On 5/11/2013 5:43 PM, K8RI wrote:
On 5/11/2013 9:10 AM, W2RU - Bud Hippisley wrote:
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.
I had this happen this week.
About 1:30 AM I sat here listening to 40 and surfing the internet.
There was a tremendous bang, but more of a "Boom" through the headset.
It was loud enough I was reaching to grab the headset off, but
immediately following the boom, it got real dark in here.
Still I saw light coming from the hall.
Troubleshooting a panel with one hand is... well... exasperating and I
was getting strange readings with my DVM and they were unstable.
It appeared the one line was low and varying
So it was out to the shop to get the Simpson 250.
Now that line was dead. I said, This can't be good.
Consumers made it out in the morning, said the problem was underground
and somebody would be out to fix it soon.
They made it at 1:00 PM.
Hmmmm...A little comma mahes such a difference
This: The one hot feed that wasn't broke at the base of the pole
(underground)
Should be:The one hot feed that wasn't, broke at the base of the pole
<sigh>
73
Roger (K8RI)
where it turns 90 deg to enter the conduit.
Around here they figure the frost line is 3 feet. Last winter it was
less than a foot.
73
Roger (K8RI)
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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