On 7/5/19 2:26 PM, Poul-Henning Kamp wrote:
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In message <28f942e8-b61d-4fa5-929d-923184828...@n1k.org>, Bob kb8tq writes:
Energy flow is indeed inside the cable if things are set up and operating
correctly.
Please note in this context that *nothing* about lightning strikes
works the way you would assume it does.
Cables run inside steel tubes protect the steel tube from lightning
current because copper is a better conductor than steel - in
particular when the leading flank is measured in kV/uS and the
current in kA.
Likewise, a 90 degree bend or a loop on the cable is a huge
inductance to get all that high frequency energy through, so
lightning tend to jump from bends and loops, to less inductive
paths if possible
Actually, the inductance of a bend isn't much more than the inductance
of a straight piece of wire of the same length. You have to have a
complete loop before the inductance starts to rise, and even then, a 1
turn loop doesn't have huge inductance, it's multiple loops where the
inductance starts to rise as Nturns^2.
Straight wire has an inductance of about 1 microhenry/meter (very weakly
dependent on diameter)
But a more exact calculation says that a 31.4 cm piece of wire, 10mm in
diameter has a self L of 0.26 microhenry.
A loop that is 100mm in diameter with a 10mm diameter conductor has an
inductance of about 0.15 microHenry. 100mm diameter is a length of 0.314
meters, so it actually has *less* inductance that a wire that's the
length of the perimeter.
A 1 meter diameter loop (perimeter 3.14 meters) has an inductance of 3
uH. Which is close to the self inductance of a 3.14 meter straight wire
(4 uH)
The origin of the "no sharp bends in lightning conductors" is more
related to the flashover voltage to surroundings - A sharp 90 bend has a
lower breakdown voltage than a gradual bend because the radius of
curvature is smaller.
There's also a mechanical stress effect - if you have a corner, the wire
on one side of the corner is carrying a current at right angles the
field from the other arm of the corner, and it will tend to move
(violently, given the large peak currents)
Be careful with EMI/EMC clam-on ferrites, they can explode in
lightning strikes.
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