[TowerTalk] Grounding System Question

[Jim Lux]

On 4/15/14, 6:46 PM, Tony wrote:
> Jim:
>
>> The position of the rods isn't  going to make a lot of difference in
>  > how the current flows with respect to the house.
>
> So the tower doesn't necessarily have to be concentric with the ground
> rods circling the tower?

Nope.. obviously, if one wants a perfectly even current sheet, then 
symmetry is good.  But the soil resistance probably isn't uniform (drier 
close to the house, usually, so resistance is higher).

The main thing is "lots of contact area" (why the Ufer/Concrete encased 
grounding electrode is so popular)

>
>> Is your tower on a foundation,  or planted on the bare dirt?
>
> The crank up tower It's anchored to a concrete foundation that's about 7
> feet deep. The existing ground rods are about a foot away from the edge
> of the foundation.

Then most of your practical grounding is coming from that big block of 
concrete.  Think about it.. it has something like 100 square feet of 
contact area with the soil, while a rod has what.. maybe 1 square foot 
of contact area.

So the current density of a 30kA stroke is spread out to 300A/square 
foot for the concrete. or about 2 Amps/square inch.

Compare that to the rod.. 200 Amps/square inch.  The soil around the rod 
is going to get a lot hotter (10,000 times the power dissipation: I^2 R 
and all that)  and the voltage drop is going to be a lot more  (100 
times higher gradient)






>
>> You can use strap if you find  it mechanically convenient, but from a
>  > lightning impulse standpoint, round wire works just fine.
>
> I'm glad to hear that because I already have some #2 solid and tinned
> copper wire.

Use that.

>
> Thanks Jim.
>
> Tony
>
>
>
> On 4/14/2014 9:15 PM, Jim Lux wrote:
>> 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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>>
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