In a message dated 7/20/01 6:46:08 PM Pacific Daylight Time,
SPELUNK.SUENO@prodigy.net writes:
> I'm planning on putting the arrestors at the base of my tower, bolted
> to the insides of a metallic Hoffman box or similar. From the box the coax
> and control cable runs will be in 3 inch sch 40 buried conduit to the
> house/shack. The alternative is to put the arrestors in a box just outside
> the house at the end of the conduit run and ground at that point, as most
> commercial installations have it.
>
> My thinking is that the ground field for a lightning strike originates
> at the tower, where all the underground radials connect to the tower legs.
> Here the inductance of multiple parallel ground legs will be less than at
> the entrance to the house/shack, fifty feet from the tower, where I will
> have only one radial to which to connect the arrestors' grounds. (I'm not
> planning on encircling my house with buried copper ground wires.) I'd
> rather have the arrestors far away from the house and let them do their
work
> in a low-inductance part of the ground field, rather than close to the
> house.
I wouldn't recommend this course of action. A direct hit is only ONE way
that lightning transients occur. Another significant one is an induced ground
surge from a nearby strike. It doesn't even have to be very close to do
damage. In the case of a ground surge, anything buried can be a problem and
the lightning charge has got nothing to do with a tower strike. The whole
idea of a ground system is to have everything at the same potential so that
the energy rises and falls at the same rate everywhere. When that happens,
there's no arcing which is what causes most of the damage.
In any case, the surges still come along the cables and having transient
arrestors at the tower does nothing to prevent a ground surge from getting
into the house. While the conduit may give you some Faraday shield
protection, lightning paths to ground don't always go where you want them to.
So unless your arrestors at the Single Point ground bus at the building
entry, they're not doing anything worthwhile.
The other way that lightning charges can get into your building is
induced through utility wires coming into the building. These can come from
miles away. This includes power, telephone, and TV cable; all of which need
to be grounded, protected, and connected to the SPGS.
Also the idea of tower cable shields being connected to the tower means
that you're trying to give a low resistance and inductance path to ground.
Any bend in the conductors will encourage the surge to take another path
which means you really want a straight line to ground or if you need to bend
something, make it a real gradual bend - no sharp bends at all. I get the
idea that a box at the bottom of the tower with some sort of grounding
function will interrupt the straight-line lightning path which means you're
compromising your ground system again.
You want to have the cables grounded to the tower at the bottom of the
tower before the cables bend towards the building and you want to have your
lightning arrestors at the building entry attached to the Single Point ground
bus. This is the standard commercial practice. Any other designs are not as
effective and may be totally ineffective.
Cheers, Steve K7LXC
Tower Tech
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