[TowerTalk] Grounding System Question

[Jim Lux]

On 4/14/14, 5:48 PM, Tony wrote:
> All:
>
> My crank-up tower is located a few feet from the house and each leg is
> attached to a ground rod that's about a foot or so away from the
> foundation.
>
> I'd like to add 3 more rods in a radial pattern to help "divert" a
> potential lightning strike away from the house.

The position of the rods isn't going to make a lot of difference in how 
the current flows with respect to the house. Is your tower on a 
foundation, or planted on the bare dirt? If it's bolted to a foundation, 
that will probably be the dominant current path to the surrounding soil, 
because it's got a lot of surface area in contact with the soil.


The grounds can't be
> positioned 360 degrees around the tower due to it's close proximity to
> the house so I'm not sure how critical that is.

More important is the spacing of the rods from each other. Two rods 
driven 6" apart is basically one rod.  Spacing them two rod lengths 
apart is a good place to start. If you're worried about electrical 
codes, 6 feet is the minimum spacing (of course, if you're driving 8 
foot rods and you space them twice their length, then the 6 foot rule is 
moot)


>
> Any suggestions on this subject as well as rod spacing and copper strap
> size would be appreciated.
>

You can use strap if you find it mechanically convenient, but from a 
lightning impulse standpoint, round wire works just fine.

How big the wire has to be is more about mechanical ruggedness.  AWG 6 
is what the code requires, if you're stapling it to the house or 
something, if it's hanging out #4.

"A 4 AWG or larger copper or aluminum grounding electrode conductor 
shall be protected where exposed to physical damage.

A 6 AWG grounding electrode conductor that is free from exposure to 
physical damage shall be permitted to be run along the surface of the 
building construction without metal covering or protection where it is 
securely fastened to the construction; otherwise, it shall be in rigid 
metal conduit, intermediate metal conduit, rigid nonmetallic conduit, 
electrical metallic tubing, or cable armor."


You can get a bit wrapped around the axle/prop shaft if you start trying 
to differentiate between the "grounding electrode conductor" (the wire 
from service disconnect/feeder to the FIRST ground rod) and "bonding 
jumpers" (a wire between grounded stuff, including a pair of rods).

It is ok by the code to do #4 from panel to first rod, then #6 from there.


A lot of lightning protection systems use #2 everywhere.  #2 is what's 
required for a "ground ring" and if you're in the lightning protection 
business, likely you've got a big spool of bare AWG2 on the truck, and 
you just use it for everything.


> Thanks
>
> Tony
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