? Warning: I am not an expert. . If the tower receives a healthy
>100kA bolt, my guess is that it could Zap the protection and follow the
wiring. I would not connect the tower ground to the subpanel. Since
bolts can be in the megavolts, I would have a plug/socket/20'
service-loop at the tower to disconnect the tower from the mains during
lightning storms. The same thing goes for the coax and rotor control
cables. (Type-C RF coaxial connectors are ideal for waterproof quick
disconnects.) A friend had a 45' tower which received a direct hit. The
bolt followed the "grounded" coax shield into the house and caused damage
to pretty much everything that was plugged into an electrical outlet. .
Remember that lightning acts like VLF RF.
Well, I don't want to get into a serious debate on lightning protection
because my experience on other reflectors has been that there are widely
differing opinions on the subject, even among the experts. There seem to be
two schools of thought: 1) it is possible to build a lightning protection
system that allows everything to remain connected, and 2) the only safe
course is to disconnect everything during a lightning storm.
Adherents to the former position point to the reliability of commercial
broadcast installations that are well protected with proper ground systems,
proper ground connections and proper surge protectors. Many hams who have
installed protection systems according to the specs of such companies as
Polyphaser will swear up and down that they had a ton of damage before the
installation and absolutely none after. The problem, of course, is that
"proper" can be *very* expensive and labor intensive to achieve. It involves
entire fields of ground rods interconnected with long low-impedance radial
runs, possible chemical treatment of the ground, Ufer grounds on towers,
low-impedance connections to a single-point station ground, high-rating
dc-blocked surge protectors, ground wire runs around buildings, careful
chassis bonding, bulkhead panels, anti-corrosion treatment, etc., etc., etc.
Adherents to the latter position say that it's all for naught -- NOTHING
will protect your station (or house or life) from a direct hit. However,
their assumptions are 1) that it is possible and practical to disconnect
every conductor between the tower and the house, 2) that you will always be
around to do so when a storm comes up, 3) that you will remember to do it,
4) that you will be motivated to do it even when a small harmless-looking
rainstorm starts, and 5) that you will not get killed by a lightning strike
while doing it.
As with many subjects in ham radio (SWR, antenna gain, rf grounds, parasitic
oscillations, etc.) the two main lightning protection camps have plenty of
vehement supporters and critics. The S/N ratio is often low in such
discussions and I don't think the issue is going to get resolved anytime
soon (especially on an Internet reflector.) Let's not start it up again
here.
For my own installation, I evaluated the frequency and intensity of local
thunderstorms, my particular tower/antenna arrangement, the practicality of
the complete-disconnect solution, and the affordable cost/labor for the
so-called complete-protection solution. I also did a *lot* of reading on the
subject.
First, my experience in nearly 30 years of living within 10 miles of my
present QTH is that we have a very low incidence of thunderstorms in this
area (less than 1/2 a dozen per year) and they are typically not very severe
(nothing like the light shows you see in Texas, the mid-Atlantic, Florida,
etc.) We get mostly cloud-to-cloud lightning and very few ground strokes. Of
course, severe storms can and do happen, but they are rare. While I was
concerned about trying to protect against a direct hit, my bigger concern
was to protect against damage caused by induced surges from nearby strikes.
It seemed to me that the odds of that kind of damage were much higher (in
fact, back in the 80's I lost a couple of modems from lightning induced
surges on the phone lines.)
Second, I have a 70' tubular crankup that normally lives at 50', which is
below the plentiful collection of surrounding trees. The tower is remote
controlled and I always lower it to 22' when lightning is in the area or
when I am away overnight. None of my other antennas is above tree level
(unfortunately).
Third, I realized that there would be many conductors to disconnect
(something like 55 of 'em) and that it would be difficult to find proper and
safe quick-disconnect connectors for them and to locate those connectors in
a place where they could be pulled well away from the house and kept dry
during a raging storm (Note: *please* don't send me e-mail about how you
solved this problem at your house.) There were also security and
child-proofing issues with the outdoor connections. I have a friend with 100
feet of Rohn 45 who swears by the disconnect method. He has a bunch of
quick-disconnects behind the radio table, which he thought was pretty cool
and pretty convenient. One time, during a particularly nasty storm, he kept
hearing snaps and pops from his closet. Turns out that the cables all ran
through the closet and induced surges were jumping from the cables to a
nearby water pipe. Obviously, the disconnects must be *outside* the house.
Fourth, I knew that I may not be home when a freak storm occurs and was
reluctant to rely on disconnecting the cables after *every* use of the
radio, which is the only way the complete disconnect approach can be made
truly foolproof. I know that I'm likely to grow lazy or neglectful about
such things. Also, my tower is 250' from the house, down a steep hill. I'm
not likely to get down there to disconnect the AC mains, so there would have
to be a disconnect point in some sort of panel mounted at the conduit
entrance to the house. More code issues, to be sure.
Finally, I was scared to death of the prospect of standing outside in the
rain frantically disconnecting cables while lightning was striking all
around me.
So, I took the protection approach. I built my ground and protection systems
as much as possible to Polyphaser specs, and spent more than $1,000 doing
it. 12 ground rods at the tower, in a radial pattern connected with 1/0
stranded wire (that's really big stuff), a 250' run of 1/0 stranded wire
buried 4-feet deep in the conduit trench (like a big horizontal ground rod),
Polyphaser surge protectors on every conductor on *both* ends of the
conduit, single-point ground outside the shack, massive ground panels at
both ends, corrosion protection, all ground rod connections Cadwelded, etc.,
etc., etc. As I said in my last post, I put surge protecting breakers on the
AC lines at the tower and where they enter the house.
Still, I keep my fingers crossed.
>Are the MOVs rated for 100kA, Dick?
Good question. No. More like 4kA, I'm afraid. The rotor protectors are rated
about 6.5kA and the coax protectors are rated 50kA. Pretty wimpy for a big
direct bolt, but should be enough for induced surges. The surge protecting
breakers at the tower end have no rating on them, but the instructions
specifically say they will not protect against a direct hit.
73, Dick, WC1M
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