This article explains why a Power over Ethernet (PoE) switch stops working and results in the minor alarm "PoE voltage injection in Interface."

The Ex4300-48MP switch Power over Ethernet (PoE) has stopped working and is not supplying power to some ports. The following alarm is displayed:

root@switch> show chassis alarms 
1 alarms currently active
Alarm time              Class Description
2023-12-14 11:16:30 IST Minor PoE voltage injection in Interface mge-0/0/29

The "PoE voltage injection in interface" alarm is a peculiar alarm because the port that is reporting the alarm may not be the one that has injected the power. This is because the PoE controller provides power to a group of ports and the PoE controller can go faulty depending on the voltage received on the port (not necessarily the port showing the alarm). If the switch port receives voltage > 10W, it can damage the controller as per the vendor. Often this issue is seen during power maintenance or power outage when a device may malfunction (Camera/APs etc) due to a power surge, further leading to PoE voltage injection on the switches. 

Note that there can be many other reasons such as a grounding loop on the switch or on a connected device. If the end device that is injecting the power due to grounding, earthing, or any other issue is not isolated, it will continue to damage the switch where it is connected. The issue may not appear until next occurrence of such power maintenance or outage. 

Perform the following steps to recover the device when the PoE stops working: 

• When there is failure, configure a new switch with the same configuration, code, and install in parallel to the existing switch.

• Remove the cables one by one from the existing switch and migrate to the new switch.

• While migrating the cables to the new switch, carefully check for the “PoE voltage injection in Interface” alarm on all interfaces.

• While removing the connections from the switch one by one, check whether the alarm disappears on the problem switch or has moved to the newer switch. Note down the end device and port that triggered the alarm.

• If the alarm persists, with all the connections removed from the failed switch, power cycle/hard reboot the problem switch. (Disconnect the power cable from the switch/outlet and connect it back).

• When the switch comes back online after the hard reboot, check for the alarm. If the alarm disappears, make all the connections again to the switch to check whether the port that reported the PoE issue recovered.

• After the hard reboot and reconnecting the devices, if the switch continues to raise the alarm, it means that the PoE controller is detecting the PoE inject voltage again. Check the alarm and the device that triggered it after connecting.

• If, after removing all the connections and hard rebooting the switch, the switch did not recover the PoE, then this switch may have permanent failure. 

Open a support case if you need any further assistance.

2026-02-16: Minor, non-technical changes made for search optimization

2024-09-11 : Article created