Used for the code section about less than 25 ohms on ground rods or install 2 rod. the distance is not less than 6' apart Reference is 250.56 or .58 in 99 NEC, sorryI'm not under 2002 NEC yet.
The purpose is to insure a resistance of <25 ohms. Some of our local jurisdictions will allow only 1 if we want to set up the megger and prove to them that it's <25 ohms. Needless to say it's cheaper, quicker and easier to just install 2
