Chapter 8: Power Over Ethernet Overview
198
GE-DS-242-PoE Managed Ethernet Switch User Manual
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).
Summary of Contents for GE-DS-242-POE
Page 1: ...GE Security P N 1069174 REV 1 0 ISS 22FEB10 GE DS 242 PoE Managed Ethernet Switch User Manual ...
Page 14: ...Chapter 1 Introduction 10 GE DS 242 PoE Managed Ethernet Switch User Manual ...
Page 124: ...Chapter 4 Web Based Management 120 GE DS 242 PoE Managed Ethernet Switch User Manual ...
Page 130: ...Chapter 5 Console Management 126 GE DS 242 PoE Managed Ethernet Switch User Manual ...
Page 204: ...Chapter 8 Power Over Ethernet Overview 200 GE DS 242 PoE Managed Ethernet Switch User Manual ...
Page 210: ......
