Re: Ground Rod resistence
ANSI/IEEE 142 requires the resistance between the main graounding electrode and "ground" to be less than 5 Ohms for large commercial or industrial systems and 1 ohm or less for generating or transmission stations.
We get called to do testing all the time, it is required on many new installations that meet the above criteria, we use the fall of potential method, the clamp on method can only be used for certian situations and you have to know what you are doing. Megger, has a good article on the differences between fall of potential and clamp on testing if anyone cares.