On 8/19/23 9:12 AM, jim.thom jim.thom@telus.net wrote:
Date: Sat, 19 Aug 2023 10:35:30 -0400
From: Mike H <mph@sportscliche.com>
To: k8zm@oh.rr.com, towertalk@contesting.com
Subject: Re: [TowerTalk] Shack to service entrance ground
<According the the ARRL Grounding and Bonding book, Good Practices and
<Guidelines:
<"Should I bond my tower to the station ground?
<...Once the distance (between tower and station) exceeds 40 to 50 feet,
<however, the inductance of the ground conductor will be too high for the
<bond to be effective."
<Mike WB2FKO
The tower is gonna ultimately be bonded to the station grnd anyway..... via
the braid of the coax.
IMO, bond the braid of the coax at the top of the tower, and again at the
bottom of the tower, then at the SPG.
Then run BARE, buried 2 ga CU stranded cable, from tower grnd setup to
SPG. I used 2 ga cu stranded RW-90 power cable from each tower leg...to
it's own 8' rod...and cadwelded at the grnd rod.
Burying 50-100 ft of bare #2 is like driving a bunch of ground rods,
it's a fairly effective grounding electrode in its own right.
But it's unclear what the physics of needing #2 is:
1) Even a big lightning discharge isn't carrying enough "action"
(integrated current squared * time) to come close to melting #10, much
less #2. and if it's buried even less so.
2) As just pointed out, the inductance of 100 ft (30 meters) is 30
microhenries - with a 1 microsecond rise time lightning impulse the
voltage drop is huge. So huge, it's essentially an open circuit.
V = L di/dt = 30E-6 * 20E3/1E-6 = 600 kV for a 20kA stroke current.
3) If you were concerned about line voltage/frequency faults - I would
assume there's overcurrent protection at WAY lower than #2 sorts of
ampacity.
4) If you're concerned about "power line falling and shorting to
antenna" now you're in a potential "high current, low frequency" scenario.
I suspect the popularity of AWG #2 for these kinds of applications is
a)in commercial practice you've got a big spool of wire on the truck,
and you tend to use the same thing for everything; b) AWG #2 is
mechanically rugged. That's almost certainly the case for a doc like R56
or the FAA doc - They're providing a spec to contractors that is "bullet
proof" but "buildable and biddable" - They're not as cost sensitive
about the cost of the copper, for instance, since the labor costs will
dominate.
And then once everyone uses AWG 2, things like cad-weld forms, clamps,
etc, all tend to be made for that size.
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