Jim Jarvis wrote:
> It was written, regarding UFER grounds:
>
> "Sinking the ground rod several inches below the bottom of the foundation
> hole and only running copper wire out the bottom of the concrete is no
> problem. If house foundations use copper wire for Ufer grounds, then it
> sounds safe to assume that copper embedded in concrete does not eventually
> corrode away."
>
> WRONG! You want to divert strike current around the tower foundation,
> not through it. The conductor will be evaporated, exploding the
> foundation.
I don't think so. Fusing current on a typical solid UFER ground wire is
pretty high.
When doing exploding wire experiments 10 years or so ago, I recall it
takes on the order of tens of kiloJoules to blow up a meter long segment
of AWG16 wire. AWG 4 (what you make a UFER from, typically) is more
than 10 times the area, and is about 0.25 milliohm/foot. At 100kA
(which is much higher than most lightning strikes, the power dissipation
will be
2.5 MW, but because the impulse only lasts, say, 50 microseconds, the
energy dissipated is only 125 Joules. (you can do a similar calculation
just figuring the amount of energy required to raise the wire
temperature from 20C to 1083C-Copper melting point.. about 17kJ/meter of
AWG 16 wire)
There might be a longer duration pulse in a wire (from the inductance),
but the amplitude would be correspondingly reduced, so the "action"
(I^2*time), which is what melts the wire/fuse, will actually be less.
BTW, even with the inductance of a wire carrying a lightning impulse, it
might raise the voltage along the wire (from the L di/dt), raising all
those issues with overvoltage protection, etc. However, like any other
inductor, there's no loss or heat dissipation. That's all from the
series R, and even moderate sized wire has low enough series R so that
it won't fuse with a short impulse.
Here's a table for solid copper wire in free air, 1 millisecond pulses,
using the equation from Onderdonk.
AWG Fusing Current (kiloAmps)
4 193.9
8 77.2
10 48.7
12 30.7
14 19.4
16 12.2
Most lightning strokes are MUCH shorter than 1 millisecond (50
microseconds per strokes, but there could be 4-5 strokes in a single
hit, so you want some margin, because the wire's not going to cool much
in the few tens/hundreds of milliseconds between strokes)
Heavy wire is used for another reason: the continuing current after the
main stroke can be several hundred amps, and can last for seconds.
While an AWG 16 wire can take 12.2 kA in a 1 ms pulse, if the pulse
lasts 5 seconds, the fusing current is less than 200A. There's also a
mechanical ruggedness consideration, and an allowance for conductor and
connection degradation over time. The electrical and lightning
protection codes recognize that these kinds of things are installed and
forgotten for the next 20-30 years.
I think the literature shows it's pretty well understood that any
examples of explosion or spalling are not due to the wire failing, but
due to water filling a gap between conductor and surrounding concrete,
getting heated and flashing into steam. While it might take 17 kJ to
raise a meter of AWG16 to 1083C, it only takes 1400 J to get to 100C to
boil water. So there's another reason why heavier wire is better.. more
thermal mass to take the heat pulse (and a reduced thermal load in the
first place because the resistance is lower)
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