Steve:
Thanks for the thoughtful comments (as usual.) Regarding the metallic
enclosure at the base of the tower, I plan on grounding the shield of the
coaxes to the tower leg BEFORE they turn horizontal and enter the side of
the box. The Trylon has ample flat angled pieces to which I can fasten the
ground clamps while still keeping the ground lead running in a vertical
direction (the same direction as the lightning energy from the top of the
tower.) So we agree on that point.
On the position of arrestors, I understand your argument, but with my
configuration, I believe the risk of an underground induction of lightning
current to be minimal. I've had several lengths of direct-buried coax for
my EWEs and 160M inverted L antennas in operation for about three years now,
with no lightning damage to either the house or the shack equipment. Have
heard of no lightning damage to others' homes in the neighborhood in my 18
years of living here, other than one strike to a chimney (which was
surrounded by 80-foot tall oak trees, BTW.)
In addition, the earth connection at the SPG in my tower installation
will be to a ground rod at the END of one tower radial (fifty feet long.)
If my tower were to take a direct hit, the energy on the interior of the
coax will have to be shunted to ground by the arrestor at this single,
distant ground connection (the shield's energy will be shunted to ground by
the two shield connections at the top and bottom of the tower, hopefully).
In other words, I prefer to shunt a direct tower hit to ground at the
base of the tower where there are three low-inductance connections to earth,
rather than to one ground rod at the end of a long single ground conductor.
I'm discounting as very unlikely, based on experience at AD3F, the
possibility of lightning energy from a nearby hit being coupled to the
underground portion of my coax runs.
But I still have a few weeks before I install the coax. I'll give your
advice some more thought. Thanks again.
73 de
Gene Smar AD3F
P.S. The Sch 40 conduit will be PVC, not metallic. Important distinction.
-----Original Message-----
From: K7LXC@aol.com <K7LXC@aol.com>
To: SPELUNK.SUENO@prodigy.net <SPELUNK.SUENO@prodigy.net>;
jamvet@bellsouth.net <jamvet@bellsouth.net>; towertalk@contesting.com
<towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Sunday, July 22, 2001 11:45 AM
Subject: Re: [TowerTalk] Routing cables - tower to shack
>
>In a message dated 7/20/01 6:46:08 PM Pacific Daylight Time,
>SPELUNK.SUENO@prodigy.net writes:
>
>> I'm planning on putting the arrestors at the base of my tower, bolted
>> to the insides of a metallic Hoffman box or similar. From the box the
coax
>> and control cable runs will be in 3 inch sch 40 buried conduit to the
>> house/shack. The alternative is to put the arrestors in a box just
outside
>> the house at the end of the conduit run and ground at that point, as
most
>> commercial installations have it.
>>
>> My thinking is that the ground field for a lightning strike
originates
>> at the tower, where all the underground radials connect to the tower
legs.
>> Here the inductance of multiple parallel ground legs will be less than
at
>> the entrance to the house/shack, fifty feet from the tower, where I will
>> have only one radial to which to connect the arrestors' grounds. (I'm
not
>> planning on encircling my house with buried copper ground wires.) I'd
>> rather have the arrestors far away from the house and let them do their
>work
>> in a low-inductance part of the ground field, rather than close to the
>> house.
>
> I wouldn't recommend this course of action. A direct hit is only ONE
way
>that lightning transients occur. Another significant one is an induced
ground
>surge from a nearby strike. It doesn't even have to be very close to do
>damage. In the case of a ground surge, anything buried can be a problem and
>the lightning charge has got nothing to do with a tower strike. The whole
>idea of a ground system is to have everything at the same potential so that
>the energy rises and falls at the same rate everywhere. When that happens,
>there's no arcing which is what causes most of the damage.
>
> In any case, the surges still come along the cables and having
transient
>arrestors at the tower does nothing to prevent a ground surge from getting
>into the house. While the conduit may give you some Faraday shield
>protection, lightning paths to ground don't always go where you want them
to.
>So unless your arrestors at the Single Point ground bus at the building
>entry, they're not doing anything worthwhile.
>
> The other way that lightning charges can get into your building is
>induced through utility wires coming into the building. These can come from
>miles away. This includes power, telephone, and TV cable; all of which need
>to be grounded, protected, and connected to the SPGS.
>
> Also the idea of tower cable shields being connected to the tower means
>that you're trying to give a low resistance and inductance path to ground.
>Any bend in the conductors will encourage the surge to take another path
>which means you really want a straight line to ground or if you need to
bend
>something, make it a real gradual bend - no sharp bends at all. I get the
>idea that a box at the bottom of the tower with some sort of grounding
>function will interrupt the straight-line lightning path which means you're
>compromising your ground system again.
>
> You want to have the cables grounded to the tower at the bottom of the
>tower before the cables bend towards the building and you want to have your
>lightning arrestors at the building entry attached to the Single Point
ground
>bus. This is the standard commercial practice. Any other designs are not as
>effective and may be totally ineffective.
>
>Cheers, Steve K7LXC
>Tower Tech
>
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