When you hear lighning interference on the radio, is it just a SNAP
- which would be all you get with straight DC?? No, it is a
crackle/fizz typical of an alternating current of high rate of
rise/decay ( I forget the term - something like dldt but that's not
I understand. But remember that *if* you take 17 strands of a small
diameter wire, combine them into a bundle, near DC the overall cable
will act like 17 wires in parallel and have lowered impedance, but at
high frequency the new bundle of 17 wires will not act like 17 wires
in parallel, but rather act much like a large diameter, somewhat solid
The only way to connect two points with a really low impedance is to
make the connection WIDER than LONG. Then you have a shot at lowered
impedance. And, that even means making the connection wider than you
think necessary, like 2X wider. Then your connection becomes very low
impedance acting much like a ground plane, very low impedance. Else,
if connection is longer than wide, you have an inductor. And as you
know, for an inductor it will not pass any appreciable current for a
short bit of time, no matter how large a voltage you slap across it.
You can kind of cheat a bit and approximate a wide connection by using
several connectors. That is why a well bonded wire grid structure
protects buildings/structures so well. The connections, albeit not
solid over the whole width, help lower the impedance of the connection
by making it wider than long.
The only way to have a bunch of conductors bundled, is to insulate each to
prevent interaction. Except it must withstand, how many volts !!!! Even
insulated, stranding close to other strands will have interaction
nullifying what your trying to do.
The cables I have seen on buildings are stranded aluminum at least 3/4 inch
On Jan 26, 3:14 pm, email@example.com wrote:
EXACTLY! Lowering the impedance further.
The currents flowing in a ground plane even at DC are interesting but
for a transient become even more interesting. There are currents
flowing in what you would consider the 'wrong' direction. Adjacent to
the currents flowing in the 'correct' direction. As you may know a
current in one direction [the main current flow] having currents next
to it flowing in the 'wrong' direction [one enhances this effect by
making the connection much wider than what seems necessary] cancels
magnetic fields and really lowers the impdeance that the main current
was 'seeing'. For exmple, consider the impedance between 2 points one
inch apart in the center of a ground plane that is first only 1.5 by
1.5 inches, then compare to impedance between the two points when the
ground plane extends out further to 4 by 4. Map the currents flowing.
The allegory I mentally use to envision a discharge and makes it easy
[easier] to understand lightning discharge is envision a rubber sheet.
That is ground. Then pinch a small bit and lift up. that is like a
lightning strike at the point where you pinched the sheet to llift it.
It is easy to see that with uniform impedance around [the rubber
sheet] the slope of the fields created by that discharge. You have
like a teepee. And, you can see how there is very little voltage
difference between adjacent points. Also, see why four legged animals
are more likely to die than two legged animals from an adjacent
strike. Now violate the uniform impedance of the rubber sheet by
running conductors around. Unless those conductors are concentrically
placed around the hit they will transfer high voltage gradients
unintentionally between two points of widely varying voltage and since
high voltages occur across very narrow regions SECONDARY discharges
A bit clumsy allegory but helps sometimes to envision the voltage
gradient aspects of lightning.
Also, from experience of discharges in air. It is very difficult to
get a super fast rise time of a discharge against a 'point' surface. A
broad flat surface makes a blast discharge for the same amount of
charge build up that a pointy surface produces. Thus, IMHO lightning
rods can be made to work as protection and can be much better than
nothing there at all.
I agree with Robert Macy.
I believe hams like flat braid (like a flat stranded wire) for
conductors that may carry high lighting currents.
Far as I know, the current design technique is to roll a sphere with a
radius of 30m over the building and surroundings. The sphere stays on
top of the rods. If the sphere touches the building, lightning can
You likely need fewer rods with the sphere design than the cone design.
But if you transport the Empire State building to the middle of
Nebraska, with no surrounding buildings, lightning can hit the side of
the building. Sides of buildings may also need lighting protection.
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