This makes a lot of sense. Thanks for the explanation.
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Lux
Sent: Wednesday, October 09, 2013 10:20 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Connecting Tower to Ground Rod
On 10/9/13 8:11 PM, GARY HUBER wrote:
> National Electrical Code 250.94 will apply to the Rohn Tower and
> grounding. A minimum of #6 copper is required for bonding the tower
> and ground to the electrical service ground. With the galvanized
> tower you don't want a direct copper and galvanizing connection as
> when wet there will be electrolytic corrosion. Stainless Steel, and
> Aluminum are the most commonly used metals which can be used to
> isolate the copper wire from the galvanizing.
>
> NOTE: this BONDING of GROUNDS is the minimum REQUIRED for electrical
> safety. It is NOT adequate for the dissipation of a direct lightning
> strike.
>
Actually, AWG6 is more than capable of carrying lighting stroke current.
A 100kA pulse 50 microseconds long (a big stroke) won't melt AWG10.
The idea with using "lots o' rods" is to reduce the voltage rise, although
the inductance is probably going to set a limit on that. And, of course,
rods just aren't a very good connection to the bulk soil, so you have the
"smoking rods" problem because of the high current density at the rod/soil
interface.
This is why the concrete encased grounding electrode (Ufer ground) works so
well. It has very large surface area at the soil/grounding system interface,
so the current density is low. The surface of the concrete is in intimate
contact with 20 feet of wire (minimum), so the current density across that
interface is low as well.
The usual high quality lighting ground is a ring ground, which is basically
a long continuous electrode in contact with the soil over a large surface
area. Consider a house that is 30 feet by 50 feet with a ring ground 5 feet
away all around. That grounding electrode is 200 feet long. It would take
driving a couple dozen standard 8 foot rods to get that kind of contact
area.
When it comes to dissipation, there's really no big advantage to driving
deep; it's all about conductor/soil contact area, and spreading it over a
wide area. So burying a wire 2 feet deep that's 200 feet long around your
house is a whole lot better than driving a dozen 8 foot rods with AWG 6
connecting them above the ground.
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