George That is a good question.I looked in a few of my resources and did not see any minimum value given as a point of reference. I did see a minimum of an 8 ft ground rod that shall extend 10 feet vertically in the earth.
Because that lightning protection ground rod gets bonded to the building's grounding electrode system, your results/regulation is by the NEC (more than one rod, no maximum resistance). NFPA 780 doesn't have a maximum resistance established that I'm aware of.
Larry That is a requirement from NFPA 780-Standard for the Installation of Lightning Protection Systems. 3.13.1 & 126.96.36.199 Ths standard has an illustration that shows an 8' groundrod buried 2' to make up the 10' vertical burial depth.
I have seen this done many times, as it seems the big homes and larger office buildings are all installing lightning protection these days.
If you do a radio tower you will see guys who are serious about ground electrodes. I inspected a bunch of Florida DOT "radio equipment shelters" (the little shack under the tower). They use a Ufer, coupled to ground rods (up to 40') at the corners, a ground ring and buried radials out from there. Everything is cadwelded.
Yes, Roxie and PC, I knew that. It just struck me funny the way you (PC) worded it.
About two years ago, we did the ground work for a cellular phone tower. Rods every four feet along the ground ring, which was solid #2 tinned copper, and all spliced exothermically welded; all in a 2'+ ditch.