[TowerTalk] QST's Tower in the Attic

[Eric Gustafson]

Hi Pete,

Comments below.


>Date: Fri, 21 Jan 2000 13:06:54 +0000
>From: Pete Smith <n4zr@contesting.com>
>
>At 06:35 PM 1/20/00 EST, W5USM@aol.com wrote:
>>
>>Aside from the structural debate, has not the critical question
>>of proper lightning ground been ignored?
>>
>>And how about code compliance as well as your homeowner's
>>insurance?
>>
>>Any building inspectors or insurance agents wish to comment?
>>
>>Just thinking.  No flames, please, an electrical storm could do
>>that.
>
>This doesn't strike me as any more likely to take a hit than any
>of the millions of large TV arrays on 20-30 feet of Rohn 20 that
>you see all over rural America.

Very punny Pete! (strike me)...

But it is also no _less_ likely.  And the TV arrays are
frequently hit.  Ask any homeowners insurance agent.

The question is if it _does_ get hit then how might the
consequences differ from those acruing to a strike on an antenna
system the conductive structural parts of which are all located
outside the building envelope?  Clearly, there is potential (no
pun intended) for more sevre consequences when the tower is
brought inside in much closer proximity to other conductive parts
of the house.

With the minimum single down conductor referred to in another
post, the lightning control texts all caution that any conductive
parts of the structure within 3 feet of the grounding down
conductor should be bonded to the conductor to prevent side
flashes.  This means that the experts believe that there is a
possibility of a high enough potential appearing on the ground
conductor during a strike to break down 3 feet of air.  The
potentials are going to be higher at the top of the ground
conductor than they are anywhere below that.

At a minimum, this means that a farily large clearance zone
around the tower parts in the attic (and on the floor below just
underneath the tower base) should be planned for.  You might want
to consider relocating all that gunpowder and reloading equipment
to a remote corner of the house. ;-)  It also implies that it
_might_ be a good idea to bond the heating and A/C ductwork into
the system.  That could turn out to be fairly problematic unless
a lot of bonding all over the A/C system was carried out and
returned to a good earth connection.  As each conductive
structural component is brought into the bonding scheme, the zone
where other things should be bonded expands to the extent of the
last bonded conductive system (although the potential flash
distance steadily decreases).

I don't think I'd undertake this effort on an already built
house.  Though it might be posible to do adequate mitigation if
the installation was planned for properly prior to construction.

IMHO, the only alternative worth considering on an existing
structure would be to provide multiple low impedance paths to
ground outside the structure to prevent the potential at the top
of the system from getting high enough to support any side
flashes inside the structure.

The code requirements of a single relatively small diameter
conductor to ground are simply inadequate to protect in this
situation.  When the code was written you can be sure that the
possibility of an internal ungrounded conductive tower structure
was _not_ considered.  And, the code assumes proper lead dress
and entry point suppression for the feedline and any control
wiring.

If I were doing one of these, I would not consider less than four
large surface area conductors connecting to four earth connection
points which are symmetrically disposed around the mast
penetration point.  Also, the feedline and control lines should
be routed _outside_ the structure along the path of one of the
down conductors to the house service entry point and brought
inside at that location through a suppressor panel.  Yes, I realize
that this negates one of the motivations for having the tower in
the attic in the first place.

Another alternative would be to move the family into the garage,
overinsure the house, leave the reloading stuff near the tower
base, and avoid occupying the house during storm season.  Then
since you would be able to afford to relocate to a place that
permits you to have a real tower if you were struck, you could be
sure that it would never happen.

Of course, all of this is modulated by the relative probability
that _anything_ in the area will get hit by lightning.  Only the
individual considering the attic tower can evaluate his relative
risk level.

73, Eric  N7CL

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