Bill:
I agree with you that a direct hit on an exposed conductor, such as a
telephone line or coax cable, can be very dangerous, even if part of this
conductor is underground. My neighborhood has all utilities underground, so
that is still a concern to most who live here. (My house was recently
converted to fiber-optic cable for TV, phone and internet service, so at
least those conductors are no longer an issue for me. In fact, I removed
the abandoned twisted pairs and the TV drop cable. But I still have the
power line entering the house.)
My issue is with the notion that a lightning strike can induce
dangerous voltages INSIDE a piece of coax cable whose shield is grounded at
both ends and whose center conductor is not connected to an outside antenna.
This is the case with my tower and the switch at its base. The coax that
leads from the switch into my shack is NOT connected to anything outside of
the box at the tower and 45 of its fifty foot length are underground. My
thesis is that this arrangement does not require another lightning arrestor
where the cable enters the shack, although, as I wrote before, its shield is
grounded at the SPG panel inside another steel box outside the shack. So if
a strike does induce a voltage on the shield of this underground piece, as
least it will be grounded at the SPG before it enters the shack.
My issue is not with whether lightning can still affect underground
conductors - it certainly can - but with the notion that the center of an
underground coax run needs additional protection from another arrestor. Now
if this underground coax were routed directly from the tower top into the
SPG with no intervening switch, then I would agree that an arrestor would be
needed.
However, I see nothing wrong with adding this final arrestor at the SPG
to the coax that leads from the antenna switch. As I said -
belts-and-suspenders.
73 de
Gene Smar AD3F
----- Original Message -----
From: "Bill Aycock" <billaycock@Centurytel.net>
To: "Gene Smar" <ersmar@verizon.net>
Sent: Saturday, January 09, 2010 11:32 AM
Subject: Re: [TowerTalk] Lightning Arrestor Question
> Gene, Jerry, Jim and all-
> My personal (and frightening) experience tells me that we should NOT rely
> on the folklore that "underground" is safe.
> My first damage at this QTH was through an underground Telephone wire,
> over a distance of over a mile and a quarter. The local (Telephone wall
> socket) flash was followed , several seconds later by the sound, and later
> confirmation that the hit had been on a distro box at the (other) end of
> my road. The last 1650 feet from a pedestal to my house was an unbroken
> line,underground, not in pipe.
> Underground is better, but still has hazards.
> Bill--W4BSG
>
> ----- Original Message -----
> From: "Gene Smar" <ersmar@verizon.net>
> To: "Jerry" <grf@uneedspeed.net>; "Tower Talk" <towertalk@contesting.com>
> Sent: Saturday, January 09, 2010 9:42 AM
> Subject: Re: [TowerTalk] Lightning Arrestor Question
>
>
>> Jerry:
>>
>> My coax switching arrangement is identical to yours, i.e., the switch
>> is at the tower and the coax runs enter through a steel box. I chose to
>> install arrestors at each coax where it enters this box, before it
>> connects
>> to the switch. In this way I have protected all my incoming coax lines
>> BEFORE they hit the switch. I have thus also protected the switch in
>> case a
>> strike hits while an antenna is connected to it.
>>
>> At the shack end I have no additional protectors. The underground
>> run
>> to the tower is fifty feet and I cannot convince myself that sufficient
>> lightning energy will be coupled to the underground coax's center
>> conductor
>> to be of concern. (I also ground the coax shields at a SPG panel outside
>> the shack.) Others on Towertalk will disagree, saying that a ground
>> strike
>> near my coax run COULD induce energy INSIDE the coax, where another
>> arrestor
>> would be helpful at the shack end. You decide for yourself whether this
>> belts-and-suspenders approach is for you.
>>
>> If you follow my approach, you ought to make sure that you have an
>> adequate lightning ground system protecting your tower and that you
>> provide
>> a positive connection to it from within the steel box the switch is in.
>> Don't rely on mounting hardware to provide this connection through the
>> tower
>> steel; it could corrode and there goes your ground connection. I left
>> excess length on my tower leg ground wires so I could extend at least one
>> into the box. I grounded it to the back of the inside of the box with
>> appropriate hardware. I ran this wire into the box through a plastic
>> insulating that I filled with electrician's putty once the wire entered
>> it.
>> I suppose you could ground the wire adequately to the exterior of the
>> steel
>> box as well, but I preferred to bring it inside where it would come into
>> contact with the steel box in an environment protected from direct
>> rainfall.
>>
>> 73 de
>> Gene Smar AD3F
>>
>>
>> ----- Original Message -----
>> From: "Jerry" <grf@uneedspeed.net>
>> To: "Tower Talk" <towertalk@contesting.com>
>> Sent: Saturday, January 09, 2010 9:50 AM
>> Subject: [TowerTalk] Lightning Arrestor Question
>>
>>
>>>I have a box at the base of my tower housing my DX Eng. antenna switch
>>> box that also provides a central entry point for all the antenna cables
>>> (prior to going underground to the house). Will placing an arrestor
>>> only on the output coax from the antenna switch to the house be
>>> sufficient or do I need to place an arrestor on each antenna line
>>> entering the box and switch (the shields are all grounded to the ground
>>> system in this box)? Additionally, do I need to place an arrestor at
>>> the house's central grounding point (where it enters the shack)?
>>> Overkill? The grounding system is all tied together (tower, switch box,
>>> house entry point, and home's electrical system).
>>> Thanks for any help/ideas you can give me.
>>> Jerry France
>>> K7LY
>>> Lake Havasu City, AZ
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>> _______________________________________________
>>
>>
>>
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>>
>
>
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