On 12/28/2014 11:05 PM, Roger (K8RI) on TT wrote:
One further thought: I would not place an additional arrestor at the
base of the tower where it would likely see a much higher voltage than
at the grounding plate (CPG) at the house entrance.
73
Roger (K8RI)
I agree with David, but I've not read all of the thread.
You want the arrestor as close to the thing it's protecting as
practical. The common point ground at the station entrance is the
best compromise. I'd phrase it a bit differently than David, but the
result is the same. The effects of a spike coming in as induced
voltage in the house wiring depend on a number of things. An induced
spike will likely create a substantial spike on the power, phone,
cable, and the antennas. You have the direction to the source, or
lightening bolt. The induced voltages will travel a bit slower on the
wiring than the EMP through the air. So distance and direction will
determine the phase differences between all these conductors connected
to the rig.
The pulse "induced" into the antenna(S) from a nearby strike sees the
coax and tower as parallel conductors while the center conductor of
the coax acts like one plate of a very long capacitor so the voltage
across the coax is much less at the base of the tower than at the
top. With the coax braid grounded to the tower at the top and botton,
both are "Earthed" at the base of the tower. With the coax between
the tower and shack the coax agan acts like a long capacitor reducing
the center conductor to shield even farther. This in itself reduces
the work the arrestor has to do substantially. (Another reason for
mounting the arrestor at the CPG.
Beyond that being the best place to put the arrestor, lightening
protection can never guarantee 100% protection. The induced voltages
come at the rig from all directions with their phase differences and
amplitudes varying from almost nothing to those we don't want to think
about.
Lightening protection is a crap shoot, BUT, a properly placed
lightening arrestor along with a well laid out ground system of
sufficient size pushes the odds in our favor. It can be substantially
in our favor, however you soon reach the point of diminishing returns
where where it takes a LOT invested to only give a little increase in
protection.
I had no damage from many direct strikes to the tower (17 visually
verified strikes and who knows how many were not seen) along with
major close strikes that did costly damage to the neighbors. The one
was a multiple strike that hit all around me, but ignored the tower
and rigs...BUT...Last summer a nearby strike got into my CAT 5 gigabit
network and did major damage to several computers. I ended up
rebuilding two BIG computers, while the router and switch were
fried.. One was a complete rebuild from the motherboard up. The
other, a smaller computer, but still larger than what most would have
was a complete replacement. The one computer hooks into the station,
but I've found no damage to the station.
Unfortunately I use OEM software which meant not only new hardware,
but new operating systems as well. The good news was the station had
no problems, or at least none I've found so far.
73
Roger (K8RI)
On 12/27/2014 1:58 PM, David Robbins wrote:
Yes, it can make a big difference. Lightning arresters are NOT just
protecting you from a strike on your antenna. They also protect you
from a strike on the power line or even just to the ground nearby
that causes the ground system voltage to rise while the voltage on
the center of the coax doesn't... if the arrester is not very close
to the radio it delays the arrester action which can make your radio
the easiest path from ground to the center conductor... in hv
engineering terms its called a backflashover, which is caused when
lightning hits the ground system causing a flashover to a power
conductor.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf
Of David Gallatin via TowerTalk
Sent: Saturday, December 27, 2014 18:49
To: TowerTalk@contesting.com
Subject: [TowerTalk] Arrestor placement
Hello all,Somehow I have gotten the idea that one's lightning
arrestors are best placed at the bottom of the tower. This seems
logical to me. Then I came across another piece that said, no,
arrestors are to be placed at the shack entrance panel. Which is it?
Or does it matter? 73, David, AA9G
ex W5DCG and KC9EEV
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73
Roger (K8RI)
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