Skip K3CC wrote:
> If everything is perfect in the world with your grounding systems how
> fast is fast enough to prevent damage from a direct lightning strike of over
> 100,000 A at a freq near 10 Gigs. The heat that can produced from a
> strike is that greater then the temperature of the Sun.
>
> A little over 10 years I lived in Kempton, PA atop a ridge at 1000 ft ASL.
> My tower was grounded OO wire from each leg about 6 ft up from the base.
>
> We had an indirect strike of lightning come through our 400A underground
> service, fuse the line and destroy all electrical and electronic devices
> plugged into the outlets. Cost over $30,000 w/o the electrical service.
> The above ground transformer box was completely destroyed and the concrete
> pad gone. that service was for about 6 homes
>
> I guess this is why I can't believe you can protect yourself from a direct
> lightning hit.
>
You have to realize not all lightning is created equal. In fact what you
witnessed is probably and exception called a "super strike". The average
lightning bolt is probably an order of magnitude less than that at maybe
10,000A and of a very short duration.
My tower http://www.rogerhalstead.com/ham_files/Tower26.htm has taken an
average of three *verified* direct strikes a year since it was put up.
None last summer and 5 the previous summer. I have no idea as to how
many it's actually taken. Since the present ground system was installed
I've had no equipment damage including 5 computers of which 4 are on a
CAT5e Gigabit network which has one run between the garage and shop
about 10 feet to the right of the tower in the photo. In addition there
is one rotator control line, one remote antenna switch control line, two
RG-6 coax cables for the satellite receiver and two RG-6 cables to a
pair of UHF antennas (with antenna mounted preamps) near the top of the
tower, and 5 runs of LMR-400 in the underground PVC conduit between the
tower and when the cables enter the house through the grounded bulkhead
connector. I rarely disconnect any equipment or turn off computers when
storms come through. BTW the CAT5e runs with all those other cables for
about 20 feet while in the basement. It then goes into the garage, up
the wall, across the ceiling and down the North wall, through the wall
and then underground to the shop.
> In the 60's I worked for a radio station WAEB. I remember what we went
> through grounds and megging the system. I was told the important thing was
> to keep the strike outside the transmitter/studio building.
>
> I can't disagree with most of the steps that are taken, It takes a lot of
> copper in the ground, taking reading with a megger, and trying to stay away
> from the RF ground system. However, I think they will only prevent
> transients, not direct strikes
>
>
I'd have to disagree as I've seen repeated direct strikes to my tower
with no damage. However I hasten to add that any lighting is a crap
shoot. Although rare there is little one can do about one of the super
strikes. Fortunately they are a tiny fraction of a percent so the work
to protect a station is well worth the effort. IIRC Prior to installing
the ground system (32 or 33 8' ground rods cad welded to over 600 feet
of bare #2), adding the grounded bulkhead connector and grounding all
coax shields on the tower at the top and bottom, I lost one repeater
antenna (the repeater survived), one run of 7/8" Heliax, one 2-meter
rig, several runs of 9913 coax, and one computer (that was less than two
weeks old). Since then I've lost one N connector and one Polyphaser, but
I can't trace their failure back to any particular storm. Let me
restate that as "I think" I can trace the Polyphaser to one week end
when we had numerous storms. OTOH I was standing out by the road,
talking with a neighbor while listening to a storm way off in the
distance. We could hear the thunder but nothing loud. Then a bolt of
lightning struck the power pole that was no more than 12 feet to the
North of where I was standing. We both made hasty exits heading for
home. Another bolt struck the next pole to the North and a pine tree
about 200 yards to the East. It literally blew the pine tree apart with
pieces over three feet long embedded over a foot into the ground over 50
feet from the base of the tree. The neighbor to the North who's house
was fed off that transformer lost a TV, sump pump, water heater,
refrigerator and "a bunch of other stuff". I had no damage from the one
that stuck the transformer that feeds my house and shop. Even though the
tower is much taller and about 130' from the pole it didn't get hit.
(that we saw)
73
Roger (K8RI - ARRL Life Member)
www.rogerhalstead.com
N833R (World's oldest Debonair)
> de Skip K3CC
>
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