Here's the same thing written down one more time.
TSC Tech Talk | Grounding Quote:
Don't break the AC ground!!!
With just one path to ground, there can't be a ground loop. Can there be a ground loop with one audio cable joining a console to a power amplifier? Yes! A ground connection through the AC cables and the chassis of the two units campletes the second ground connection.
One way to break this ground loop is to lift the AC ground on one piece of equipment, typically the power amplifier. This removes the safety AC ground. The system now relies upon the audio cable to provide the ground ... a practice that is hazardous!!! You also put at risk your multi-pair snake, console, post rack equipment, and most important the client. I do not endorse the use of AC ground lifts for any system ... anywhere. Don't do it.
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Let's say you're using a preamp that has the safety ground disconnected. Someone made a pair of XLR cables for the outputs in where the other end of pin 1 (screening/ground) is disconnected. These types of XLR cables are perfectly safe, and I do them myself.
Now the unit is floating happily. There is no reference to anything.
Some failure happens, the mains transformer starts to melt down as the heater wirings, for example, began dumping current to a short circuit.. Insulation breaks on the transformer and the mains voltage enters the chassis. From there it goes to the microphone through the pin 1 (which is connected as it's supposed to be connected!)..
The user touches the microphone and 230 volts go through the user, finding the shortest path. I hope the user was wearing rubber boots & gloves!
I've got jolted with 230v a few times. It's never funny. I hate the feeling of 50Hz buzzing through my body, and I'm happy to be alive.