On Tue, Jul 17, 2012 at 11:16:07AM -0600, Matlock, Kenneth L wrote:
That brings up an interesting question. I assumed the ground potential stays the same between 2 points, but have there been any studies to see if it's actually DC, or if there's an AC component to it?
Thaat's not a safe assumption, since most power companies use earth grounds for their distribution systems. That means that potential between two points, and the current through the ground between those two points, may vary depending on what's happening in the electrically-near parts of the power distribution system. That's not a happy thought, but it is Real Life. It's one of the reasons we went to fiber between widely-separated buildings in our field sites. In my experience, there are AC and DC components both. They're generally -- but not always -- negligible, unless something goes wrong or one end of the line takes a lightning strike, in which case "ground" can rise to bunchty KV.
If there's an AC component in the ground at either end (or both) that may introduce EM into adjacent pairs across the cable. And are they more or less than the EM ungrounded pairs would pick up?
Whatever is picked up by ungrounded pairs should be common-mode -- the same on both wires in the pair. Even if it is induced into the "live" pairs in the bundle, it shouldn't affect signalling. In theory, that is. -- Mike Andrews, W5EGO mikea@mikea.ath.cx Tired old sysadmin