Fortunately, wiring used in lightning protection does not have to carry
sustained current so the thermal mass of the wire can be used as a thermal time
constant to increase its carrying capacity.
Using "Lightning and Lightning Protection", 1979 as my reference, the impulse
carrying capacity of #6AWG copper wire is 200 KA for 300 microseconds, fusing
at 300 KA in the same time. The same text states that 99% of strikes are less
than 60,000 amperes and with shorter durations.The Codes may require heavier
wire, but it seems that 4 or 6 AWG is sufficient for most eventualities.
-Mike-
From: Grant Saviers <grants2@pacbell.net>
To: Jim Thomson <jim.thom@telus.net>; towertalk@contesting.com
Sent: Monday, February 2, 2015 9:30 AM
Subject: Re: [TowerTalk] [Bulk] Antenna to Shack Ground Connection
I'm considering what gage wire to use for grounding tower, ground rods
and Ufer guy anchors together, and need about 500' so cost is an issue.
The best pricing for solid bare copper I have found is $1.80/ft 2ga;
$0.85/ft 4ga; and $0.42/ft 6ga.
I suspect the code requirement for 2ga on 200/400a entry panels is to
handle the maximum interruption current of the affiliated main breaker.
The fusing currents for 2/4/6 awg copper are 72Ka/36Ka/28Ka respectively
which makes some sense. (although with the usual 2 to 4 ohm rod to
ground connection, how can current ever get beyond a few hundred amps?
Not so if the ground is domestic water supply iron or copper.)
However, in the case of a lightning strike is fusing current important
since currents are likely an order of magnitude larger or more? My
historical examination of HV tower grounds often found big wire 2/0, 4/0
etc. so is that needed for a strike and it really doesn't matter much
between 2/4/6 ga, they fuse? Or is the strike current so short that
fusing isn't a concern? Then 6 ga would be ok to tie stuff together.
Is there some engineering basis for 2ga for ground rod and tower to
shack wiring?
73,
Grant KZ1W
On 2/2/2015 3:17 AM, Jim Thomson wrote:
> Date: Sun, 1 Feb 2015 20:44:30 -0500
> From: Hans Hammarquist via TowerTalk <towertalk@contesting.com>
> To: towertalk@contesting.com
> Subject: [TowerTalk] Antenna to Shack Ground Connection
>
> ## The braid wont handle the current......so u still require 2 ga bare,
> stranded copper wire between base of tower and shacks spg. The 2 ga
> stranded, bare wire is buried in the dirt aprx 1-3 feet.
> Then its in direct contact with the dirt. Even if the braid of coax used,
> the coax would still have to be bonded to both top and bottom of the tower.
> It still wouldn’t work, since the braid is covered
> with insulation..and not in contact with the soil. Around here, the
> electrical inspector will go crazy if you don’t install the bare, stranded,
> min 2 ga copper wire between base of tower and shacks spg.
>
> Jim VE7RF
>
>
>
>
> Brian,
>
>
> Yes, you are right all the shield of the feed-lines are connected (or should
> be connected) tween the tower and the shack ground. The risk of getting the
> shields fused (burned off) is reduced with a separate grounding wire between
> the tower and the shack. Large enough your shack might even survive a direct
> hit.
>
>
> Again, the ground wire should be run close to the feed lines in order to
> minimize induced current from near strikes.
>
>
> Hans - N2JFS
>
>
>
> -----Original Message-----
> From: Brian Carling <bcarling@cfl.rr.com>
> To: Bill Turner <dezrat@outlook.com>
> Cc: Towertalk <towertalk@contesting.com>
> Sent: Fri, Jan 30, 2015 11:10 pm
> Subject: Re: [TowerTalk] Fwd: Antenna to Shack Ground Connection
>
>
> Most of us have that already in the form of out coax feedline's outer
> conductor.
> Typically this is grounded at the antenna and also in the shack. Am I right?
>
> Best regards - Brian Carling
> AF4K Crystals Co.
> 117 Sterling Pine St.
> Sanford, FL 32773
>
> Tel: +USA 321-262-5471
>
>
> _______________________________________________
>
>
>
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