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Was there any upgrades to the NMC or DCE / DDFs around this time. Usually there are only a few reasons for unplugged sensors:
DDF Change during a DCE upgrade, or manual DDF import.
Upgrade of device firmware and the device no longer reports that SNMP value.
The sensor on the device is unplugged or "offline". This applies mainly to sensors that you can physically plug in, such as
What occured around the time that the device sensor started reporting unplugged?
In this case it seems that the device may not be reporting the value back, so DCE is just showing the sensor(s) as unplugged.
In regards to the alarm: Did the device report any alarms on its web interface for the breaker being tripped? How long was the alarm active for on the PDU?
You can also reach out through your local technical support, and as long as you have a software support contract, they should be able to look at certain information to help pin-point the issue, such as the essential alerts alarm file for the PDU within DCE to determine if it is an alarm that will trigger an alert.
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