Ideally a ground
should be of zero ohms resistance. The NEC has stated that "A single
electrode consisting of a rod, pipe, or plate which does not have a resistance
to ground of 25 ohms or less shall be augmented by one additional
electrode..." Once you have added the supplemental ground you have met the
requirement for the NEC.
This does not
mean that the value of the ground now has to be 25 ohms or less. The ground
resistance values objectives vary from industry to industry.
Telecommunications
industry has often used 5 ohms or less as their value for grounding and
bonding. The goal in ground resistance values is to achieve the lowest ground
resistance value possible that makes sense economically and physically
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