[TowerTalk] Ground Rods

[jimlux]

David Robbins wrote:
> normally  when measuring ground resistances a higher voltage source is used to overcome the drift problems that you are seeing.  There are also special methods used that require either a large remote ground or 3 rods spaced certain distances apart where a current is passed between the outer 2 and the voltage is measured between them to calculate the resistance and ground resistivity.  
> 
> where i work i have done full scale ground impulse testing for lightning protection of high voltage power lines, and i write software that uses those results and other related published works to predict ground performance during lightning strokes.  When struck by lightning the ground resistance decreases rapidly due to underground corona and streamers that effectively increase the diameter of the rods or radials.  as an example, a pair of 25.4mm by 4m rods in sand should measure around 75 ohms... when the tower is struck with about a 10ka stroke about 7ka may make it to the tower ground which causes the ground to ionize and reduce the effective resistance to less than 30 ohms.
> 
> This is an older version of the software i work on, look for the slides talking about the Weck model for grounding:
> http://ewh.ieee.org/soc/pes/lpdl/archive/lpdw.pdf
> There are many other references, try to google for "weck lightning ground".
> 


I think the whole idea in lightning grounding is mostly to keep two 
things reasonably low:
the energy dissipation density (i.e. no hot spots to start a fire or do 
other Bad things)
the voltage gradient (volts per unit distance - to avoid breakdown, 
sideflash, etc.)

This applies over the whole system, not just the ground.

Both goals are met by "spreading things around" because reducing the 
current density also reduces the voltage gradient, whatever the 
conducting medium.