I just went through this exact exercise last week. A different method was recommended in IRC, and I built the following checker to suit:

 

Conditions:

 

sensor_event ne up

sensor­_event ne ignore

 

Devices:

 

*

 

Entities:

 

sensor_class eq state
sensor_descr match *power supply*

 

This uses the built-in state logic to see if the power supply is alive or not. At the moment, I know that Brocade power supplies will go into ‘alert’ state if they are missing or down. I’m assuming the same for Cisco, and that they’d also alert if there were max/min power violations as well. It might not be exact – as in “power supply undervolt!” – but I figure if a PSU is anything but “up” then it deserves to be looked at.