[TowerTalk] right sizing strap for tower connections

[jimlux]

On 10/16/17 5:10 PM, Jeff Blaine wrote:
> In the discussion of the ground connection, a mix of guys commented or 
> sent pics of round conductors where others are using strap.  The general 
> comment was that strap is better which I think means lower R & L for the 
> run.

Strap has lower AC resistance - good if you're providing a ground 
connection for a transmitting tower and you want to minimize loss.

Strap and round have about the same inductance.

For a fast pulse like lightning (about 1 microsecond rise time), 
inductance will dominate as far as *voltage rise* goes, but the 
*heating* is from resistance.

But in lightning protection, the heating is small, compared to the 
energy needed to melt the conductor.  That's why you see AWG 10 wire 
used to ground telephone poles and such - it will carry the lightning 
stroke current.. sure the top of the pole is at a much higher voltage 
than the bottom, but the current still flows.

So if you need to keep two things at the same voltage - short is better, 
the shape doesn't matter.  If you want low loss (it's part of a 
transmitting antenna), worry about resistance and use strap.

For the most part, it's a lot easier to drill holes and bolt flat 
conductors than round ones.




> 
> At the outset I was planning to use 3-gauge stranded insulated wire 
> which I have a lot of on hand.  Wire would run from the tower leg clamp, 
> over to the edge of the concrete and then down to a copper ring bus 
> where the individual runners to the rest in-ground stuff was.

Sounds like a decent plan.


> 
> But for the conductor run from tower leg to ground system I am wondering 
> what width of strap would be better than the 3 gauge stranded wire.  
> While I realize that wider is better on the strap width, I wonder what 
> width of strap would approximate the 3-gauge wire?

For lightning protection, it doesn't matter if you use wire or strap - 
so use what you have and can securely attach.


> 
> 73/jeff/ac0c
> www.ac0c.com
> alpha-charlie-zero-charlie
> 
> -----Original Message----- From: Clay Autery
> Sent: Tuesday, October 17, 2017 7:32 AM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Fwd: Grounding connection to tower legs
> 
> On 10/16/2017 6:18 PM, jimlux wrote:
>> On 10/16/17 3:44 PM, Clay Autery wrote:
>>
>> 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,
> Well...  For me?  When I have many thousands of dollars in equipment at
> risk, there is no such thing as "overkill".  :)
> 
> Anything I can do to pump as much of the strike into the ground prior to
> entry into the shack, and thence to reduce the step potential, I will
> do.  :)
> It doesn't take that much "extra" to do a job right.
> 
> Who said anything about a "long strap"?  I was responding to the man's
> question about the clamp...
> But since you bring it up.... yes...  a strap as wide as I can make
> work, AND as short as possible and of sufficient mechanical strength to
> stand up to the forces imparted. Apologize...  I assumed that those were
> givens.  <big smile>
> 
> 73,
> 
> ______________________
> Clay Autery, KY5G
> MONTAC Enterprises
> (318) 518-1389
> 
> 
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