[TowerTalk] Disconnecting cables and lightning (questions --

[Tom Rauch]

> Date:          Thu, 07 May 1998 08:56:22 -0400
> From:          Pete Smith <n4zr@contesting.com>

> Polyphaser and ICE make much of the ability of their line protectors,
> properly installed, to protect against lightning hits even if the lines
> remain connected throughout the event.

Do they pay for equipment damage if the protection fails?

By far most of the energy travels on the outside of the cables, 
and the bulk of energy INSIDE the cable is common mode...not 
differential mode that kind that fires the gap.

What that means is even if you solidly ground the shield to the 
center, a lot of damage will still occur. This is especially true if 
the coax connects to a small antenna element.

> 1.  Am I wrong?

No.
 
> 2.  Would I be better off grounding each of the conductors coming from the
> tower at the entry bulkhead, instead of letting them float, as now, even
> though I know the grounding of the bulkhead is relatively poor in terms of
> inductance?

No.
 
> 3.  What about a disconnect and/or grounding panel at ground level, near
> the AC service entry?  Worth doing, for the sake of the better, and more
> unified ground?

That would be better. That way in case you forget to remove the 
cable, the bulk of a stroke would probably bypass your equipment.
   
> 4.  Some people have written here recently about cable grounding schemes
> using relays to switch their cables from the shack to ground.  It seems to
> me that with the close spacing of relay contacts, the ground they're
> switching to better be pretty darned good, or arc-over and equipment damage
> will occur regardless.  Wrong again?

Nope, you are correct. Let me give just one example....

I had a 130 ft grounded tower, with 110 #8 copper radials most over 
150 feet long , in a wet marshy location. The feedline was buried in 
very wet black acidic soil, and the antenna had just such a relay 
scheme using large contactors like AM broadcast stations use (so my 
tiny transmitter wouldn't blow them up). My main power disconnect 
was a similar contactor, and I switched these contactors to an off 
position when I wasn't using my station. Kerchunk! Some VERY big 
relays!

The shunt fed tower, dc blocked by a 700 pF 15 kV vacuum 
variable, had lightning arc through the vacuum cap (melting the 
plates) and followed the cable into the house. The common shield 
connection across the relays carried the current to the chassis of my 
equipment, where it eventually arced from the power line ground 
connections to the chassis of the equipment, blowing out all kinds of 
components.

It also coupled, probably by common mode coupling or a relay arc, to 
the center conductor of the coax and melted the line bypass 
caps and antenna relay board in my T4XC.

Since that day, I do like you and disconnect the cables outside the 
house. I also, having lost a vacuum cap capable of withstanding 30 
kV peak and several hundred amperes of RF current on a well 
dc-grounded shunt fed tower, completely lost faith in the elixir that 
a dog-gnat sized dc blocking cap would do squat in a hit. 

The CBer down the road has a large glass jug filled with pennies and 
saltwater grounding his tower and rig. He puts his cables in a glass 
jar wrapped with foil, and collects the lightning. He says it works, 
because he has never had any equipment damage since he installed
that system. 

73, Tom W8JI
w8ji.tom@MCIONE.com

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