Managing free pairs to prevent DSL sync. loss

Mike Andrews mikea at
Tue Jul 17 17:45:35 UTC 2012

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 at
Tired old sysadmin 

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