On 10/11/16 10:43 PM, Grant Saviers wrote:
To paraphrase, it depends on what the definition of a "good bond" is.
It seems to me that at the overlapped section ends, two legs are always
in contact at one level and at least one at the other level. Wind, tilt
from gravity, or off center tension from the hoist cables insure the
tower sections lean against each other. The wear marks I've seen on the
inside of crank up legs have the galvanizing missing where the "sliders"
are wearing when the tower is extended. So my bet is the tower legs
are the primary ground path, not the cables. So the path inductance is
probably about the same as similarly sized fixed towers.
The inductance is driven by the length, not the width, of the conductor,
so I would expect the voltage drop along the tower from a lightning
impulse to be basically the same for the "through the tower" path vs the
"through the coax shield" path.
AC Resistance through the tower is another story - the tower is steel,
not copper, so the bulk resistivity is higher, and because it's steel,
the skin depth is smaller. There's also the "section to section joint
resistance" (which I suspect is fairly low). And since we're talking
about an AC signal here (lightning), the capacitance between the
overlapped sections is also significant - although I've not even back of
the enveloped what it might be.
For copper, the impedance of the inductance (1uH/meter - 6 ohms/meter at
1MHz) dominates over the copper resistance (whether AC or DC - fraction
of an ohm/meter). But for steel?
Carbon steel has a resistivity about 10 times that of copper, and if I
assume a relative permeability of 100, the skin depth is just under a
0.001" at 1 MHz. There's probably enough magnetic field during a
lightning transient to saturate the steel, so maybe a mu of 1 is a
better approximation - that's 10 mil skin depth, then.
(250 microns) - or maybe we should look at the zinc coating?
I do these kind of things by scaling from something familiar - AWG 10
copper wire is 0.1" in diameter and has a resistance of 1 ohm/1000 ft.
The steel is 10 times as resistive - AWG 10 wire DC resistance is 10
ohms/1000 ft, and has a cross section of (.05)^2*pi =0.00785 square
inches. A tower has 3 faces that are 12" long, and with a 0.001 skin
depth, it's got a cross sectional area of 12*3*.001 square inches or
.048 square inch. That's about 6 times the cross sectional area of our
AWG 10 wire.
So, ultimately, the AC resistance of a tower, at 1 MHz, is probably
around 1.6 milliohm/ft or 5 mOhm/meter.
The inductance still way dominates at 6 ohms/meter.
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