Randy,
I didn’t mean to pick on you. Yes, in years past the neutral conductor was referred to as a ground, and also the “common” conductor. As Bren stated, it is there for safety reasons. When I was an apprentice back in the early 90’s, my mentor(s) and the journeymen that drug me around construction sights were very adamant about nomenclature and the neutral was never, never called a ground. That was beat into me, so I naturally try to make that point clear. It would sound as if your friend is old school. Nothing wrong with that and I’m sure he could teach us all a thing or two. It’s just a different world now. Safety is # 1 in the electrical trade nowadays.
Getting back to your problem, I’m clueless to why the lighting circuit would be affecting your HT circuit. I had thought there may be some stray voltage or inductence on the neutral leg from the lighting circuit if they shared the same neutral. You said they don’t, so if this hum is being caused by stray voltage or inductance, it would have to be transmitted from the neutral buss in your panel. I just can’t see this happening. It may just be coincidence that the hum went away when you turned the light off. Can you check this again and confirm that it is a repeatable symptom? If it is, I’ll go to the second floor wehn I get back to the office and talk to an EE/PE I know. He’s nothing short of brilliant and if there is a correlation, he’ll know what it is. This fella is one of the few engineers that came from the trade and then got his degree. RE: an engineer with common sense - a rarity.
Bren,
When they say “main panel”; they are referring to the service panel from the power provider (meter base/service panel/line disconnect). And yes, ground rods are still used, and also required for residential applications. Depending on the service / load, the rod can be different sizes and driven to different depths. You also tie the ground rod to the water main coming into the home/facility and the meter base/panel. In industrial applications, this varies. Many plants have a grounding ‘grid’ for lightning protection. You tie the facility into this grid, but again, it’s at the service panel/disconnect to the load side facility.
