RickH wrote:
On Oct 15, 12:59 pm, bud-- wrote:
GoHabsGo wrote:
"Dave" wrote in
rica:
Was talking with someone the other day and mentioned that I had just
grounded the TV antenna (the mast, actually) to protect against
lightning strikes, and they said that was not such a good idea because
lightning is more likely to strike a path that goes straight to
ground. Now I am not sure what to do. Anyone have any input on this
topic? Ideas are gratefully received...
Antennas should be grounded in two ways. First the mast should be grounded
and second, the cable feeding from the antenna should be grounded to a
grounding block that is wired to a ground source before entering the home.
Grounding is not only for lightning strikes. Wind blowing over the tines
creates static electrical charge that will be discharged through the ground
wire.
Larry
The NEC (if you are in the US) requires the earthing Larry describes. It
must be to the same earthing system as the power and telephone.

A separate ground rod that is not tied to the power earthing system is a
code violation and bad idea. I wouldn't even use a rod if it is bonded
to the power earthing system. With a close strike, like to a tree, the
rod can be thousands of volts from the earthing system used for power.
That voltage shows up at TVs and anything else connected to the antenna
and power.

The earthing is not for a direct lightning strike - it is totally
inadequate. If you expect the antenna (and house systems) to survive a
direct strike you would have to use the much more elaborate protection
used by hams.

--
bud--


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You guys really can't figure out this is gargbage from the retarded
google newsreader that is of no interest to anyone?


The problem is that you never want to bring the coax into the house
unless it and the mast were grounded in some way shape or form.
Grounding them inside the house is not good. So if you dont have easy
access to the service ground (maybe service meter is way over on the
other side of house). Then in that case the ground rod is your only
choice. You would not want to run a ground wire through the house to
the other side, just to get a service ground. With that you would be
bringing the lightining into the house.



An "average" lightning strike is 20,000 amps. If you have a ground rod
with a resistance of 10 ohms to earth it is a near miracle. The
lightning strike will then lift the isolated ground rod 200,000 volts
above "absolute" earth potential. (There can be arcing across the ground
surface away from the rod.) The rest of the house electrical will be
many thousands of volts from that. The coax will "be bringing lightning
into the house".

If you have lightning rods (now called air terminals), the down
conductor will be far larger than you would use on the TV antenna. An
there will probably be at least 2 of them. The earthing system of a
lightning rod system *must* be connected to the earthing system for the
building electrical. The NEC has a note referencing NFPA780, the
standard on lightning rod installations. The standard may require metal
6 feet from lightning rod conductors be bonded to the rod conductors
(for instance a rooftop A/C unit on a flat roof). There may be enough
voltage from the rod conductors to the metal to cause a side flash
across 6 feet, even with the rod and building earthing systems connected.

The provisions of the NEC for TV antennas, which are more than what is
used on some installations, are not intended to protect from a direct
lightning strike to the antenna. If you want to protect from a direct
strike find out what hams use for protection.

In gfretwell's post, a large conductor goes to the house grounding
electrode system. The house grounding system will be raised above
"absolute" earth potential, but all the wiring rises together. (That
also requires service panel suppressors (which are required by NFPA780)
and good connection to phone and cable entry protectors.)

--
bud--