> None of the tower sections are Cadwelded. They are bolted together.
> How is it that these bolts make an adequate lightning
> connection but one
> "has" to Cadweld the ground wire to the tower at the base?
In a nutshell, there's a lot of contact area between the legs where they
mate, whether flanged, pin and cup, plate splice, or whatever. The
combination of gravity plus all of the fasteners keeps those mating surfaces
in tight contact. Any resistance (and inductance) along the length of the
tower (and that includes distributed L and R, not just what happens at the
section joints) is only going to impede current flow, which in and of itself
isn't a "bad thing", although it does have the potential (no pun intended)
for forcing more current to flow on other conductors running up the tower in
parallel, such as transmission lines. What happens along the length of the
tower in terms of L, R, and current distribution doesn't matter much as long
as you do your job correctly at the base...
Down at the bottom of the tower, there are two basic goals. First, force
everything to the same potential. This is done by bonding everything
together - tower, ground system, coax/control cable shields, etc. Second,
conduct as much of the strike directly to earth via a low-R and low-L ground
system. Any R or L at this critical point at the base of the tower will
result in potentials which can result in equipment damage as some of the
strike energy takes the unwanted path through your equipment en route to
ground. In other words, you want the bonding and ground system to be
as-perfect at the base of the tower as possible. Cadwelding the conductors
to the tower gives you the low-R bond you want, with a lifespan that should
equal that of the tower.
This is obviously a very simplified explanation for the sake of brevity.
--- Jeff WN3A
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