[TowerTalk] Ground Rods

[Jim Lux]

At 08:07 AM 5/4/2004 -0700, Bill Turner wrote:
>On Mon, 3 May 2004 18:38:26 -0600, Grillo's wrote:
>
> >One of the calculations I was looking at can drop impedance from 300 
> down to 5 ohms with using only 4 of these rods.
>
>_________________________________________________________
>
>Does it really matter to a multi-million volt lightning strike whether
>the ground is 300 ohms or five?  The difference would seem to be
>inconsequential.
>
>This reminds me of the school of thought that says the connection to
>your ground rods must be tight.  As if lightning, having traveled
>thousands of feet through the air, is going to be stopped by a gap of a
>few thousands of an inch.

The voltage in a lightning stroke is immaterial (mostly because it's spread 
over many thousands of meters of air), but the current is what does the 
damage (or, in some cases, the di/dt, which induces a voltage in nearby 
conductors). The goal is to keep the voltages in your system low enough to 
be reasonably handled.

Figure 25kA for a typical lightning stroke.. 25kA at 5 ohms is 125V.. not 
enough to flash over in air and well within the transient suppressing 
ability of many devices.
25kA @ 300ohms is 7500V, more than enough to flashover to adjacent 
conductors, and well beyond most transient suppression devices.


So.. reasonably low impedance is a "good thing", although the practical 
difference between 5 ohms and 10 ohms isn't all that signficant.

Jim, W6RMK