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5.3 Start-up
Once line detection and optional classification stages are completed, the PSE must switch from low voltage to its full
voltage capacity (44-57 Volts) over a minimal amount of time (above 15 microseconds).
A gradual startup is required, as a sudden rise in voltage (reaching high frequencies) would introduce noise on the data
lines.
Once provision of power is initiated, it is common for inrush current to be experienced at the PSE port, due to the PD’s input
capacitance. A PD must be designed to cease inrush current consumption (of over 350 mA) within 50 ms of power provi-
sion startup.
5.4 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.
5.5 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).
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 im-
pedance 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.