[TowerTalk] copper or galvanized ground rods in red SC clay

[jimlux]

On 1/10/16 6:46 AM, Kevin Kidd wrote:
> Which doesn't mean that 2 closely spaced rods are worse than 1 rod but that
> they just aren't as effective as 2 properly spaced rods.  This assuming
> that the rods are firmly bonded together with a lo-z conductor.
>
> According to most of the lightning and grounding literature, you can
> imagine the soil dissipation area of individual rods as half of a ball the
> radius of the length of the ground rod starting at the soil surface and
> sweeping down to the tip of the rod.  Overlapping these "balls" of
> dissipation is a waste of rods.  IE, rods should be spaced 2x the rod
> length for maximum effectiveness.  I tend to believe that in most
> situations, 3ea properly spaced 8ft rods are more effective than 1ea  24ft
> rod.\


Interesting.. I don't have the three rod equation to hand, but I can say 
that a 16 ft rod (5/8") has a DC/line frequency resistance that is 60% 
of an 8 ft rod.

So two 8 ft rods, 8ft apart is essentially the same as a single 16 ft rod.


>
> On the other hand, in some soil types (ie, sand), you supposedly get some
> benefit from closely spaced rods in that the multiple rods aren't as prone
> to glass coating in huge strikes.  I have exactly zero experience with that
> type of grounding failure although we have pulled a few glass coated rods
> out of the ground.

This is the "current density" issue.  In the substation grounding 
standards there's a lot of discussion of this, because they can get 
essentially continuous faults at kiloamps.



>
> I own several different types of ground testers, 3 wire, 4 wire, clamp-on,
> etc.  I don't trust any of them to tell me anything other than DC
> resistance to ground.  Your eye and knowledge of lo-z materials is a better
> judge of proper RF grounding than any tester that I know of.
>
> We build broadcast grounding to _be_ lo z and trust that it is going to
> remain so.  If I am doing an inspection and see a long piece of #6 wire
> being used as an RF or lightning ground, it gets replaced with strap.


In broadcast practice, you're really interested in low loss, so low AC 
resistance is wanted, hence the strap is useful.  I'm not sure that's 
the case for lightning grounds, where inductance effects dominate.  It 
is the general case that a good low-loss RF ground will also be a 
low-loss lightning ground.  On the other hand, the peak current in a 
lightning stroke is probably higher.  I don't know about the average 
current  (or more important, the action: the I^2 * time) which is what 
melts wires.



  We
> typically use 4in wide copper strap with all brazed or exothermic bonded
> connections.  In general, Low Z = short as possible, large surface area and
> no sharp bends.  And yes we sometimes have to use smaller than desired
> conductors simply because that is all that will work in that situation.
> These small conductors are kept very, very short and as straight as
> possible on it's way to the main ground strap.
>
> Regards,
>
> On Sun, Jan 10, 2016 at 12:37 AM, Jim Brown <jim at audiosystemsgroup.com>
> wrote:
>
>> The reason for rod spacing is to minimize inductive coupling between them,
>> which reduces their effectiveness.
>>
>> 73, Jim K9YC
>>
>
>
>
> Kevin C. Kidd, CSRE/AMD
> AM Ground Systems Company  -  WD4RAT
> kkidd at kkbc.com  --  866-22-RADIO -- 866-227-2346
> www.amgroundsystems.com
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