They can be connected while powered on.   Same is true for the EPS/RPS on POE switches.
I am planning to purchase an RPS-600 to supply redundant power to four PowerConnect 6248 switches that are currently in use.  I was wondering if the RPS-600 can be connected to the 6248 switches while they are powered on?  I have looked over the documentation for both devices, and there does not appear to be a clear answer.  I would rather not shut down any of the switches, as they are carrying iSCSI and NFS traffic.  Does anyone have experience with this?
Dell PowerConnect 6248 and RPS-600