At 12:23 PM 3/20/99 +0000, David Robbins wrote:
>
>this has been brought up before, did anyone have documentation showing
>that this happens? pictures, magazine or newspaper reports, references
>to failure reports, etc??
>
>Gary McDuffie, Sr. wrote:
>> Running a ground wire THROUGH the base could lead to instant explosion
>> of the base during a lightning strike. The moisture in the concrete
>> turns to steam and expands greatly, destroying the base.
Maybe we can nip this thread in the bud. From The "Grounds" for Lightning
and ERMP Protection, Second Edition, by Roger R. Block, published by the
Polyphaser Corporation:
"... Mr. Ufer came up with the idea of using steel reinforcement rods
embedded in concrete foundations as a ground .... one of the most important
tests performed was under actual lightning conditions. The test was to see
if the Ufer ground would turn the water inside the concrete into steam and
blow the foundation apart. Results indicated that if the Ufer wire were
long (20-feet minimum) and kept approximately 3-inches from the bottom and
sides of the concrete, no such problems would occur (In my many years of
experience, I have only seen one tower base with cracks that could be
considered as lightning induced ..... For those who are afraid to use the
Ufer, think about this: The heating of the concrete is more likely if the
current is high or concentrated in a given area. This is known as current
density "J." The more surface area you have to spread out the current, the
less the current density. Your tower's anchor bolts are in the concrete
anyway. If your ground system is poor, the current density surrounding the
bolts will be high and can blow apart your concrete. At least if you tie
in your rebar, the area is increased and the current density is decreased."
and later
"It stands to reason that the outermost sections of the rebar structure
should be bonded together, not just tied. If tied, a poor connection could
cause an arc. Because the temperatures are very high and are very
localized, they could cause deterioration of the concrete (cracking and
carbonizing) in that area. In fact '...it has been found that these wires
are surprisingly effective electrical connections... One might think that
the ties would fail under fault conditions. However, it should be
remembered that there are a large number of these connections effectively
in parallel, cinched tightly...' (IEEE Seminar Notes 1970
The book goes on to diagram a tower base with a pigtail connecting the
tower itself to the outermost rebar "box," and that "box" is connected to
another pigtail that exits the tower base below ground level for connection
to a radial system.
73, Pete Smith N4ZR
n4zr@contesting.com
Loud is.
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