To: <towertalk@contesting.com>
>Date: Fri, 12 Jun 1998 00:40:05 -0400
>From: "Guy L. Olinger" <k2av@qsl.net>
>
>K7LXC@aol.com wrote:
>>
>> In a message dated 98-06-11 19:36:07 EDT, n7cl@sparx.mmsi.com
>> writes:
>>
>> << Also, you might compare the total square feet of land that
>> you have antennas on to the total square feet of land
>> encompassing the area where you have seen the lightning
>> hitting. It just might not have been your turn yet. >>
>>
>> Ouch! The cold slap of reality strikes again (no pun
>> intended).
>>
>> Cheers, Steve K7LXC
>>
>
>I keep seeing these (apparently) contractory anecdotes. Assuming
>no one is telling bald-faced lies, any theory must include *all*
>the anecdotal material to count.
I don't think anyone is fabricating anecdotes. Lightning
behaviour is so apparently weird that this isn't generally
necessary.
I also don't think that the aecdotes are at all contradictory.
All the accounts that I have read on here so far are completely
consistent with what I have perceived over the last 50ish years
as reality.
The only contradiction I see is in the assignment of causality to
account for the observations.
>
>I suspect that there is a real reason that certain towers never
>get hit, and others get hit all the time.
>
There are many possible different reasons. Any one or a number
of which may or may not apply to a particular installation. Some
have to do with meteorology, topology and physics. And some have
to do with human perception.
It has been my experience on the commercial side of life that
customers who sustained damage were aware that they had been hit.
Those that didn't were convinced that they had never been hit.
!!! Warning anecdotal material to follow !!!
We have a customer that colocated the majority of his radio
infrastructure with the dispatch tower where the dispatchers sit
to observe the mining operation and deal with exceptional
circumstances by talking to the equipment operators on the voice
radio net.
This is a steel structure that sits up about 15 feet off the
ground on steel support beams buried in a concrete slab. It is
located about 3/4 of the way up the edge of their pit. That's
right, the top of this tower is several hundred feet down in a
hole. So, of course, the prevailing wisdom was that this site
couldn't get struck.
Consequently, the lightning diversion and grounding scheme at the
tower were quite literally a disaster. The tower is filled with
radio and some very high dollar computer equipment. It has both
wireless and dedicated line wired communication links with
various other locations on the mine property. It looked ike it
had been wired by Rube Goldberg.
It seemed that every summer in the monsoon season, any time
lightning struck anywhere in southern Arizona, they had extensive
damage to just about everything in the tower. Radios, modems,
and serial I/O ports were particularly susceptable. Of course,
they thought that this must have all been induced damage by
nearby strikes since this site _can't_ cet hit.
Then one day they actually took a direct hit to one of the base
station radio antennas at the site. The dispatcher doesn't
remember seeing the flash or hearing the report. The emergency
medical team found him more than 8 feet away from (behind) the
chair he had been sitting in. His shoes were still on the floor
in front of the chair. The dispatcher was unconscious but OK
other than some burnt skin. Every last piece of electronic
equipment at the site was irreparably fried including the
emergency standby generator. Not even the microwave oven
survived and it was unplugged at the time.
This event finally convinced the mine management that the cost of
a lightning protection system for the dispatch tower might not be
excessive after all. They asked us to specify a protection
scheme for their tower. We had been urging them to do this for
years since we frequently had to eat the cost of our damaged
equipment that was installed there.
We went to the site. analyzed why we thought they were getting so
much trouble, and recommended a course of action to correct the
problem. They implemented _most_ of our recommendations.
Including all of the really important for human safety ones.
Our recommendations included air terminals (lightning rods),
antenna relocation, single point grounding scheme, and line
protectors. We also recommended some inside the facility wiring
rerouting to minimize loop coupling to magnetic pulses.
That was eight years ago. From that time to this, they have lost
ZERO electronic equipment at that site (due to lightning).
So, how did the mine managers perceive this situation? Were they
jubilant that we had completely solved their problem at such a
small relative cost? No. They were pissed at us for making them
spend all that money unnecessarily. Why? Beacuse lightning no
longer strikes there. They didn't ascribe this good fortune to
our corona points deionizing the air and stopping the lightning.
They thought that their probabilities had run out and they really
weren't going to get hit as much as they had been. In their
minds, the previous experience had been a statistical anomaly.
If we hadn't been pestering them, they would have held out a
little longer and not been required to spend the money. After
all, the dispatcher really wasn't hurt all that badly. They
hadn't been required to pay out any death benefits.
Fortunately, I had installed a lightning strike counter on one of
the tower support legs without putting it on their budget or
telling anyone about it. It was out of sight and out of mind. I
actually paid the 50 bux for it out of my own pocket. It wasn't
even on OUR budget. I left it in place until they called up
after the second monsoon season with no damage. They wanted us
to pay for the lightning protection work.
I went to their site and took one of the managers out to the
tower. I showed him the counter which I had zeroed two seasons
ago. It now registered 35 events. Their tower had been struck
35 times in a little less than two years with enough energy to
ring the strike counter's chimes. I removed my counter. We
didn't have to pay for the work.
!!! End of anecdotal material !!!
