Ground lightning is the equalization of charge between cloud and ground. It
happens all over the world many times a minute.
What mainly determines where lightning will strike are local conditions.
This includes wind, humidity air pressure etc. The charge from ground
objects are usually referred to as "streamers" that travel upward as charge
builds.
The charge from the clouds produces what are called "step leaders" which
travel downward in jagged steps of around 150' steps. The local conditions
have an effect on which way the next step goes. When the step gets close
enough to a streamer there is a connection and a strike path results in a
plasma trail that has very low impedance.
The 150' steps are where the "rolling ball theory" comes from. This gives
some idea of how lightning can strike the side of a tall tower rather than
the top as the step leader is jumping around at 150' intervals. If it is
near a tower the next "step" may be sideways rather than straight to the top
of the tower.
This is also where the "cone of protection" comes from. If this imaginary
"rolling ball" with a 150' (step leader is in 150' steps)radius is rolled up
so that it touches a tower it will show that everything below that point of
contact goes out away from the tower at curved angle of the circumference of
the ball.
The area from that point on the tower to the point where the ball touches
the ground is said to be within the "cone of protection".
In other words lightning will most likely hit at the 150' point or higher
and objects within the "cone" are less likely to be struck.
This theory also says that any tower over 150' can be hit at any point above
the 150' point and not necessarily at the top.
Once there is a strike and current comes down the tower it needs to go to
earth. The ground system needs to be spread out (radials) to allow the
charge to rapidly be absorbed. There is propagation time delay in the earth
just like a transmission line has delay.
If you try to dump all of the charge into a single spot the earth can not
absorb it as fast as it is coming in and voltage will become high at that
point. If there are multiple paths for the charge to travel in the earth it
will be dissipated without as large of a charge build up at the entry point.
This is one reason why ground rods and radials should supplement a Ufer
ground.
73
Gary K4FMX
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
> Roger (K8RI) on TT
> Sent: Thursday, January 29, 2015 4:21 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Lightning. What is it?
>
> I'll try to do this justice, but it is covered fairly well in a number
> of locations on the web with the Polyphaser site being one of the best.
>
> Lightening is a discharge between two moving, charged areas: Cloud to
> cloud and cloud to ground.
>
> In the case of cloud to ground we have two "moving" charged areas. The
> emphasis is on moving!
> Because the charged areas are of opposite polarity lightening is
> mistakenly thought of as a DC event.
>
> They also vary in intensity from a few hundred to hundreds of thousands
> of Amps
>
> 1st the area on the ground is as dynamic as that in the clouds. It
> varies is size, shape, speed, intensity, and direction.
> This charge is not homogeneous varying from zero at the edges to a
> maximum at the center. Instead the intensity varies widely in Jouls
> over the entire area so the tower at or near the center could be at a
> low point, or even close to zero while areas all around it may be at
> much higher potentials leading the lightning to strike much shorter
> objects...
>
> The charge is "in the ground", Towers, trees, buildings, power line,
> virtually any conductive object. Said object does not even have to be
> elevated, although elevated objects do present a closer target and are
> struck more often. But think for a minute. When a lightening bolt
> travels a mile or two, is it really going to have a lot of concern for a
> path a 100 feet shorter.
>
> Lightening bolts are initiated by "feeders" coming down from the clouds
> and up from the ground. When these feeders connect a lightening stroke
> is initiated. This initial stroke may be cloud to ground, or ground to
> cloud. It will be followed by alternating strokes up and down. The
> rise and fall times determine the RF frequencies which "usually" center
> around 1 MHz.. What the strike hits may have as much to do with where
> the object is located in the ground field as it does the height. OTOH
> The odds favor the taller object getting struck..
>
> The dynamics of a lightening stroke are complicated as is the
> development of the feeders. Anyone who has seen a lightening bolt
> realizes they do not follow a straight path. Lightening bolts contain
> tremendous currents. So much so that the bolt may be self quenching and
> as we see it flicker, it may follow different paths. I've seen large
> towers struck, with the bolt getting off part way down, The magnetic
> field becomes so strong that the lightening bolt seeks an easier path to
> ground.
>
> Although the dynamics of the bolt may be complicated, they follow simple
> rules of physics and the parts, we as hams are interested in can be
> simplified to useful levels if we don't "over think" what's happening.
> There is no preventing lightening strikes. ALL we can do, is mitigate
> the effects of the lightening strike, direct hits and nearby strikes
> that can induce very high voltages in electrical wires. Give it a more
> conductive path to ground and use devices to short voltage pulses to
> ground and keep them out of our fragile and expensive equipment. The
> Franklin system including lightening rods, porcupines, and other devices
> just provide a path to ground that is more attractive to lightening than
> the structure.
>
> Lightening protection systems, often called Franklin systems are named
> after Benjamin Franklin who is given credit for inventing the lightening
> rod although the use of lightening protection systems predates the
> "Franklin Lightening rod", by hundreds, or possibly thousands of years,
> but Ben is given credit for proving lightening is just a form of
> electricity. https://en.wikipedia.org/wiki/Lightning_rod
>
> It's worth repeating: There are no devices that will prevent lightening
> strikes. We can only use good installation and grounding practices to
> mitigate damage from lightening strikes. We can build fairly effective
> systems, but none give 100% protection. The more effective the system
> the more expensive it become. Beyond a good system, building a more
> effective one rapidly approaches diminishing returns for the investment.
>
> --
>
> 73
>
> Roger (K8RI)
>
>
> ---
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