no, fusing current is not important for lightning since the total energy is
actually relatively small compared to what is possible with a power line fault
lasting several cycles needed for a breaker to operate. Even though the peak
current may be higher for lightning it is such a short event that it won't heat
even relatively small wire enough to melt it. remember lightning strokes are
measured in microseconds... power fault currents for grounding purposes are
specified in seconds, often 2-3 seconds depending on the distance to the
protective device and it's settings.
also consider this, those top static lines above hv power lines that are
designed to intercept lightning strokes are usually made out of something
between 1/4" and 1/2" hs or ehs guy wire. Newer installations are using a
fiberoptic cable wrapped with essentially a single layer of steel similar to
the outer layer of guy wire.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Grant
Saviers
Sent: Monday, February 02, 2015 17:30
To: Jim Thomson; towertalk@contesting.com
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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