<3717319D.DFE23C18@erols.com> <199904161807.LAA17332@toontown.mmsi.com>
<3717BA82.726BD29D@erols.com>
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Hi Dave,
What kind of static discharge device do you have in mind? What
makes you think the lightning current is going to go down the
wire that you want it to?
Tom is correct. I think you are worrying about the wrong part of
the system. I don't think that lightning is a good reason to not
use a crank-up (even though I don't like them for other reasons).
The crank-up tower is not in much danger even though it will
carry most of the fault current for two reasons.
First, the connections between sections in any crankup that I
have seen have been pretty good. Good enough so that it seems to
be OK to feed them as vertical radiators. I haven't seen any
feedpoint impedances jumping around due to intermittent tower
section connections in any wind conditions from zero through very
gusty. Nor have I seen any evidence of RF burns at junction
points. I have heard stories of cable / pulley damage from a
lightning strike but I have never observed even a single
instance of this even on crank-up towers that I know for sure
have been hit. In fact, those towers also didn't exhibit any
evidence of having high resistance heating effects at the section
joints after a strike.
Ask yourself "How come the tower sections which are not bolted
together don't fall over sideways under the loads that are on
them?". It isn't because the loads are all perfectly balanced
enough that there are no sideways forces that the tower joints
are resisting. The tower sections are resisting the lateral
forces by making contact with one another at the joints. Usually
at fairly high localized presures.
Second, even if nature should conspire to produce an open
connection at exactly the same instant that lightning strikes
your tower, as Tom said, the lightning that just successfully
broke down a 20,000+ foot column of air to get to your tower
isn't going to have any trouble at all ionizing a gap that might
be from a few thousandths of an inch to a quarter inch (though I
doubt one this big) across.
Regardless of the connection mechanism during a fault, the tower
is going to be a much superior low impedance conductor for the
fault current than any wire you are likely to be willing to hang
of to the side of it. So it will carry the majority of the fault
current.
In terms of protecting the cables, I would make sure that:
1. All the cables are shielded.
2. All the shields are bonded to the tower system at the top.
3. All the shields are bonded to the tower system at the bottom
before leaving the tower area.
4. Equipment which will be connected 100% of the time has
appropriate surge suppression devices on the cable inner
conductors.
5. The fault current has a low impedance path to somewhere safe
at the bottom of the tower.
73, Eric N7CL
To: <towertalk@contesting.com>
>Date: Fri, 16 Apr 1999 18:32:34 -0400
>From: Dave Jordan <wa3gin@erols.com>
>
>Eric,
>
>SURE...but we're not talking about solving the concern but
>perhaps reducing some of the volunerabilities that might cause
>damage to the cables, etc. I plan to install a static discharge
>device for the top of the tower, run the down lead to the ground
>grid and forget about it. I don't see any other options other
>than installing a non-crank-up tower. Do YOU ?
>
>Eric Gustafson wrote:
>
To: <towertalk@contesting.com>
>> >Date: Fri, 16 Apr 1999 08:48:29 -0400
>> >From: Dave Jordan <wa3gin@erols.com>
>> >
>> >Why can't a person simply run a ground wire from the top
>> >section/mast of the crank-up down along with the coax and
>> >terminate the wire to the ground system at the base?
>> >
>> >===================
>>
>> You _can_ do that. But until the surface area of the ground wire
>> and coax shield are larger than the surface area of the tower
>> structure, the tower will still be carrying the majority of the
>> stroke current.
>>
>> 73, Eric N7CL
>>
Snip...
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