[Towertalk] Grounding mast to tower

[K7LXC@aol.com]

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