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@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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