Our hall has a projector hooked up to a PC via a VGA cable but we're seeing horizontal bars on the projector image when we turn on the lights in the hall. I'm think it's a ground loop problem. I'm trying to eliminate ground loop between PC and projector via a VGA cable. I've tried hooking them to the same power socket which didn't eliminate the horizontal bars on the projector image. I disconnected the safety ground inside the projector plug and the horizontal bars on the image went away, but I believe this is not a long term safe option. but does seem to indicate that the ground loop is the cause of the horizontal flicker/bars. Is it possible to disconnect the ground signals inside the VGA cable to eliminate the ground loop without making it unsafe/losing the picture entirely? (pins 5-8, pin 10?) What would be the consequences of doing so? I don't want to degrade my image quality, or risk damage. I'm also not super keen to "experiment"if I might damage something, and also re-soldering the vga cables is fiddly work.
That approach is common in audio and industrial sensor signals for the reasons you've identified. The screen is usually left connected at the transmit end and disconnected at the receive end so that may be worth a try. Get a short male-female cable to hack and install it at the projector end.
We're short a few details on your setup. What kind of lights? Fluorescent, incandescent, dimmers, etc.
Ground loops cause problems because of either a) a difference in ground voltages between the signal source and destination or b) because of induced currents in the loop due to EM interference. Something that may be worth a try is to power the PC and projector from the same socket. This should eliminate any ground potential differences. A temporary extension lead should be good enough to give an indication. If you can run the mains cable close to the video cable it will reduce the cross-sectional area of the loop and that may help too. (I know this is usually not considered good practice and you may get some switched mode power supply interference.)
What you really want is a video 'longitudinal stop coil' sometimes known as a 'video Humbucker', basically a big common mode choke on the video lines. It acts as a current balun, and push comes to shove you could build one out of a VGA cable and suitably large power transformer core by winding as many turns as possible of the vga cable onto the core.
One other thing that may help is to run a heavy gauge earth conductor in parallel to the video cable (Tape it to the side of the cable so the loop formed has minimum area), The objective is to bond the projector and the source so that the flowing current (Not a problem in itself) develops little voltage between the two because the parallel earth conductor is of such low impedance.
In really nightmare cases, both can be used, but the need seldom occurs.