On May 11, 2013, at 7:51 41AM, "K1TTT" <K1TTT@ARRL.NET> wrote:
> Consider this example... my neighbor had a broken ground in the wire
> overhead from the transformer to his entrance panel. There are ground rods
> at the pole and at his entrance as required. When a big load on one side of
> the line started, like a refrigerator, that leg browned out but the other
> leg of the 220 would get up to about 200v and blow out stuff on that circuit
> due to the overvoltage. Obviously there wasn't enough return current
> through the ground, even over that relatively short distance, to keep the
> supply balanced.
This can happen in an _underground_ distribution system, as well. Our home --
originally a seasonal summer camp -- was first electrified in the '50s with an
underground single-phase 110-0-110 3-wire feed from a shared distribution
transformer to the entrance panel. The shortest path from the distribution
wires and transformer to the camp was diagonally across the dirt driveway and
part of my parking area.
It's well known by contractors and life-long residents here in the frozen north
that snow-plowing a driveway and driving on it all winter long "drives the
frost line deeper into the ground", but back in the '50s all the properties in
our neighborhood were seasonal so underground utilities were seldom more than a
foot or two below the surface. (Our present building codes assume a frost
depth of at least 4.5 feet.) By 2000, probably half of the dwellings,
including ours, had become year-round.
And so, about five years ago, we experienced the same fate as Dave's neighbor.
In our case, the constant shifting of the driveway frost line eventually caused
the buried neutral to break in half. (According to the power company crew, the
shifting frost line causes rocks and gravel beneath the surface to move enough
to repetitively stress the wire until it snaps.) As Dave noted, if the ground
characteristics at 60 Hz are high-resistance, a big load (in our case, probably
either our refrigerator compressor or our well pump turning on) on one side of
the line will cause the other leg to rise to nearly the full 220 volts of the
single-phase feed. We lost a microwave and a bunch of other household items
in that little "event".
Once the power company located the problem, I immediately hired an excavation
contractor and an electrician to re-route my entrance cable so it skirted my
entire driveway and parking area. They were required to coordinate with
someone from the power company who had to be on-site for the re-connection to
the distribution transformer and back-filling of the trench. My difficulties
in getting all three people to show up at the same time were seemingly
never-ending. Let me simply say that scheduling that meeting was the most
difficult part of the process (other than ponying up "mucho" dollars), but I
have never regretted having the power line installation finally done right.
Bud, W2RU
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