[TowerTalk] Ground Rods

[Jim Brown]

On Tue, 4 May 2004 11:32:42 EDT, Cqtestk4xs at aol.com wrote:

>Could it be that the low resistance required is to 
>allow the tower to bleed off the charge easily and thereby reducing the chance 
>of a direct hit?  Is there anybody out there who is more of a tech than I am, 
>who would know for sure?

Research published by IEEE says that the energy content in lightning has a broad peak around the middle of the AM 
broadcast band. Simple circuit analysis says that the highest priority should be given to achieving a low INDUCTANCE 
path AND a low resistance for that energy to be discharged as quickly as possible so that the voltage on your system 
wiring doesn't build any higher than need be. Inductance is reduced by providing multiple, parallel, paths to earth, and 
keeping all of them as short as possible. Resistance is mostly reduced by getting better contact with mother earth. 

To understand this, remember  that one way lightning protection works is to offer a good discharge path for energy at 
moderate levels before it can achieve the higher levels that does the damage. 

And also remember that there is lots of resistance between ground electrodes driven into the earth at different points. 
As others have noted, it is also wise to bond those multiple paths together OUTSIDE your premises. The last thing you 
want is high potential differences across wiring within your building!  

I concur with W8JI that when possible, all grounds, including the power system ground, should be bonded at a 
common point outside the building. 

Also remember that with all of this, we are simply playing the percentages and trying to put them as much in our favor as 
possible by good engineering. Because of the huge potential energy in a ligthing strike, any given storm condition can 
cause a lightning strike with damage, despite our best implementation of good practice. That good engineering can 
raise the odds in our favor that we survive without damage, or experience less damage. 

It is far too simplistic to say that a lightning strike has enough energy to jump around whatever obstacle is in its path. 
We can reduce the energy remaining in a strike by giving it a better and faster means of discharge than through our 
equipment (or house). It's all Kirchoff's law (with XL in the equation) for the division of current in parallel circuits, and 
Ohm's law for the voltage across an impedance due to current flow.  

Jim Brown K9YC