Re: grounding electrodes
Originally posted by pierre: In reality, there is generally not a "low resistance" at the ground rod(s). Adding one or two rods does not really mean a whole lot in the scheme of grounding (grounding of standard type buildings).
I suspect you are right, in that adding a second rod might not change anything significant, especially if the first one had a high contact resistance. But I will continue to assert that two rods will have (about) half the resistance of one rod. The real question, as I stated above, is whether the benefit you get from the second rod is worth the cost. I do not know that answer.
Originally posted by pierre: So what is the reason for the second ground rod? Why or how did that 25 ohms come into play.
I do not know the history. I have three guesses. They are probably all three wrong, and they are not entirely consistent with each other. But it's OK to say something untrue, when you warn the reader that it might be untrue.
My
first guess really is along the lines that I had mentioned above. The code committee members probably thought about requiring a certain maximum resistance. But then they realized that it could not be reasonably achieved in all areas of the country. The "two rods" was probably just a simple way to get a reasonable installation, without making the rule any more complicated than it needed to be.
My
second guess is that the "25 ohms" was the compromise, not the "two rods." In other words, the committee probably wanted to just say "install two rods." But then some member said "What if the resistance of the first is low enough? Why should we make them install a second if the first one has a low enough resistance." So rather than making everyone install two, they added the bit about "one rod is all you need, if the ground resistance is low enough."
There was probably some debate about how low is low enough, before they settled on 25 ohms. My
final guess is that the number 25 was just a "reasonably small number" and a "reasonably round number." The explanation that Mike shared at the G&B Seminar (as I described above) is more likely to be the experiment that showed that 25 ohms was achievable, and not likely to be the experiment that determined that 25 ohms was a good value to select for the rule.
