Note: I'm not an electrical engineer.

I've been researching this topic and have not seen anything that seems definitive or consistent. Working on an arcade game with an older crt-type monitor. The monitor will get faint interference lines (sometimes called "herringbone" or mori lines), which seems to come from a ground loop.

If I disconnect the earth ground of the monitor (ground wire connected directly to the monitor chassis to the third ground plug on the AC plug), the interference goes away.

I've read that tying the GND and FG on the power supply (see below that came from a Midway manual) is a way to rectify this issue as well. But have not found a good explanation on why this works, if this is safe, or some will say it will destroy the equipment.

Any help or references would be greatly appreciated! Really would like to understand this.

 

I'm not sure of the entire set up that you have, but it sounds like a ground loop is very possible. Ideally, you want the earth ground to be a "star-connected" configuration such that there are are no loops, which will act as both receivers and transmitters of electromagnetic interference. So you want the monitor ground connection to be routed back to the power supply and connected to the ground connection there.

 

Not a "ground loop" issue but certainly a ground noise issue.
As for the connection addition shown, my GUESS is that the black wires on the third terminal are supply negative connection, and that the green-yellow wire on the other third-in terminal is the "safety ground" pin on the mains plug. So that addition is tying the safety ground to the supply negative. That is a legitimate sort of fix. If you are seriously concerned then you can make that connection with a low current fuse, half an amp or 1/4amp.

I have a similar issue with a church sound system, in that the mains feed runs in a conduit along with several circuits of light power controlled by triac dimmers. There would be a very audible line-buzz in the speakers. Breaking the green wire ground stops then noise completely. With no access to the sound system internal common, breaking the green wire ground is the only fix available other then using a different conduit run.
No shack danger created because no grounded objects near the microphones.

 

Not a "ground loop" issue but certainly a ground noise issue.
As for the connection addition shown, my GUESS is that the black wires on the third terminal are supply negative connection, and that the green-yellow wire on the other third-in terminal is the "safety ground" pin on the mains plug. So that addition is tying the safety ground to the supply negative. That is a legitimate sort of fix. If you are seriously concerned then you can make that connection with a low current fuse, half an amp or 1/4amp.

I have a similar issue with a church sound system, in that the mains feed runs in a conduit along with several circuits of light power controlled by triac dimmers. There would be a very audible line-buzz in the speakers. Breaking the green wire ground stops then noise completely. With no access to the sound system internal common, breaking the green wire ground is the only fix available other then using a different conduit run.
No shack danger created because no grounded objects near the microphones.

Makes sense. I've seen some connections of the GND and safety ground connected via a capacitor (maybe a X or Y rated one). I've have not seen a schematic of this or type/value of the capacitor. I'm guessing this was done suppress noise.

Here's another example I found. Different manufacture (Namco instead of Midway).

 

Another optio, if no conductive parts of the monitor can be touched, would be to disconnect that ground connection of the monitor. That might also work. No promise though.