Daveawalker wrote: ↑
Tue Oct 10, 2017 2:16 pm
... As they are running on the hospital ups there should not be spikes...
The ups might isolate the hospital from outside problems, but I would expect things such as MRI scanners to put noise on the internal system.
As wally333 says, the Pi is normally floating, and most modern TVs also.
Connecting either to "earth" could cause them to have various noisy signals induced into them, depending on your definition of "earth".
The "protective earth" connection on a standard domestic-style socket is merely a piece of wire that, at some point in the system, is connected to a metal stake buried in the ground (probably at the UPS)
The wires themselves can act as aerials for all sorts of nasties.
Are the Pis interconnected, and if so how? Wired network? I would guess not, since the A doesn't have a built-in network port.
I would look into galvanically isolating them, if possible.
Just out of interest, one site I worked had video equipment in two areas about 100m apart, with baseband SDI video signals going between the two. Same UPS, same "earth", but the signals were unusable because of a couple of volts at 50Hz on them. Expensive in-line video transformers, isolating the grounds, cured it.