[TowerTalk] Fwd: Grounding connection to tower legs

[jimlux]

On 10/16/17 3:44 PM, Clay Autery wrote:
> Nope... not me...  Not nearly enough contact for my tastes...
> 
> I want a TRUE galvanized or stainless clamp that was tailored as closely 
> to the leg diameter as I could get and AT LEAST as wide as the strap I 
> am using...  Like 2" or better wide...  Think wide turbocharger clamps...
> Then I want either a properly wound intermediate copper shim that is 
> fully tinned on one side and bright copper on the other, OR a properly 
> formed copper strap which is fully tinned on the contact side.
> Copper strap... as wide as I an afford and get formed to fit the leg, 
> preferably with nice smooth transitions.
> 
> Then I want a suitably compatible conductive filler paste to exclude any 
> moisture form minute gaps anywhere in the clamp system.... preferably 
> where the carrier durable enough not to wash out.  (I used to make stuff 
> like this).
> 
> https://www.fastenal.com/products/details/0427932
> 
> This kind of design, but say 3 times wider....  3 fasteners.... Or 
> simply stack 3 high.
> 
> Whatever you can do to reduce resistance/heating/inductance/corrosion at 
> the junction.
> 
> 73



Just off the cuff, I think this would be overkill for *lightning 
protection*, which is why you don't see this approach in the usual 
commercial radio installation handbooks (e.g. R56)

you don't worry so much about low resistance or contact area - if it's 
reasonably close, the lightning current will arc from one to the other 
and a high current spark is VERY low impedance.

And with a long strap, if it does arc underneath, you've got somewhere 
for pressure to build up.

What *is* important is mechanical strength - as Jim K9YC pointed out, 
when you have high di/dt and high I, the magnetic forces can be very 
high - this is, I believe, the reason for "no coiled wire in lightning 
down conductors" -

while a AWG10 wire will easily carry the stroke current without melting, 
it's not strong enough to hold together if it's in a configuration where 
it will be torn apart.

I used to shrink quarters by discharging 10kJ or so in a few 
microseconds through 10 turns of AWG10 wire wrapped around a broomstick. 
  When the current pulse goes through that coil, the EM forces cause the 
coil to expand to the point where the coil fragments, arcs, and turns 
into little blobs of molten copper and solid pieces.

The worst case is if you have parallel conductors, close together, 
carrying a high current.   For instance, two conductors, 0.1 feet apart, 
carrying 20kA, will have a force of several hundred pounds (per foot of 
length) pushing them together.

in most systems, the current is being carried by parallel conductors 
that are already close together (e.g. the tower leg and a parallel 
grounding conductor), so squishing them together isn't a problem...  The 
real problem comes with dynamic effects - it's not a steady push, it's 
more like getting hit with a hammer, so things vibrate, and since most 
lightning consists of multiple strokes, you can see how resonance and 
dynamic effects can really break things up.

On the other hand, you don't need to go overboard. I'm not seeing 1" 
bolts on this kind of thing - On the vast majority of lightning 
installations I've seen, it's more like 1/2" or 3/8", and AWG 2 (just 
over 1/4") up to 2/0 (just under 3/8" diameter)

So those standard ground rod clamps (1/4" bolt tightening against the 
rod into a saddle) seem to be about right.

A hose clamp - yeah, they're not super strong in tension against the 
worm screw, and they might loosen with temperature cycling.  15 psi in a 
2" diameter rubber hose with a band clamp is going to put 30 pounds 
tension on the clamp - not too challenging in the radiator hose application.