Weird AC Voltages in a PoE Camera System

We are switching the Gravesend Inn video systems over from an old, all-analog system to a modern, IP-based system with ACTI ACM-1431 cameras powered over the Ethernet line using PoE (Power over Ethernet).  I got the cameras a few months ago and did a lot of testing with them on a plastic cart in the shop, and everything worked great. I wanted to use one for a show right at the end of the semester, and discovered that the ACTI mounting bracket had a convenient 1/2" hole that fit a lighting c-clamp perfectly.  I clamped the camera up to our lighting grid, ran the cable down to a Netgear PoE switch in our sound booth, and plugged it in. It didn't work. I went back up to check for lose wires and so on, and couldn't find anything. Working with a student, we tested the cable, and then eventually plugged into another port on the switch, and it didn't work either. I disconnected the camera from the grid to get better access to the wiring, and we plugged it again (to another port) and it worked. Right about that time, I got a shock from the camera through to my other hand which was holding onto the grid.  I knew that PoE runs at 48VDC, so I figured it was some issue with that and a loose wire.  Then I surprised my student with a big spray of sparks as I went to clamp the camera back on to  the (grounded) pipe grid, and of course, the camera stopped working again.  I got out the voltmeter and found some very interesting things, which I have documented here in this video:

I forgot to mention in the video that I tried moving the cameras around to the various PoE sources but saw similar results.  I also got the same thing when using D-Link PoE extractors (which will be our standard set up--we need the extra power to power a microphone at each camera position).  Also, only the bigger Cisco Small Business switch shows a low-resistance path to the ground pin on the incoming power plug--all the others use external supplies that apparently float off the ground.

I assume most people don't see these kind of things because they have PoE devices that are never connected to ground, or have a plastic case.  I guess I could go in and decouple the camera electronics from its case but I'm not too excited about that idea.

I think we'll be OK for the Gravesend Inn since I'm planning to use the Cisco Small Business switches you see at the right in the video, which show only a tiny (and seemingly reasonable) leakage voltage, but I'd like to get to the bottom of this.  Does anyone know anything about why we'd be seeing 50 or 60V AC on a device that's powered from 48VDC?  Please post a comment if you do!