It is possible if not likely that a significant fraction of the
towers that have "never" been hit are simply towers that actually
have been hit but took it well. And it is possible that towers
that apparently get hit all the time are installations that have
the characteristic that either:
1. They are under constant observation so strikes are likely to
be observed even if no damage occurs.
or
2. They sustain or induce observable damage and make the owner
aware of the strike regardless of whether the strike event
was actually observed.
>Remember the story I told about my 70 ft tower at K4VDL topped by
>a 33' vertical that was never hit and a 12 foot mimosa tree 70
>feet away converted to toothpicks by a strike. Do folks *really*
>want to ascribe that to random behavior? By some opinions here,
>the tower should have been 100 feet more attractive than the
>mimosa.
How do you determine that the tower assembly has never been hit?
The tree made a permanent and easily observable record of its hit
status. The tower may not do this. Most don't.
Also, charged clouds are not static (pun only marginally
intended) phenomena. The clouds are typically moving along as
they are accumulating a potential difference with the earth. As
the cloud moves along the horizontal plane, the region of
oppositely charged earth under the cloud is also moving along
under it. What if the cloud was approaching from the direction
of the Mimosa tree and earth displacement charged region just
simply got to the Mimosa tree before it got to the tower? Sure,
mine is also only a theory but it explains the observation as
well as the others.
>
>My elmer was the head engineer at WCTT (AM & low end of the
>band). Lessons were at the transmitter house out at a place
>lovingly called "the swamp". I think the tower was 450 foot or so
>quarter wave. Also think the swamp was 10% copper. Many summer
>nights watched the lightning gap at the bottom of the tower arc
>with an almost metronome regularity. Now and then lightning
>would hit the top of the tower and throw sparkles fifty feet from
>the gap balls. The gap balls got quite pitted and had to be
>replaced a couple times a summer. Lightning never took
>transmitter off the air while I was there.
>
>*HOW* do both stories fit under the same theory?
>
If the tower had been a grounded base design, how would you have
been aware of all the lightning activity? Other than the spark
gap balls, what damage was sustained by the tower or equipment?
Snip...
>It would also say that lightning rods work fine on barns and
>houses, and get melted above the critical height.
I can tell you from my own personal experience that lightning
rods do not get melted at any height even when struck (except for
a tiny spot right at the tip where the plasma attaches) by
lightning. They do not protect the barn by preventing a strike.
They protect the barn by arranging for the hot plasma of the
strike to touch the rod which can easily withstand the brief heat
pulse rather than coming into contact with a more flammable part
of the structure. And by dumping the current of the discharge
between earth and cloud via a path that can handle the load
without failure. This is not magic or new. It has been well
known and documented since the time of Ben Franklin.
We (here at work) routinely use side protruding lightning rods at
heights above 500 feet (actually at 100 feet and up) to protect
side mounted antennas. We use them because they are effective,
cheap, obtainable locally in virtually any part of the world, and
don't blow apart like the porcupines when they do get hit. I'm
not certain that the porcupines would blow apart either if
properly installed. But I have seen some blow apart when hit.
>
>In the fireball story I told, where lightning came in the
>telephone line, he *did* have a small tower & a beam, which was
>not struck.
How was the determination made that this tower assembly was not
struck?
>He was on a mild knob with flat area around, and the knob took a
>lot of strikes. Why not his tower? This would fit with the
>theory above.
It will also fit just about any of the considerable number of
other theories that have been proposed. This is one of the
problems we have when trying to develop a theory to assign
causality to individual occurances of very low probability events
associated with a phenomenon that has an extremely high degree of
variability. Most of us are not very good at intuitively
analyzing random statistical processes. Myself included.
>If the theory is true, I sure would like to know what the balance
>height is, or what it depends on. It's above 100 feet and below
>450 feet.
>
>I also would *guess* that ground conditions underneath the tower
>could raise or lower this height, and so could local geographic
>features, such as the ridge in one account. A local ridge would
>raise the critical height in an adjoining valley. Being on an
>isolated knob in otherwise flat territory would lower the
>critical height.
>
Many things can and do affect the relative probability of a
strike occurring at a particular geographic location. Some are
fixed and some are variable. Some are easily observable and some
are not within the ordinary awareness of human observers. So
far, I have never done a radio communications equipment
installation in a location for which that probability had been
lowered to ZERO by any combination of natural circumstances or
the diligent efforts of human beings.
I have never seen a mitigation scheme that could prevent a strike
from happening. Including porcupines at any height or quantity.
I have only seen mitigation strategies that could successfully
prevent damage in the event of a strike.
>As you can hear on HBO, "That's just my opinion, I could be
>wrong."
>
>73, y'all
>
>Guy.
>
Don't get me wrong. I'm not an expert on all the nuances of
lightning phenomena. I don't have a "unified lightning theory"
to explain all the various apparently weird circumstances
surrounding all the reported events. I don't think anyone really
does. Certainly not anyone that isn't doing current formal
scientific research on the subject.
Mostly, I just don't worry about trying to assign a cause for any
of the weird things I have observed lightning to do (and not do).
Frequently, I find myself saying "How the Heck did it do
_that_!!!?" And then (usually very soon afterwards) "Boy am I
gald we had a good protection system installed on that site".
73, Eric N7CL
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