I purchased a rod potentiometer and was surprised to see it came with a 4th wire. The manufacturer said it was a ground, though the wiring diagram didn’t even mention it.
What’s the purpose of a separate ground wire?
I’m intending to have the potentiometer referenced to my ground plane and 3.3 V. I assume I should just tie the extra ground to my ground plane too.
The fourth wire is a ground. In the old days of analog volume controls, an unearthed metal cased pot was more susceptible to mains hum pick up. These days there are lots more noise sources which could cause grief. This noise pickup issue applies irrespective of operating voltage. For high voltages, there are safety reasons for grounding metal objects.
Sometimes you want your circuits to be "grounded" to other reference voltages rather than 0V, therefore, you can use the external grounding to ensure electrical safety
