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Chapter 8: Power Over Ethernet Overview
Operation
During normal operation, the PSE provides 44-57 VDC, able to support a
minimum of 15.4 watts power.
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.
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.
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).
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GE-DS-242-PoE Managed Ethernet Switch User Manual
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