[TowerTalk] Naive question about lightning protection

[Mark .]

Hi Sylvan,
Let me add to Tom's excellent post about providing a lightning protection 
ground for an insulated tower. You would construct the grounding system just 
the same as any other tower: a lightning ground to carry strike current into 
a large volume of earth, outlined in previous posts.
The difference is, as Tom pointed out, that there is no direct connection to 
the tower, but instead, a controlled air gap. The voltage induced by your 
transmitter will not create an arc, but a static streamer or a direct strike 
will easily ionize the air gap, jump it, and create a path for the strike 
current to dissipate.
  Since you disconnect when you are not operating, you can augment the 
protection by applying booster cable clamps, or other easily implemented 
shorts (invent one!), across the air gap, for full DC continuity and static 
dissipation.

One easy way to create a discharge gap is to bend the end of a heavy ground 
cable into a "V", with the point of the "V" aimed at the conductor to be 
protected (tower leg). The sharper the point, the earlier an arc will form, 
improving response time. The wire is bent closer to, or farther away from, 
the other conductor to adjust the air gap. Open wire feedlines can also be 
protected in this way, although some amount of surge will pass before the 
gap arcs over. (The Wireman sells a device like this for ladder line that 
uses a spark plug, I think).
These arcs have the potential to generate a lot of heat, depending on the 
amount of current being shunted, and may perhaps vaporize or even melt small 
conductors, especially during a direct strike (some day, Alice!). Certainly, 
there will be som pitting. If you use a sharp point, you may have to 
resharpen it. You might even be able to read the evidence of discharges by 
examining pitting on the points. So you will know if that overhead CRAAACK 
last night while you were in bed, hit your tower!

Does anyone have a feel for minimum sizes? I would offer an initial guess 
that you should stick to a large size, maybe #6 gauge or more.

  Using a parallel copper pipe as Tom suggests, certainly seems like a great 
idea, since its extra mass will limit the heating during the brief 
discharge, and provide more surface area for parallel discharge arcs, 
reducing the overall inductance.
An AM broadcast tower in my area uses large, solid balls on either side of 
such discharge gaps at the bases. I think there are three sets, and I have a 
trusted source who maintains a repeater there, who tells me that you can see 
arcs jumping on a regular basis in windy and stormy weather, dissipating 
static charges. Neat stuff!
Hopefully Sylvan, there will be some more ideas from those more experienced, 
lurking here.

regards,

  --...MARK_N1LO...--
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