WOW!!! Understandably, describing what an electrode function does in any unstabilized Hi-voltage event and its difference from Fault safety grounding is a big order to a PV audience that deals in DC predominately. Maybe I got that in reverse importance.
Anyway, IMO the key is to describe grounding two ways as, 1) an Electrical load and equipment Ground-Fault system that is connected to 2) an Earth Embedded Ground-electrode System through the common single point ground of the Service panel disconnect main jumper bonded terminal bars. (Emphasizing that the embedded ground-electrode system does not provide an electrical circuit ground-fault function.)
The Equipment ground-fault system path through the equipment grounding does provide a low impedance path back to the AC System source for operation of over-current protection breakers or fusing.
As far as the electrode system quality between a UFER and Supplemental Electrodes, both can only function to dissipate unstablized high voltages of unpredictable transients through parallel paths intended to earth. The NEC [250.52] stipulates that the UFER and metal water pipe common electrodes are to be used as available. If the UFER is not available, then the supplemental rods are required. Any other 'tapped' electrodes present are also mandated to be bonded to either common electrodes. Now, which one is better? The UFER wins IMO due to less cost and time to install.
In determining vertical soil driven electrodes versus a horizontal soil electrode effectiveness, according to data from Sandia Lab Lightning tests, the National Lightning Safety Institute has verified that approximately 85% of a stroke dissipates over the earth surface compared to a 15% depth penetration. Google up National Safety Institute or Sandia Labs testing data. Its been a while. The url didn't connect.
Edit: url changed
Last edited by gndrod; 01-12-08 at 08:41 AM.
rbj, Seattle...Safety is a Professional Courtesy.