[TowerTalk] Shack to service entrance ground

[Lux, Jim]

On 8/19/23 9:12 AM, jim.thom jim.thom at telus.net wrote:
> Date: Sat, 19 Aug 2023 10:35:30 -0400
> From: Mike H <mph at sportscliche.com>
> To: k8zm at oh.rr.com, towertalk at 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.