Description: The network cable between the AP and the PoE switch is 80 m
long and uses category 6 network cables. Currently, the AP is installed using
the same network cable. When the AP is fixed on the wall of third floor or the
first floor, the PoE switch can supply power to the AP. However, when the AP is
fixed on the wall of the second floor, the PoE cannot supply power to the AP.
1. The same AP can be supply power normally on the third floor and the
first floor. Therefore, the fault is not caused by the AP or PoE.
2. Configure the PoE legacy enable command on the PoE switch to check whether the power supply between the AP and
the switch is compatible.
3. Install the plastic expansion screws on the wall of the second floor,
and then ground the PoE switch. The AP can supply power normally.
The voltage on the second floor is special. When the AP is installed on
the wall of the second floor, there is static electricity on the wall. As a
result, the PoE switch cannot detect the voltage difference between the
interface of the PoE switch and the interface of the AP, therefore, it is
determined that the interface is not connected to the powered device.
Ground the AP and PoE switch separately to prevent the voltage difference
between the AP and the PoE switch.
The PoE power supply process is as follows:
1. The PSE periodically outputs a small voltage with limited current on
the port, so as to detect the presence of the PD device. If a resistor with a
specific resistance is detected, it indicates that the terminal is connected to
a powered device that supports the IEEE 802.3af
standard or IEEE 802.3at standard. (A specific resistance of the 19kΩ~26.5kΩ.
The normal voltage is 2.7V~10.1V, and the detection period is 2 seconds.)
2. Power Supply Capability Negotiation: The PSE classifies the PD and
negotiates the supply power. The power supply capability can be negotiated in
either of the following modes: Parses the detected specific resistor and
negotiates the power supply capability through LLDP.
3. Start power supply: In the startup period (generally less than 15μs),
the PSE supplies power to the PD from the low voltage until the 48 V DC voltage
4. Normal power supply: After the voltage reaches 48 V, the PSE provides
stable and reliable 48 V DC power for the PD. The power consumption of the PD
does not exceed 30 W. For the S5720-14X-PWH-SI-AC, the value is 90W. For the
S5720-16X-PWH-LI-AC, the value is 60 W.
5. Power-off: During power supply, the PSE continuously monitors the
current input of the PD. When the current consumption of the PD drops below the
minimum value or the current increases sharply, for example, the PD is removed,
or the power consumption of the PD is overloaded, short-circuited, or exceeds
the power supply load of the PSE, the PSE will disconnects the power supply and
repeats the detection process.