2 existing and operational NV1638P 's stopped providing POE power for no apparent reason. The 2 units were in different closets and had different AC power circuits, there was no electrical activity (i.e., storms, etc) prior to the units failing. The switches still function for DATA but no POE, I have trouble believing both power supplies failed the same way at relatively the same time.
I'd believe it. Loss of the PoE supply is a fairly common failure mode of these switches. Even with no surges, some designs of switching supplies fail such that they continue to function until power is interrupted, then fail to start up again. So a momentary power outage may have done you in even if no surge. Were the switches UPS-backed?