
Chapter 8: Power Over Ethernet Overview
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GE-DS-82 and 82-PoE Ethernet Managed Switch User Manual
Power Overloads
The IEEE 802.3af standard defines handling of overload conditions. In the event of an
overload (a PD drawing a higher power level than the allowed 12.95 Watts), or an
outright short circuit caused by a failure in cabling or in the PD, the PSE must shut
down power within 50 to 75 milliseconds, while limiting current drain during this
period to protect the cabling infrastructure. Immediate voltage drop is avoided to
prevent shutdown due to random fluctuations.
overload (a PD drawing a higher power level than the allowed 12.95 Watts), or an
outright short circuit caused by a failure in cabling or in the PD, the PSE must shut
down power within 50 to 75 milliseconds, while limiting current drain during this
period to protect the cabling infrastructure. Immediate voltage drop is avoided to
prevent shutdown due to random fluctuations.
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, 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.
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, 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.
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.
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.