[Top] [All Lists]

[TowerTalk] Strategy for Lightning Protection

To: <>
Subject: [TowerTalk] Strategy for Lightning Protection
From: ("Dick Green".)
Date: Wed, 9 Jul 1997 15:44:50 -0400
Hello, all. As the thread on lightning protection goes by on this
reflector, I just happen to be designing the cable runs and protection
system for my new antenna farm. I really like my house and my radios, so I
want the protection system to be the best I can do. I'd appreciate any
comments and some answers to a few questions. Here's a summary of what I've
come up with so far:

The antenna farm will consist of a 72' base-rotated motorized tubular
crankup with tribander, a 40M 4-square vertical phased array, an 80 meter
inverted vee (later, a broadband "DX Special"), and a GAP Titan multiband
vertical. I'll probably add a beverage later.

The cable run is about 250 feet from the operating position in the house.
There will be three PVC conduits buried four feet deep: one for 240V AC,
one for two runs of 9913 (PVC large enough to replace 9913 with hardline
later), and one for four multiwire control cables (5-position antenna
switch, tower motor control and limit switches, 4-square direction switch,
and rotator.) Each control cable will be six or eight wires. 

QUESTION: should I use shielded control cables (my gut says "yes", what do
you folks think?)

At the antenna site, the conduits will terminate at a wooden panel with two
plastic termination boxes: a small one for AC and a larger one for
everything else. There will be four ground rods surrounding and connected
to the tower and rotator.

The AC will be split into two 120V circuits, one for the tower raising
motor and one for a utility socket (a licensed electrician will do this

QUESTIONS:  Should the AC ground be connected to one of the tower ground
rods? Should there be a surge suppressor on each circuit? I think the code
will require a GFP breaker anyway. Is this adequate?

Each of the two coax runs will attach to an SO-239 feedthrough barrel
exiting the box. One connector will be grounded (it's a spare) and the
other connector will be attached to the antenna switch with a short run of
9913. Separate runs of 9913 will go from the switch to the 4-square, vee
and GAP, and one run of RG-213 will go up the tower. I plan to put a gas
discharge unit on each feedthrough connector at the plastic termination
box. I guess I can put the units on the outside of the box, with direct
connections to one of the tower ground rods, or I can put them on the
inside of the box and pass their ground straps through a hole in the bottom
of the box to a ground rod. I like the latter idea to minimize the number
of connectors exposed to the elements. 

QUESTIONS: Should I put the discharge units inside or outside the box? Can
I daisy chain them with one large ground strap or should there be a
separate strap on each protection unit? 

If possible, I would like to handle the four control lines the same way. My
installer said that there are multiwire lightning protection units for this
purpose (I assume they contain some sort of suppressor devices like MOVs.)
Also, I'd like to use feedthrough connectors for the control lines. 

QUESTIONS: Does anyone know anything about multiwire surge suppressors and
feedthrough connectors? Are they weatherproof? Are they available?

At the house end of the cable run, there will be another pair of
termination boxes, one for the AC and one for all the other cables. There
will be one or more ground rods as close to the panel as possible
(depending on whether there's gravel right next to the house.) This will be
the main station ground.

The 240V AC line will run into the basement (probably to a junction box)
and along the ceiling to the main service panel.

QUESTIONS: Should the AC line ground be attached to the station ground?
Should there be a surge suppressor in the termination box (again, there
will probably be a GFP breaker in the main panel)?

The other cables will be attached to gas discharge units or surge
suppressors inside the plastic termination box. The protection units will
be attached to the station ground rod via separate ground strap.

Question: (Same as before) Can I daisy chain one large ground strap or
should there be a separate strap on each protection unit? 

The back of the box will be cut out, exposing the space between the studs.
At the back of the space, there will be a copper bus panel mounted in the
drywall of the shack room. The bus panel will have (if possible)
feedthrough connectors just like the ones in the box down by the tower. The
bus panel will have one heavy strap to the station ground. Each piece of
radio equipment will have its chassis ground connected via a short strap to
a copper bar (or pipe) on the back of the desk, which will in turn be
connected with a heavy strap to the bus panel. The AC and Telco grounds
currently installed in the room will be attached to the bus panel as well.

The tower ground rods are 250 feet from the station ground rods and the AC
and telco ground rods are at least 150 feet from the station ground (and
350 feet from the tower ground rods.)

QUESTION: At these distances, is it worth trying to tie all these grounds
rods together?

My installer thinks this type of lightning protection will be more than
adequate, and that I should not try to disconnect the cables when there's a
big storm in the area. His theory is that I could get killed if there's a
strike while I'm disconnecting the cables. My worry is that a direct hit or
close strike would put so much juice on the lines that the ground system
wouldn't be able to handle it. My guess is that such an event would put a
lot of energy on the bus panel, which would in turn cause the adjacent
portions of the house to burst into flames.

QUESTIONS: Wouldn't it be best to disconnect the lightning protection units
at the bus panel and pull the whole mass of cables and protection units
(with ground straps still attached,) back and away from the house? Wouldn't
I have to disconnect the station ground strap from the bus panel to keep
the juice off it? Since I can't disconnect the 240VAC line, won't the heavy
juice still make it to the bus panel that way? Should I just leave
everything connected?

QUESTIONS: What do you think of the design, and any other comments?

73, Dick, WC1M

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>