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>Date: Thu, 11 Jun 1998 10:48:49 -0400
>From: Pete Smith <n4zr@contesting.com>
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Snip (Discussion about putting safety ground below frost line.)
>>Probably OK for Western Washington, but here in the frozen
>>tundra of the Northeast, four feet deep is the standard for
>>getting under the frost line.
>
>More to the point, what does the frost line have to do with
>buried wire, anyway? If you don't bury radials that deep, why
>would you have to do so with a perimeter ground wire? For that
>matter, all of my feed and control lines come across the lawn in
>two runs of PVC conduit, which is buried only about 3 inches
>deep, and it has stayed put so far through one very hard winter
>and two average ones.
>
>Am I missing something?
>
Pete,
The missing information here is that frozen electrolytes are
insulators. When the ground freezes, its conductivity drops to a
vanishingly small value.
All you disbelievers out there can do a simple experiment to
confirm this. I probably shouldn't reveal this as it has
provided a nice supplementary income for me over the years
collecting on $50 sucker bets. For some reason people feel
strongly enough about this one that they are frequently willing
to bet money on it. But in the interest of promoting safety,
here goes.
1. Fill a small tupperware (any good plastic) container with
water to which you have added just barely enough salt to
change the taste of the water. It doesn't have to be
actually salty to the taste. You could just watch the
conductivity of the water with an ohm meter and stop when it
is easy to measure the water conductivity with the meter.
The salt is just to provide ions to the electrolyte for
charge carrying.
2. Suspend two insulated wires with 1/4 inch of the insulation
stripped off the end in the water so that the stripped ends
are completely submerged. Separate the wire ends by an inch
or so.
3. Measure and note the resistance indicated by an ohm meter
connected to the free ends of these wires.
4. Place the assembly (w/o the meter) in the freezer and freeze
the water completely solid.
5. Once the water is frozen solid, measure the resistance
between the wires as before. You should see an open
circuit. Absent an external leakage path, while the water
remains frozen, no current will flow between the wires
through the ice.
This is why large ground mounted verticals with not enough
radials under them show resonant frequency changes when the
ground freezes. The antenna thinks is is farther from the ground
plane when the earth is frozen than it does when it isn't.
For a perimeter lightning ground in earth, it is a good idea to
place it below the frost line. This is true even though in most
locations lightning is not a high probability event during the
time of year when the ground is frozen. I have customer
installations that have been hit out of a snow storm cloud at
latitudes farther north than 55 degrees (frost line down beyond
14 feet or so). So the probability is not actually zero even
though it may be small.
73, Eric N7CL
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