[TowerTalk] Connecting Tower to Ground Rod

[Jim Lux]

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.