In a message dated 4/2/02 6:42:22 AM Pacific Standard Time, goudpj@mac.com
writes:
> > The mast is metal, the TB and its holding bolts are metal, it's
> > attached to the metal tower - seems like that's enough for me.
>
> Well, the only contact between the mast and the tower is through the
> point contacts across the balls, or rollers, in the bearing, which
> doesn't seem like a low impedance path.
There are over a dozen ball bearings in the Rohn thrust bearings so I
suspect that it's a pretty good contact. Even if there are some ohms of
resistance, there is virtually no gap for the strike to arc across so you
still wind up with a decent path.
Look at it like this - where else is the strike going to go? Putting a
strap from the mast to the tower won't work because of the inductive
resistance (?) so you're stuck with that path anyway.
>
> Sure, that's the plan, but if the tower is grounded and the mast
> isn't well grounded to the tower, the surge current from a strike to
> an antenna or the mast has to jump the connection point to get to
> tower ground so a transient high potential can exist.
>
Believe me - those bearing contact points (which wouldn't suffer any real
damage due to a strike anyway) would be the least of my worries. The main
point of a ground system is to keep harmful transients out of the building so
I think you're getting too hung up on something at the extreme OTHER end of
the scenario.
> > Everything except the
> > insulated elements are connected to the tower and to the tower
> ground
> > so they should be okay. You get arcing when you have potential
> > differences between conductors - that's why everything in the ground
> > system is tied together so they all rise and fall at the same rate.
> > Then you don't have any arcing.
>
> Understood. But if the mast isn't well grounded to the tower, there
> will be arcing. I'm assuming that even the minimal contact to the
> tower via the thrust bearing and rotator bearing is adequate to keep
> the mast grounded to the tower, just that it isn't a low impedance
> path for surge currents and thus some arcing could conceivable occur.
>
It's well grounded - okay? Yes - lightning is pretty unpredictable and
yes - some arcing could conceivably occur but I'd suggest spending your time,
energy, and money on the more important *other* end of the system.
Cheers,
Steve K7LXC
TOWER TECH -
Professional tower services for industry and amateurs
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